.\"Copyright (C) 2009-2014 Kaz Kylheku . .\"All rights reserved. .\" .\"BSD License: .\" .\"Redistribution and use in source and binary forms, with or without .\"modification, are permitted provided that the following conditions .\"are met: .\" .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in .\" the documentation and/or other materials provided with the .\" distribution. .\" 3. The name of the author may not be used to endorse or promote .\" products derived from this software without specific prior .\" written permission. .\" .\"THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR .\"IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED .\"WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. .TH "TXR" 1 2014-06-11 "Utility Commands" "TXR Text Processing Language" "Kaz Kylheku" .SH NAME txr \- text processing language (version 90) .SH SYNOPSIS .B txr [ options ] query-file { data-file }* .sp .SH DESCRIPTION .B TXR is a language oriented toward processing text from files or streams, using multiple programming paradigms. A .B TXR script is called a query, and it specifies a pattern which matches (a prefix of) an entire file, or multiple files. Patterns can consists of large chunks of multi-line freeform text, which is matched literally against material in the input sources. Free variables occurring in the pattern (denoted by the @ symbol) are bound to the pieces of text occurring in the corresponding positions. If the overall match is successful, then .B TXR can do one of two things: it can report the list of variables which were bound, in the form of a set of variable assignments which can be evaluated by the .B eval command of the POSIX shell language, or generate a custom report according to special directives in the query. Patterns can be arbitrarily complex, and can be broken down into named pattern functions, which may be mutually recursive. TXR patterns can work horizontally (characters within a line) or vertically (spanning multiple lines). Multiple lines can be treated as a single line. In addition to embedded variables which implicitly match text, the .B TXR query language supports a number of directives, for matching text using regular expressions, for continuing a match in another file, for searching through a file for the place where an entire sub-query matches, for collecting lists, and for combining sub-queries using logical conjunction, disjunction and negation, and numerous others. Furthermore, embedded within TXR is a powerful Lisp dialect. TXR Lisp supports functional and imperative programming, and provides data types such as symbols, strings, vectors, hash tables with weak reference support, lazy lists, and arbitrary-precision (bignum integers). .SH ARGUMENTS AND OPTIONS Options which don't take an argument may be combined together. The -v and -q options are mutually exclusive. Of these two, the one which occurs in the rightmost position in the argument list dominates. The -c and -f options are also mutually exclusive; if both are specified, it is a fatal error. .IP -Dvar=value Bind the variable .IR var to the value .IR value prior to processing the query. The name is in scope over the entire query, so that all occurrence of the variable are substituted and match the equivalent text. If the value contains commas, these are interpreted as separators, which give rise to a list value. For instance -Da,b,c creates a list of the strings "a", "b" and "c". (See Collect Directive bellow). List variables provide a multiple match. That is to say, if a list variable occurs in a query, a successful match occurs if any of its values matches the text. If more than one value matches the text, the first one is taken. .IP -Dvar Binds the variable .IR var to an empty string value prior to processing the query. .IP -q Quiet operation during matching. Certain error messages are not reported on the standard error device (but the if the situations occur, they still fail the query). This option does not suppress error generation during the parsing of the query, only during its execution. .IP -d Invoke the interactive txr debugger. See the DEBUGGER section. .IP -v Verbose operation. Detailed logging is enabled. .IP -b This is a deprecated option, which is silently ignored. In TXR versions prior to 90, the printing of variable bindings (see -B option) was implicit behavior which was automatically suppressed in certain situations. The -b option suppressed it unconditionally. .IP -B If the query is successful, print the variable bindings as a sequence of assignmnents in shell syntax can be evaled by a POSIX shell. If the query failes, print the word "false". Evaluation of this word by the shell has the effect of producing an unsuccessful termination status from the eval command. .IP "-l or --lisp-bindings" This option implies -B. Print the variable bindings in Lisp syntax instead of shell syntax. .IP "-a num" This option implies -B. The decimal integer argument specifies the maximum number of array dimensions to use for list-valued variable bindings. The default is 1. Additional dimensions are expressed using numeric suffixes in the generated variable names. For instance, consider the three-dimensional list arising out of a triply nested collect: ((("a" "b") ("c" "d")) (("e" "f") ("g" "h"))). Suppose this is bound to a variable V. With -a 1, this will be reported as: V_0_0[0]="a" V_0_1[0]="b" V_1_0[0]="c" V_1_1[0]="d" V_0_0[1]="e" V_0_1[1]="f" V_1_0[1]="g" V_1_1[1]="h" The leftmost bracketed index is the most major index. That is to say, the dimension order is: NAME_m_m+1_..._n[1][2]...[m-1]. .IP "-c query" Specifies the query in the form of a command line argument. If this option is used, the query-file argument is omitted. The first non-option argument, if there is one, now specifies the first input source rather than a query. Unlike queries read from a file, (non-empty) queries specified as arguments using -c do not have to properly end in a newline. Internally, TXR adds the missing newline before parsing the query. Thus -c "@a" is a valid query which matches a line. Example: # read two lines "1" and "2" from standard input, # binding them to variables a and b. Standard # input is specified as - and the data # comes from shell "here document" redirection. txr -B -c "@a @b" - < hello.txr #!/usr/bin/txr @(bind a "Hey") @(output) Hello, world! @(end) $ chmod a+x hello.txr $ ./hello.txr Hello, world! When this plain hash bang line is used, txr receives the name of the script as an argument. Therefore, it is not possible to pass additional options to txr. For instance, if the above script is invoked like this $ ./hello.txr -B the -B option isn't processed by txr, but treated as an additional argument, just as if "txr -B" had been executed directly. This behavior is useful if the script author wants not to expose the txr options to the user of the script. However, the hash bang line can use the -f option: #!/usr/bin/txr -f Now, the name of the script is passed as an argument to the -f option, and TXR will look for more options after that, so that the resulting program appears to accept TXR options. Now we can run $ ./hello.txr -B Hello, world! a="Hey" The -B option is honored. On some oeprating systems, it is not possible to pass more than one argument through the hash bang mechanism. That is to say, this will not work. #!/usr/bin/txr -B -f To support systems like this, TXR supports special argument called --args. With --args, it is possible to encode multiple arguments into one argument. The --args option must be followed by a separator character, chosen by the programmer. The characters after that are split into multiple arguments on the separator character. The --args option is then removed from the argument list and replaced with these arguments, which are processed in its place. Example: #!/usr/bin/txr --args:-B:-f The above has the same behavior as #!/usr/bin/txr -B -f on a system which supports multiple arguments in hash bang. The separator character is the colon, and so the remainder of tha rgument, -B:-f, is split into the two arguments -B -f. .SS Whitespace Outside of directives, whitespace is significant in TXR queries, and represents a pattern match for whitespace in the input. An extent of text consisting of an undivided mixture of tabs and spaces is a whitespace token. Whitespace tokens match a precisely identical piece of whitespace in the input, with one exception: a whitespace token consisting of precisely one space has a special meaning. It is equivalent to the regular expression @/[ ]+/: match an extent of one or more spaces (but not tabs!) Thus, the query line "a b" (one space) matches texts like "a b", "a b", et cetera (arbitrary number of tabs and spaces between a and b). However "a b" (two spaces) matches only "a b" (two spaces). For matching a single space, the syntax @\e can be used (backslash-escaped space). It is more often necessary to match multiple spaces, than to exactly match one space, so this rule simplifies many queries and adds inconvenience to only few. In output clauses, string and character literals and quasiliterals, a space token denotes a space. .SS Text Query material which is not escaped by the special character @ is literal text, which matches input character for character. Text which occurs at the beginning of a line matches the beginning of a line. Text which starts in the middle of a line, other than following a variable, must match exactly at the current position, where the previous match left off. Moreover, if the text is the last element in the line, its match is anchored to the end of the line. An empty query line matches an empty line in the input. Note that an empty input stream does not contain any lines, and therefore is not matched by an empty line. An empty line in the input is represented by a newline character which is either the first character of the file, or follows a previous newline-terminated line. Input streams which end without terminating their last line with a newline are tolerated, and are treated as if they had the terminator. Text which follows a variable has special semantics, discussed in the section Variables below. A query may not leave unmatched material in a line which is covered by the query. However, a query may leave unmatched lines. In the following example, the query matches the text, even though the text has an extra line. Query: Four score and seven years ago our Text: Four score and seven years ago our forefathers In the following example, the query .B fails to match the text, because the text has extra material on one line. Query: I can carry nearly eighty gigs in my head Text: I can carry nearly eighty gigs of data in my head Needless to say, if the text has insufficient material relative to the query, that is a failure also. To match arbitrary material from the current position to the end of a line, the "match any sequence of characters, including empty" regular expression @/.*/ can be used. Example: Query: I can carry nearly eighty gigs@/.*/ Text: I can carry nearly eighty gigs of data In this example, the query matches, since the regular expression matches the string "of data". (See Regular Expressions section below). Another way to do this is: Query: I can carry nearly eighty gigs@(skip) .SS Special Characters in Text Control characters may be embedded directly in a query (with the exception of newline characters). An alternative to embedding is to use escape syntax. The following escapes are supported: .IP @\e A backslash immediately followed by a newline introduces a physical line break without breaking up the logical line. Material following this sequence continues to be interpreted as a continuation of the previous line, so that indentation can be introduced to show the continuation without appearing in the data. .IP @\e A backslash followed by a space encodes a space. This is useful in line continuations when it is necessary for leading spaces to be preserved. For instance the two line sequence abcd@\ @\e efg is equivalent to the line abcd efg The two spaces before the @\e in the second line are consumed. The spaces after are preserved. .IP @\ea Alert character (ASCII 7, BEL). .IP @\eb Backspace (ASCII 8, BS). .IP @\et Horizontal tab (ASCII 9, HT). .IP @\en Line feed (ASCII 10, LF). Serves as abstract newline on POSIX systems. .IP @\ev Vertical tab (ASCII 11, VT). .IP @\ef Form feed (ASCII 12, FF). This character clears the screen on many kinds of terminals, or ejects a page of text from a line printer. .IP @\er Carriage return (ASCII 13, CR). .IP @\ee Escape (ASCII 27, ESC) .IP @\exHEX A @\ex followed by a sequence of hex digits is interpreted as a hexadecimal numeric character code. For instance @\ex41 is the ASCII character A. .IP @\eOCTAL A @\e followed by a sequence of octal digits (0 through 7) is interpreted as an octal character code. For instance @\e010 is character 8, same as @\eb. .PP Note that if a newline is embedded into a query line with @\en, this does not split the line into two; it's embedded into the line and thus cannot match anything. However, @\en may be useful in the @(cat) directive and in @(output). .SS Character Handling and International Characters .B TXR represents text internally using wide characters, which are used to represent Unicode code points. The query language, as well as all data sources, are assumed to be in the UTF-8 encoding. In the query language, extended characters can be used directly in comments, literal text, string literals, quasiliterals and regular expressions. Extended characters can also be expressed indirectly using hexadecimal or octal escapes. On some platforms, wide characters may be restricted to 16 bits, so that .B TXR can only work with characters in the BMP (Basic Multilingual Plane) subset of Unicode. .B TXR does not use the localization features of the system library; its handling of extended characters is not affected by environment variables like LANG and L_CTYPE. The program reads and writes only the UTF-8 encoding. If .B TXR encounters an invalid bytes in the UTF-8 input, what happens depends on the context in which this occurs. In a query, comments are read without regard for encoding, so invalid encoding bytes in comments are not detected. A comment is simply a sequence of bytes terminated by a newline. In lexical elements which represent text, such as string literals, invalid or unexpected encoding bytes are treated as syntax errors. The scanner issues an error message, then discards a byte and resumes scanning. Certain sequences pass through the scanner without triggering an error, namely some UTF-8 overlong sequences. These are caught when when the lexeme is subject to UTF-8 decoding, and treated in the same manner as other UTF-8 data, described in the following paragraph. Invalid bytes in data are treated as follows. When an invalid byte is encountered in the middle of a multibyte character, or if the input ends in the middle of a multibyte character, or if a character is extracted which is encoded as an overlong form, the UTF-8 decoder returns to the starting byte of the ill-formed multibyte character, and extracts just that byte, mapping it to the Unicode character range U+DC00 through U+DCFF. The decoding resumes afresh at the following byte, expecting that byte to be the start of a UTF-8 code. Furthermore, because TXR internally uses a null-terminated character representation of strings which easily interoperates with C language interfaces, when a null character is read from a stream, TXR converts it to the code U+DC00. On output, this code converts back to a null byte, as explained in the previous paragraph. By means of this representational trick, TXR can handle textual data containing null bytes. .SS Regular Expression Directives In place of a piece of text (see section Text above), a regular expression directive may be used, which has the following syntax: @/RE/ where the RE part enclosed in slashes represents regular expression syntax (described in the section Regular Expressions below). Long regular expressions can be broken into multiple lines using a backslash-newline sequence. Whitespace before the sequence or after the sequence is not significant, so the following two are equivalent: @/reg \e ular/ @/regular/ There may not be whitespace between the backslash and newline. Whereas literal text simply represents itself, regular expression denotes a (potentially infinite) set of texts. The regular expression directive matches the longest piece of text (possibly empty) which belongs to the set denoted by the regular expression. The match is anchored to the current position; thus if the directive is the first element of a line, the match is anchored to the start of a line. If the regular expression directive is the last element of a line, it is anchored to the end of the line also: the regular expression must match the text from the current position to the end of the line. Even if the regular expression matches the empty string, the match will fail if the input is empty, or has run out of data. For instance suppose the third line of the query is the regular expression @/.*/, but the input is a file which has only two lines. This will fail: the data has line for the regular expression to match. A line containing no characters is not the same thing as the absence of a line, even though both abstractions imply an absence of characters. Like text which follows a variable, a regular expression directive which follows a variable has special semantics, discussed in the section Variables below. .SS Variables Much of the query syntax consists of arbitrary text, which matches file data character for character. Embedded within the query may be variables and directives which are introduced by a @ character. Two consecutive @@ characters encode a literal @. A variable matching or substitution directive is written in one of several ways: @NAME @{NAME} @*NAME @*{NAME} @{NAME /RE/} @{NAME (FUN [ ARGS ... ])} @{NAME NUMBER} The forms with an * indicate a long match, see Longest Match below. The last two forms with the embedded regexp /RE/ or number have special semantics, see Positive Match below. The identifier t cannot be used as a name; it is a reserved symbol which denotes the value true. An attempt to use the variable @t will result in an exception. The symbol nil can be used as a variable name, but it has special semantics, described in a section below. When the @NAME form is used, the name itself may consist of any combination of one or more letters, numbers, and underscores. It may not look like a number, so that for instance 123 is not a valid name, but 12A is valid. Case is sensitive, so that @FOO is different from @foo, which is different from @Foo. The braces around a name can be used when material which follows would otherwise be interpreted as being part of the name. When a name is enclosed in braces, the following additional characters may be used as part of the name: ! $ % & * + - < = > ? \e ^ _ ~ The rule holds that a name cannot look like a number so +123 is not a name, but these are valid names: a->b, *xyz*, foo-bar. The syntax @FOO_bar introduces the name "FOO_bar", whereas @{FOO}_bar means the variable named "FOO" followed by the text "_bar". There may be whitespace between the @ and the name, or opening brace. Whitespace is also allowed in the interior of the braces. It is not significant. If a variable has no prior binding, then it specifies a match. The match is determined from some current position in the data: the character which immediately follows all that has been matched previously. If a variable occurs at the start of a line, it matches some text at the start of the line. If it occurs at the end of a line, it matches everything from the current position to the end of the line. .SS Negative Match If a variable is one of the plain forms @NAME, @{NAME}, @*NAME or @*{NAME}, then this is a "negative match". The extent of the matched text (the text bound to the variable) is determined by looking at what follows the variable, and ranges from the current position to some position where the following material finds a match. This is why this is called a "negative match": the spanned text which ends up bound to the variable is that in which the match for the trailing material did not occur. A variable may be followed by a piece of text, a regular expression directive, a function call, a directive, another variable, or nothing (i.e. occurs at the end of a line). These cases are discussed in detail below. .SS Variable Followed by Nothing If the variable is followed by nothing, the negative match extends from the current position in the data, to the end of the line. Example: pattern: "a b c @FOO" data: "a b c defghijk" result: FOO="defghijk" .SS Variable Followed by Text For the purposes of determining the negative match, text is defined as a sequence of literal text and regular expressions, not divided by a directive. So for instance in this example: @a:@/foo/bcd e@(maybe)f@(end) the variable @a is considered to be followed by ":@/foo/bcd e". If a variable is followed by text, then the extent of the negative match is determined by searching for the first occurrence of that text within the line, starting at the current position. The variable matches everything between the current position and the matching position (not including the matching position). Any whitespace which follows the variable (and is not enclosed inside braces that surround the variable name) is part of the text. For example: pattern: "a b @FOO e f" data: "a b c d e f" result: FOO="c d" In the above example, the pattern text "a b " matches the data "a b ". So when the @FOO variable is processed, the data being matched is the remaining "c d e f". The text which follows @FOO is " e f". This is found within the data "c d e f" at position 3 (counting from 0). So positions 0-2 ("c d") constitute the matching text which is bound to FOO. .SS Variable Followed by a Function Call or Directive If the variable is followed by a function call, or a directive, the extent is determined by scanning the text for the first position where a match occurs for the regular expression, call or directive. (For a description of functions, see FUNCTIONS.) Note that the given variable and the function or directive are considered in isolation. This means, for instance, that @var@(skip)text is a degenerate form. The @(skip) will be processed alone, without regard for the trailing text and so consume the input to the end of the line. The right way to express the most probable intent of this is @{var}text. Another degenerate case is @var@(bind ...), or in general, a variable followed by some directive not used for matching text. Watch out for the following pitfall: @a @b@(bind have_ab "y") The intent here is that the variable b captures everything after the space to the end of the line, and then the variable have_ab is set to "y". But since @(bind) always succeeds, b captures an empty string, and then the whole line fails if there is any material after the space. The right way to do this is: @a @b@(eol)@(bind have_ab "y") That is to say, match an explicit @(eol) after the variable. This will search for the end of the line and capture the spanning text into b, as intended. The bind then happens afterward. .SS Consecutive Variables If an unbound variable specified a fixed-width match or a regular expression, then the issue of consecutive variables does not arise. Such a variable consumes text regardless of any context which follows it. However, what if an unbound variable with no modifier is followed by another variable? The behavior depends on the nature of the other variable. If the other variable also has no modifier, this is a semantic error which will cause the query to fail. A diagnostic message will be issued, unless operating in quiet mode via -q. The reason is that there is no way to bind two consecutive variables to an extent of text; this is an ambiguous situation, since there is no matching criterion for dividing the text between two variables. (In theory, a repetition of the same variable, like @FOO@FOO, could find a solution by dividing the match extent in half, which would work only in the case when it contains an even number of characters. This behavior seems to have dubious value). An unbound variable may be followed by one which is bound. The bound variable is replaced by the text which it denotes, and the logic proceeds accordingly. Variables are never bound to regular expressions, so the regular expression match does not arise in this case. The @* syntax for longest match is available. Example: pattern: "@FOO:@BAR@FOO" data: "xyz:defxyz" result: FOO=xyz, BAR=def Here, FOO is matched with "xyz", based on the delimiting around the colon. The colon in the pattern then matches the colon in the data, so that BAR is considered for matching against "defxyz". BAR is followed by FOO, which is already bound to "xyz". Thus "xyz" is located in the "defxyz" data following "def", and so BAR is bound to "def". If an unbound variable is followed by a variable which is bound to a list, or nested list, then each character string in the list is tried in turn to produce a match. The first match is taken. An unbound variable may be followed by another unbound variable which specifies a regular expression or function call match. This is a special case called a "double variable match". What happens is that the text is searched using the regular expression or function. If the search fails, than neither variable is bound: it is a matching failure. If the search succeeds, than the first variable is bound to the text which is skipped by the search. The second variable is bound to the text matched by the regular expression or function. Examples: pattern: "@foo@{bar /abc/}" data: "xyz@#abc" result: foo="xyz@#", BAR="abc" .SS Consecutive Variables Via Directive Two variables can be de-facto consecutive in a manner shown in the following example: @var1@(all)@var2@(end) The @(all) directive does nothing other than assert that all clauses must match. It has only one clause, @var2. So this is equivalent to just @var1@var2, except that if both variables are unbound, no semantic error is identified in this situation. Such a situation is handled as a variable followed by a directive. Of course @var2 matches everything at the current position, and so @var1 ends up with nothing. Example 1: b matches at position 0 and a gets nothing: pattern: "@a@(all)@b@(end)" data: "abc" result: a="" b="abc" Example 2: *a specifies longest match (see Longest Match below), and so a gets everything: pattern: "@*a@(all)@b@(end)" data: "abc" result: a="abc" b="" .SS Longest Match The closest-match behavior for the negative match can be overridden to longest match behavior. A special syntax is provided for this: an asterisk between the @ and the variable, e.g: pattern: "a @*{FOO}cd" data: "a b cdcdcdcd" result: FOO="b cdcdcd" pattern: "a @{FOO}cd" data: "a b cdcdcd" result: FOO="b " In the former example, the match extends to the rightmost occurrence of "cd", and so FOO receives "b cdcdcd". In the latter example, the * syntax isn't used, and so a leftmost match takes place. The extent covers only the "b ", stopping at the first "cd" occurrence. .SS Positive Match There are syntax variants of variable syntax which have an embedded expression enclosed with the variable in braces: @{NAME /RE/} @{NAME (FUN [ARGS ...])} @{NAME NUMBER} These specify a variable binding that is driven by a positive match derived from a regular expression, function or character count, rather than from trailing material (which is regarded as a "negative" match, since the variable is bound to material which is .B skipped in order to match the trailing material). In the /RE/ form, the match extends over all characters from the current position which match the regular expression RE. (see Regular Expressions section below). In the (FUN [ARGS ...]) form, the match extends over characters which are matched by the call to the function, if the call succeeds. Thus @{x (y z w)} is just like @(y z w), except that the region of text skipped over by @(y z w) is also bound to the variable x. See FUNCTIONS below. In the NUMBER form, the match processes a field of text which consists of the specified number of characters, which must be nonnegative number. If the data line doesn't have that many characters starting at the current position, the match fails. A match for zero characters produces an empty string. The text which is actually matched by this construct is all text within the specified field, but excluding leading and trailing whitespace. If the field contains only spaces, then an empty string is extracted. This syntax is processed without consideration of what other syntax follows. A positive match may be directly followed by an unbound variable. .SS Symbol nil as a Variable If the symbol nil is used as a variable, it behaves like a variable which has no binding. Furthermore, no binding is created. @nil allows the variable matching syntax to be used to skip material, in ways different from and complementary to @(skip). .SS Regular Expressions Regular expressions are a language for specifying sets of character strings. Through the use of pattern matching elements, regular expression is able to denote an infinite set of texts. .B TXR contains an original implementation of regular expressions, which supports the following syntax: .IP . (period) is a "wildcard" that matches any character. .IP [] Character class: matches a single character, from the set specified by special syntax written between the square brackets. Supports basic regexp character class syntax; no POSIX notation like [:digit:]. The regex tokens \es, \ed and \ew are permitted in character classes, but not their complementing counterparts. These tokens simply contribute their characters to the class. The class [a-zA-Z] means match an uppercase or lowercase letter; the class [0-9a-f] means match a digit or a lowercase letter; the class [^0-9] means match a non-digit, et cetera. There are no locale-specific behaviors in TXR regular expressions; [A-Z] denotes an ASCII/Unicode range of characters. The class [\ed.] means match a digit or the period character. A ] or - can be used within a character class, but must be escaped with a backslash. A ^ in the first position denotes a complemented class, unless it is escaped by backslash. In any other position, it denotes itself. Two backslashes code for one backslash. So for instance [\e[\e-] means match a [ or - character, [^^] means match any character other than ^, and [\e^\e\e] means match either a ^ or a backslash. Regex operators such as *, + and & appearing in a character class represent ordinary characters. The characters -, ] and ^ occurring outside of a character class are ordinary. Unescaped / characters can appear within a character class. The empty character class [] matches no character at all, and its complement [^] matches any character, and is treated as a synonym for the . (period) wildcard operator. .IP "\es, \ew and \ed" These regex tokens each match a single character. The \es regex token matches a wide variety of ASCII whitespace characters and Unicode spaces. The \ew token matches alphabetic word characters; it is equivalent to the character class [A-Za-z_]. The \ed token matches a digit, and is equivalent to [0-9]. .IP "\eS, \eW and \eD" These regex tokens are the complemented counterparts of \es, \ew and \ed. The \eS token matches all those characters which \es does not match, \eW matches all characters that \ew does not match and \eD matches nondigits. .IP empty An empty expression is a regular expression. It represents the set of strings consisting of the empty string; i.e. it matches just the empty string. The empty regex can appear alone as a full regular expression (for instance the .B TXR syntax @// with nothing between the slashes) and can also be passed as a subexpression to operators, though this may require the use of parentheses to make the empty regex explicit. For example, the expression a| means: match either a, or nothing. The forms * and (*) are syntax errors; though not useful, the correct way to match the empty expression zero or more times is the syntax ()*. .IP nomatch The nomatch regular expression represents the empty set: it matches no strings at all, not even the empty string. There is no dedicated syntax to directly express nomatch in the regex language. However, the empty character class [] is equivalent to nomatch, and may be considered to be a notation for it. Other representations of nomatch are possible: for instance, the regex ~.* which is the complement of the regex that denotes the set of all possible strings, and thus denotes the empty set. A nomatch has uses; for instance, it can be used to temporarily "comment out" regular expressions. The regex ([]abc|xyz) is equivalent to (xyz), since the []abc branch cannot match anything. Using [] to "block" a subexpression allows you to leave it in place, then enable it later by removing the "block". .IP (R) If R is a regular expression, then so is (R). The contents of parentheses denote one regular expression unit, so that for instance in (RE)*, the * operator applies to the entire parenthesized group. The syntax () is valid and equivalent to the empty regular expression. .IP R? optionally match the preceding regular expression R. .IP R* match the expression R zero or more times. This operator is sometimes called the "Kleene star", or "Kleene closure". The Kleene closure favors the longest match. Roughly speaking, if there are two or more ways in which R1*R2 can match, than that match occurs in which R1* matches the longest possible text. .IP R+ match the preceding expression R one or more times. Like R*, this favors the longest possible match: R+ is equivalent to RR*. .IP R1%R2 match R1 zero or more times, then match R2. If this match can occur in more than one way, then it occurs such that R1 is matched the fewest number of times, which is opposite from the behavior of R1*R2. Repetitions of R1 terminate at the earliest point in the text where a non-empty match for R2 occurs. Because it favors shorter matches, % is termed a non-greedy operator. If R2 is the empty expression, or equivalent to it, then R1%R2 reduces to R1*. So for instance (R%) is equivalent to (R*), since the missing right operand is interpreted as the empty regex. Note that whereas the expression (R1*R2) is equivalent to (R1*)R2, the expression (R1%R2) is .B not equivalent to (R1%)R2. .IP ~R match the opposite of the following expression R; i.e. match exactly those texts that R does not match. This operator is called complement, or logical not. .IP R1R2 Two consecutive regular expressions denote catenation: the left expression must match, and then the right. .IP R1|R2 match either the expression R1 or R2. This operator is known by a number of names: union, logical or, disjunction, branch, or alternative. .IP R1&R2 match both the expression R1 and R2 simultaneously; i.e. the matching text must be one of the texts which are in the intersection of the set of texts matched by R1 and the set matched by R2. This operator is called intersection, logical and, or conjunction. .PP Any escaped character which does not fall into the above escaping conventions, or any unescaped character which is not a regular expression operator, denotes one-position match of that character itself. Any of the special characters, including the delimiting /, can be escaped with a backslash to suppress its meaning and denote the character itself. Furthermore, all of the same escapes as are described in the section Special Characters in Text above are supported - the difference is that in regular expressions, the @ character is not required, so for example a tab is coded as \et rather than @\et. Octal and hex character escapes can be optionally terminated by a semicolon, which is useful if the following characters are octal or hex digits not intended to be part of the escape. Precedence table, highest to lowest: .TS tab(!); l l l. operators!class!associativity (R) []!primary! R? R+ R* R%...!postfix!left-to-right R1R2!catenation!left-to-right ~R ...%R!unary!right-to-left R1&R2!intersection!left-to-right R1|R2!union!left-to-right .TE The % operator is like a postfix operator with respect to its left operand, but like a unary operator with respect to its right operand. Thus a~b%c~d is a(~(b%(c(~d)))), demonstrating right-to-left associativity, where all of b% may be regarded as a unary operator being applied to c~d. Similarly, a?*+%b means (((a?)*)+)%b, where the trailing %b behaves like a postfix operator. In .B TXR, regular expression matches do not span multiple lines. The regex language has no feature for multi-line matching. However, the @(freeform) directive allows the remaining portion of the input to be treated as one string in which line terminators appear as explicit characters. Regular expressions may freely match through this sequence. It's possible for a regular expression to match an empty string. For instance, if the next input character is z, facing a the regular expression /a?/, there is a zero-character match: the regular expression's state machine can reach an acceptance state without consuming any characters. Examples: pattern: @A@/a?/@/.*/ data: zzzzz result: A="" pattern: @{A /a?/}@B data: zzzzz result: A="", B="zzzz" pattern: @*A@/a?/ data: zzzzz result: A="zzzzz" In the first example, variable @A is followed by a regular expression which can match an empty string. The expression faces the letter "z" at position 0 in the data line. A zero-character match occurs there, therefore the variable A takes on the empty string. The @/.*/ regular expression then consumes the line. Similarly, in the second example, the /a?/ regular expression faces a "z", and thus yields an empty string which is bound to A. Variable @B consumes the entire line. The third example requests the longest match for the variable binding. Thus, a search takes place for the rightmost position where the regular expression matches. The regular expression matches anywhere, including the empty string after the last character, which is the rightmost place. Thus variable A fetches the entire line. For additional information about the advanced regular expression operators, NOTES ON EXOTIC REGULAR EXPRESSIONS below. .SS Directives The general syntax of a directive is: @EXPR where expr is a parenthesized list of subexpressions. A subexpression is a symbol, number, string literal, character literal, quasiliteral, regular expression, or a parenthesized expression. So, examples of syntactically valid directives are: @(banana) @(a b c (d e f)) @( a (b (c d) (e ) )) @("apple" #\eb #\espace 3) @(a #/[a-z]*/ b) @(_ `@file.txt`) A symbol is lexically the same thing as a variable name (the type enclosed in braces in the @{NAME} syntax) and the same rules apply: it can consist of all the same characters, and must not look like a number. Tokens that look like numbers are treated as numbers. .SS Special Symbols Just like in the programming language Lisp, the names nil and t cannot be used as variables. They always represent themselves, and have many uses, internal to the program as well as externally visible. The nil symbol stands for the empty list object, an object which marks the end of a list, and boolean false. It is synonymous with the syntax () which may be used interchangeably with nil in most constructs. .SS Keyword Symbols Names whose names begin with the : character are keyword symbols. These also may not be used as variables either and stand for themselves. Keywords are useful for labeling information and situations. .SS Character Literals Character literals are introduced by the #\e syntax, which is either followed by a character name, the letter x followed by hex digits, the letter o followed by octal digits, or a single character. Valid character names are: nul, alarm, backspace, tab, linefeed, newline, vtab, page, return, esc, space and pnul. This convention for character literals is similar to that of the Scheme language. Note that #\elinefeed and #\enewline are the same character. The #\epnul character is specific to TXR and denotes the U+DC00 code in Unicode; the name stands for "pseudo-null", which is related to its special function. For more information about this, see the section "Character Handling and International Characters". .SS String Literals String literals are delimited by double quote respectively. A double quote within a string literal is encoded using \e" and a backslash is encoded as \e\e. Backslash escapes like \en and \et are recognized, as are hexadecimal escapes like \exFF or \exxabc and octal escapes like \e123. Ambiguity between an escape and subsequent text can be resolved by using trailing semicolon delimiter: "\exabc;d" is a string consisting of the character U+0ABC followed by "d". The semicolon delimiter disappears. To write a literal semicolon immediately after a hex or octal escape, write two semicolons, the first of which will be interpreted as a delimiter. Thus, "\ex21;;" represents "!;". If the line ends in the middle of a literal, it is an error, unless the last character is a backslash. This backslash is a special escape which does not denote a character; rather, it indicates that the string literal continues on the next line. The backslash is deleted, along with whitespace which immediately precedes it, as well as leading whitespace in the following line. The escape sequence "\e " (backslash space) can be used to encode a significant space. Example: "foo \e bar" "foo \e \ bar" "foo\ \e bar" The first string literal is the string "foobar". The second two are "foo bar". .SS Word List Literals A word list literal (WLL) provides a convenient way to write a list of strings when such a list can be given as whitespace-delimited words. There are two flavors of the WLL: the regular WLL which begins with #" (hash, double-quote) and the splicing list literal which begins with #*" (hash, star, double-quote). Both types are terminated by a double quote, which may be escaped as \e" in order to include it as a character. All the escaping conventions used in string literals can be used in word literals. Unlike in string literals, whitespace (tabs, spaces and newlines) is not significant in word literals: it separates words. Whitespace may be escaped with a backslash in order to include it as a literal character. Example: #"abc def ghi" --> notates ("abc" "def" "ghi") #"abc def ghi" --> notates ("abc" "def" "ghi") #"abc\ def ghi" --> notates ("abc def" "ghi") A splicing word literal differs from a word literal in that it does not produce a list of string literals, but rather it produces a sequence of string literals that is merged into the surrounding syntax. Example: (1 2 3 #*"abc def" 4 5 #"abc def") --> (1 2 3 "abc" "def" 4 5 ("abc" "def")) The regular WLL produced a single list object, but the splicing WLL expanded into multiple string literal objects. .SS String Quasiliterals Quasiliterals are similar to string literals, except that they may contain variable references denoted by the usual @ syntax. The quasiliteral represents a string formed by substituting the values of those variables into the literal template. If a is bound to "apple" and b to "banana", the quasiliteral `one @a and two @{b}s` represents the string "one apple and two bananas". A backquote escaped by a backslash represents itself, and two consecutive @ characters code for a literal @. There is no \e@ escape. Quasiliterals support the full output variable syntax. Expressions within variables substitutions follow the evaluation rules of TXR Lisp when the quasiliteral occurs in TXR Lisp, and the rules of the TXR pattern language when the quasiliteral occurs in the pattern language. Quasliterals can be split into multiple lines in the same way as ordinary string literals. .SS Quasiword Lists Literals The quasiword list literals (QLL-s) are to quasiliterals what WLL-s are to ordinary literals. (See the above section Word List Literals.) A QLL combines the convenience of the WLL with the power of quasistrings. Just as in the case of WLL-s, there are two flavors of the QLL: the regular QLL which begins with #` (hash, backquote) and the splicing list literal which begins with #*` (hash, star, backquote). Both types are terminated by a backquote, which may be escaped as \e` in order to include it as a character. All the escaping conventions used in quasiliterals can be used in QLL. Unlike in quasiliterals, whitespace (tabs, spaces and newlines) is not significant in QLL: it separates words. Whitespace may be escaped with a backslash in order to include it as a literal character. Note that the delimiting into words is done before the variable substitution. If the variable a contains spaces, then #`@a` nevertheless expands into a list of one item: the string derived from a. Example: #`abc @a ghi` --> notates (`abc` `@a` `ghi`) #`abc @d@e@f ghi` --> notates (`abc` `@d@e@f` `ghi`) #`@a\ @b @c` --> notates (`@a @b` `@c`) A splicing QLL differs from an ordinary QLL in that it does not produce a list of quasiliterals, but rather it produces a sequence of quasiliterals that is merged into the surrounding syntax. .SS Numbers TXR supports integers and floating-point numbers. An integer constant is made up of digits 0 through 9, optionally preceded by a + or - sign. Examples: 123 -34 +0 -0 +234483527304983792384729384723234 An integer constant can also be specified in hexadecimal using the prefix #x followed by an optional sign, followed by hexadecimal digits: 0 through 9 and the upper or lower case letters A through F: #xFF ;; 255 #x-ABC ;; -2748 Similarly, octal numbers are supported with the prefix #o followed by octal digits: #b777 ;; 511 and binary numbers can be written with a #b prefix: #b1110 ;; 14 A floating-point constant is marked by the inclusion of a decimal point, the exponential "e notation", or both. It is an optional sign, followed by a mantissa consisting of digits, a decimal point, more digits, and then an optional exponential notation consisting of the letter "e" or "E", an optional "+" or "-" sign, and then digits indicating the exponent value. In the mantissa, the digits are not optional. At least one digit must either precede the decimal point or follow. That is to say, a decimal point by itself is not a floating-point constant. Examples: .123 123. 1E-3 20E40 .9E1 9.E19 -.5 +3E+3 Examples which are not floating-point constant tokens: . (consing dot) 123E (the symbol 123E) 1.0E- (floating point 1.0 followed by symbol E-) .e (consing dot followed by symbol e) In TXR there is a special "dotdot" token consisting of two consecutive periods. An integer constant followed immediately by dotdot is recognized as such; it is not treated as a floating constant followed by a dot. That is to say, 123.. does not mean 123. . (floating point 123.0 value followed by dot token). It means 123 .. (integer 123 followed by .. token). Dialect note: unlike in Common Lisp, 123. is not an integer, but the same as 123.0. .SS Comments Comments of the form @; were already covered. Inside directives, comments are introduced just by a ; character. Example: @(foo ; this is a comment bar ; this is another comment ) This is equivalent to @(foo bar). .SS Directives-driven Syntax Some directives not only denote an expression, but are also involved in surrounding syntax. For instance, the directive @(collect) not only denotes an expression, but it also introduces a syntactic phrase which requires a matching @(end) directive. So in other words, @(collect) is not only an expression, but serves as a kind of token in a higher level phrase structure grammar. Usually if this type of "syntactic directive" occurs alone in a line, not preceded or followed by other material, it is involved in a "vertical" (or line oriented) syntax. If such a directive is embedded in a line (has preceding or trailing material) then it is in a horizontal syntactic and semantic context (character-oriented). There is an exception: the definition of a horizontal function looks like this: @(define name (arg))body material@(end) Yet, this is considered one vertical item, which means that it does not match a line of data. (This is necessary because all horizontal syntax matches something within a line of data.) Many directives have a horizontal and vertical syntax, with different but closely related semantics. A few are still "vertical only", and some are horizontal only but in future releases, these exceptions will be minimized. A summary of the available directives follows: .IP @(eof) Explicitly match the end of file. Fails if unmatched data remains in the input stream. .IP @(eol) Explicitly match the end of line. Fails if the the current position is not the end of a line. Also fails if no data remains (there is no current line). .IP @(next) Continue matching in another file or other data source. .IP @(block) Groups to gether a sequence of directives into a logical name block, which can be explicitly terminated from within using the @(accept) and @(fail) directives. Blocks are discussed in the section BLOCKS below. .IP @(skip) Treat the remaining query as a subquery unit, and search the lines (or characters) of the input file until that subquery matches somewhere. A skip is also an anonymous block. .IP @(trailer) Treat the remaining query or subquery as a match for a trailing context. That is to say, if the remainder matches, the data position is not advanced. .IP @(freeform) Treat the remainder of the input as one big string, and apply the following query line to that string. The newline characters (or custom separators) appear explicitly in that string. .IP @(fuzz) The fuzz directive, inspired by the patch utility, specifies a partial match for some lines. .IP @(some) Multiple clauses are each applied to the same input. Succeeds if at least one of the clauses matches the input. The bindings established by earlier successful clauses are visible to the later clauses. .IP @(all) Multiple clauses are applied to the same input. Succeeds if and only if each one of the clauses matches. The clauses are applied in sequence, and evaluation stops on the first failure. The bindings established by earlier successful clauses are visible to the later clauses. .IP @(none) Multiple clauses are applied to the same input. Succeeds if and only if none of them match. The clauses are applied in sequence, and evaluation stops on the first success. No bindings are ever produced by this construct. .IP @(maybe) Multiple clauses are applied to the same input. No failure occurs if none of them match. The bindings established by earlier successful clauses are visible to the later clauses. .IP @(cases) Multiple clauses are applied to the same input. Evaluation stops on the first successful clause. .IP @(require) The require directive is similar to the do directive: it evaluates one or more TXR Lisp expressions. If the result of the rightmost expression is nil, then require triggers a match failure. See the TXR LISP section far below. .IP "@(if), @(elif), @(else)" The if directive with optional elif and else clauses is a syntactic sugar which translates to a combination of @(cases) and @(require) .IP @(choose) Multiple clauses are applied to the same input. The one whose effect persists is the one which maximizes or minimizes the length of a particular variable. .IP @(empty) The @(empty) directive matches the empty string. It is useful in certain situations, such as expressing an empty match in a directive that doesn't accept an empty clause. The @(empty) syntax has another meaning in @(output) clauses, in conjunctio with @(repeat). .IP "@(define NAME ( ARGUMENTS ...))" Introduces a function. Functions are discussed in the FUNCTIONS section below. .IP @(gather) Searches text for matches for multiple clauses which may occur in arbitrary order. For convenience, lines of the first clause are treated as separate clauses. .IP @(collect) Search the data for multiple matches of a clause. Collect the bindings in the clause into lists, which are output as array variables. The @(collect) directive is line oriented. It works with a multi-line pattern and scans line by line. A similar directive called @(coll) works within one line. A collect is an anonymous block. .IP @(and) Separator of clauses for @(some), @(all), @(none), @(maybe) and @(cases). Equivalent to @(or). Choice is stylistic. .IP @(or) Separator of clauses for @(some), @(all), @(none), @(maybe) and @(cases). Equivalent to @(and). Choice is stylistic. .IP @(end) Required terminator for @(some), @(all), @(none), @(maybe), @(cases), @(collect), @(coll), @(output), @(repeat), @(rep), @(block) and @(define). .IP @(fail) Terminate the processing of a block, as if it were a failed match. Blocks are discussed in the section BLOCKS below. .IP @(accept) Terminate the processing of a block, as if it were a successful match. What bindings emerge may depend on the kind of block: collect has special semantics. Blocks are discussed in the section BLOCKS below. .IP @(try) Indicates the start of a try block, which is related to exception handling, discussed in the EXCEPTIONS section below. .IP "@(catch), @(finally)" Special clauses within @(try). See EXCEPTIONS below. .IP "@(defex), @(throw)" Define custom exception types; throw an exception. See EXCEPTIONS below. .IP @(assert) The assert directive requires the following material to match, otherwise it throws an exception. It is useful for catching mistakes or omissions in parts of a query that are sure-fire matches. .IP @(flatten) Normalizes a set of specified variables to one-dimensional lists. Those variables which have scalar value are reduced to lists of that value. Those which are lists of lists (to an arbitrary level of nesting) are converted to flat lists of their leaf values. .IP @(merge) Binds a new variable which is the result of merging two or more other variables. Merging has somewhat complicated semantics. .IP @(cat) Decimates a list (any number of dimensions) to a string, by catenating its constituent strings, with an optional separator string between all of the values. .IP @(bind) Binds one or more variables against a value using a structural pattern match. A limited form of unification takes place which can cause a match to fail. .IP @(set) Destructively assigns one or more existing variables using a structural pattern, using syntax similar to bind. Assignment to unbound variables triggers an error. .IP @(rebind) Evaluates an expression in the current binding environment, and then creates new bindings for the variables in the structural pattern. Useful for temporarily overriding variable values in a scope. .IP @(forget) Removes variable bindings. .IP @(local) Synonym of @(forget). .IP @(output) A directive which encloses an output clause in the query. An output section does not match text, but produces text. The directives above are not understood in an output clause. .IP @(repeat) A directive understood within an @(output) section, for repeating multi-line text, with successive substitutions pulled from lists. The directive @(rep) produces iteration over lists horizontally within one line. .IP @(deffilter) This directive is used for defining named filters, which are useful for filtering variable substitutions in output blocks. Filters are useful when data must be translated between different representations that have different special characters or other syntax, requiring escaping or similar treatment. Note that it is also possible to use a function as a filter. See Function Filters below. .IP @(filter) The filter directive passes one or more variables through a given filter or chain or filters, updating them with the filtered values. .IP @(load) The load directive loads another TXR file and interprets its contents. .IP @(do) The do directive is used to evaluate TXR Lisp expressions, discarding their result values. See the TXR LISP section far below. .PP .SH INPUT SCANNING AND DATA MANIPULATION .SS The Next Directive The next directive indicates that the remainder of the query is to be applied to a new input source. It can only occur by itself as the only element in a query line, and takes various arguments, according to these possiblities: @(next) @(next SOURCE) @(next SOURCE :nothrow) @(next :args) @(next :env) @(next :list EXPR) @(next :string EXPR) The lone @(next) without arguments switches to the next file in the argument list which was passed to the .B TXR utility. However, "switch to the next file" means in a pattern matching way, not in an imperative way. It is possible for the pattern matching logic to implicitly backtrack to the previous file. If SOURCE is given, it must be text-valued expression which denotes an input source; it may be a string literal, quasiliteral or a variable. For instance, if variable A contains the text "data", then @(next A) means switch to the file called "data", and @(next `@A.txt`) means to switch to the file "data.txt". If the input source cannot be opened for whatever reason, .B TXR throws an exception (see EXCEPTIONS below). An unhandled exception will terminate the program. Often, such a drastic measure is inconvenient; if @(next) is invoked with the :nothrow keyword, then if the input source cannot be opened, the situation is treated as a simple match failure. The variant @(next :args) means that the remaining command line arguments are to be treated as a data source. For this purpose, each argument is considered to be a line of text. If an argument is currently being processed as an input source, that argument is included at the front of the list. As the arguments are matched, they are consumed. This means that if a @(next) directive without arguments is executed in the scope of @(next :args), it opens the file named by the first unconsumed argument. To process arguments, and then continue with the original file and argument list, wrap the argument processing in a @(block). The variant @(next :env) means that the list of process environment variables is treated as a source of data. It looks like a text file stream consisting of lines of the form "name=value". If this feature is not available on a given platform, an exception is thrown. The syntax @(next :list EXPR) treats the expression as a source of text. The value of the expression is flattened to a list in a way similar to the @(flatten) directive. The resulting list is treated as if it were the lines of a text file: each element of the list is a line. If the lines happen contain embedded newline characters, they are a visible constituent of the line, and do not act as line separators. The syntax @(next :string EXPR) treats the expression as a source of text. The value of the expression must be a string. Newlines in the string are interpreted as line terminators. A string which is not terminated by a newline is tolerated, so that: @(next :string "abc") @a binds a to "abc". Likewise, this is also the case with input files and other streams whose last line is not terminated by a newline. However, watch out for empty strings, which are analogous to a correctly formed empty file which contains no lines: @(next :string "") @a This will not bind a to ""; it is a matching failure. The behavior of :list is different. The query @(next :list "") @a binds a to "". The reason is that under :list the string "" is flattened to the list ("") which is not an empty input stream, but a stream consisting of one empty line. Note that "remainder of the query" which is applied to the stream opened by @(next) refers to the subquery in which the next directive appears, not necessarily the entire query. For example, the following query looks for the line starting with "xyz" at the top of the file "foo.txt", within a some directive. After the @(end) which terminates the @(some), the "abc" is matched in the previous file again. @(some) @(next "foo.txt") xyz@suffix @(end) abc However, if the @(some) subquery successfully matched "xyz@suffix" within the file foo.text, there is now a binding for the suffix variable, which is visible to the remainder of the entire query. The variable bindings survive beyond the clause, but the data stream does not. The @(next) directive supports the file name conventions as the command line. The name - means standard input. Text which starts with a ! is interpreted as a shell command whose output is read like a file. These interpretations are applied after variable substitution. If the file is specified as @a, but the variable a expands to "!echo foo", then the output of the "echo foo" command will be processed. .SS The Skip Directive The skip directive considers the remainder of the query as a search pattern. The remainder is no longer required to strictly match at the current line in the current file. Rather, the current file is searched, starting with the current line, for the first line where the entire remainder of the query will successfully match. If no such line is found, the skip directive fails. If a matching position is found, the remainder of the query is understood to be processed there. Of course, the remainder of the query can itself contain skip directives. Each such directive performs a recursive subsearch. Skip comes in vertical and horizontal flavors. For instance, skip and match the last line: @(skip) @last @(eof) Skip and match the last character of the line: @(skip)@{last 1}@(eol) The skip directive has two optional arguments. If the first argument is a number, its value limits the range of lines scanned for a match. Judicious use of this feature can improve the performance of queries. Example: scan until "size: @SIZE" matches, which must happen within the next 15 lines: @(skip 15) size: @SIZE Without the range limitation skip will keep searching until it consumes the entire input source. While sometimes this is what is intended, often it is not. Sometimes a skip is nested within a collect, or following another skip. For instance, consider: @(collect) begin @BEG_SYMBOL @(skip) end @BEG_SYMBOL @(end) The collect iterates over the entire input. But, potentially, so does the skip. Suppose that "begin x" is matched, but the data has no matching "end x". The skip will search in vain all the way to the end of the data, and then the collect will try another iteration back at the beginning, just one line down from the original starting point. If it is a reasonable expectation that an "end x" occurs 15 lines of a "begin x", this can be written instead: @(collect) begin @BEG_SYMBOL @(skip 15) end @BEG_SYMBOL @(end) If the symbol nil is used in place of a number, it means to scan an unlimited range of lines; thus, @(skip nil) is equivalent to @(skip). If the symbol :greedy is used, it changes the semantics of the skip to longest match semantics, like the regular expression * operator. For instance, match the last three space-separated tokens of the line: @(skip :greedy) @a @b @c Without :greedy, the variable @c will can match multiple tokens, and end up with spaces in it, because nothing follows @c and so it matches from any position which follows a space to the end of the line. Also note the space in front of @a. Without this space, @a will get an empty string. A line oriented example of greedy skip: match the last line without using @eof: @(skip :greedy) @last_line There may be a second numeric argument. This specifies a minimum number of lines to skip before looking for a match. For instance, skip 15 lines and then search indefinitely for "begin ...": @(skip nil 15) begin @BEG_SYMBOL The two arguments may be used together. For instance, the following matches if, and only if, the 15th line of input starts with "begin ": @(skip 1 15) begin @BEG_SYMBOL Essentially, @(skip 1 ) means "hard skip by " lines, then match the query without scanning. @(skip 1 0) is the same as @(skip 1), which is a noop, because it means: "the remainder of the query must match starting on the very next line", or, more briefly, "skip exactly zero lines", which is the behavior if the skip directive is omitted altogether. Here is one trick for grabbing the fourth line from the bottom of the input: @(skip) @fourth_from_bottom @(skip 1 3) @(eof) Or using greedy skip: @(skip :greedy) @fourth_from_bottom @(skip 1 3) Nongreedy skip with the @(eof) has a slight advantage because the greedy skip will keep scanning even though it has found the correct match, then backtrack to the last good match once it runs out of data. The regular skip with explicit @(eof) will stop when the @(eof) matches. .SS Reducing Backtracking with Blocks Skip can consume considerable CPU time when multiple skips are nested. Consider: @(skip) A @(skip) B @(skip) C This is actually nesting: the second a third skips occur within the body of the first one, and thus this creates nested iteration. TXR is searching for the combination of skips which find match the pattern of lines A, B and C, with backtracking behavior. The outermost skip marches through the data until it finds A, followed by a pattern match for the second skip. The second skip iterates within to find B, followed by the third skip, and the third skip iterates to find C. If there is only one line A, and one B, then this is reasonably fast. But suppose there are many lines matching A and B, giving rise to a large number combinations of skips which match A and B, and yet no match for C, triggering backtracking. The nested stepping which tries the combinations of A and B can give rise to a considerable running time. One way to deal with the problem is to unravel the nesting with the help of blocks. For example: @(block) @ (skip) A @(end) @(block) @ (skip) B @(end) @(skip) C Now the scope of each skip is just the remainder of the block in which it occurs. The first skip finds A, and then the block ends. Control passes to the next block, and backtracking will not take place to a block which completed (unless all these blocks are enclosed in some larger construct which backtracks, causing the blocks to be re-executed. Of course, this rewrite is not equivalent, and cannot be used for instance in backreferencing situations such as: @; @; Find some three lines which are the same. @; @(skip) @line @(skip) @line @(skip) @line This example depends on the nested search-within-search semantics. .SS The Trailer Directive The trailer directive introduces a trailing portion of a query or subquery which matches input material normally, but in the event of a successful match, does not advance the current position. This can be used, for instance, to cause @(collect) to match partially overlapping regions. Example: @(collect) @line @(trailer) @(skip) @line @(end) This script collects each line which has a duplicate somewhere later in the input. Without the @(trailer) directive, this does not work properly for inputs like: 111 222 111 222 Without @(trailer), the first duplicate pair constitutes a match which spans over the 222. After that pair is found, the matching continues after the second 111. With the @(trailer) directive in place, the collect body, on each iteration, only consumes the lines matched prior to @(trailer). .SS The Freeform Directive The freeform directive provides a useful alternative to .B TXR's line-oriented matching discipline. The freeform directive treats all remaining input from the current input source as one big line. The query line which immediately follows freeform is applied to that line. The syntax variations are: @(freeform) ... query line .. @(freeform NUMBER) ... query line .. @(freeform STRING) ... query line .. @(freeform NUMBER STRING) ... query line .. The string and numeric arguments, if both are present, may be given in either order. If a numeric argument is given, it limits the range of lines which are combined together. For instance @(freeform 5) means to only consider the next five lines to to be one big line. Without a numeric argument, freeform is "bottomless". It can match the entire file, which creates the risk of allocating a large amount of memory. If a string argument is given, it specifies a custom line terminator. The default terminator is "\en". The terminator does not have to be one character long. Freeform does not convert the entire remainder of the input into one big line all at once, but does so in a dynamic, lazy fashion, which takes place as the data is accessed. So at any time, only some prefix of the data exists as a flat line in which newlines are replaced by the terminator string, and the remainder of the data still remains as a list of lines. After the subquery is applied to the virtual line, the unmatched remainder of that line is broken up into multiple lines again, by looking for and removing all occurences of the terminator string within the flattened portion. Care must be taken if the terminator is other than the default "\en". All occurences of the terminator string are treated as line terminators in the flattened portion of the data, so extra line breaks may be introduced. Likewise, in the yet unflattened portion, no breaking takes place, even if the text contains occurences of the terminator string. The extent of data which is flattened, and the amount of it which remains, depends entirely on the query line underneath @(flatten). In the following example, lines of data are flattened using $ as the line terminator. Query: @(freeform "$") @a$@b: @c @d Data: 1 2:3 4 Output (-B): a="1" b="2" c="3" d="4" The data is turned into the virtual line 1$2:3$4$. The @a$@b: subquery matches the 1$2: portion, binding a to 1, and b to 2. The remaining portion 3$4$ is then split into separate lines again according to the line terminator $: 3 4 Thus the remainder of the query @c @d faces these lines, binding c to 3 and d to 4. Note that since the data does not contain dollar signs, there is no ambiguity; the meaning may be understood in terms of the entire data being flattened and split again. In the following example, freeform is used to solve a tokenizing problem. The Unix password file has fields separated by colons. Some fields may be empty. Using freeform, we can join the password file using ":" as a terminator. By restricting freeform to one line, we can obtain each line of the password file with a terminating ":", allowing for a simple tokenization, because now the fields are colon-terminated rather than colon-separated. Example: @(next "/etc/passwd") @(collect) @(freeform 1 ":") @(coll)@{token /[^:]*/}:@(end) @(end) .SS The Fuzz Directive The fuzz directive allows for an imperfect match spanning a set number of lines. It takes two arguments, both expressions that should evaluate to integers: @(fuzz m n) ... This expresses that over the next n query lines, the matching strictness is relaxed a little bit. Only m out of those n lines have to match. Afterward, the rest of the query follows normal, strict processing. In the degenerate situation that there are fewer than n query lines following the fuzz directive, then m of them must succeed nevertheless. (If there are fewer than m, then this is impossible.) .SS The Some, All, None, Maybe, Cases and Choose directives These directives, called the parallel directives, combine multiple subqueries, which are applied at the same input position, rather than to consecutive input. They come in vertical (line mode) and horizontal (character mode) flavors. In horizontal mode, the current position is understood to be a character position in the line being processed. The clauses advance this character position by moving it to the right. In vertical mode, the current position is understood to be a line of text within the stream. A clause advances the position by some whole number of lines. The syntax of these parallel directives follows this example: @(some) subquery1 . . . @(and) subquery2 . . . @(and) subquery3 . . . @(end) And in horizontal mode: @(some)subquery1...@(and)subquery2...@(and)subquery3...@(end) Long horizontal lines can be broken up with line continuations, allowing the above example to be written like this, which is considered a single logical line: @(some)@\ subquery1...@\ @(and)@\ subquery2...@\ @(and)@\ subquery3...@\ @(end) The @(some), @(all), @(none), @(maybe), @(cases) or @(choose) must be followed by at least one subquery clause, and be terminated by @(end). If there are two or more subqueries, these additional clauses are indicated by @(and) or @(or), which are interchangeable. The separator and terminator directives also must appear as the only element in a query line. The choose directive requires keyword arguments. See below. The syntax supports arbitrary nesting. For example: QUERY: SYNTAX TREE: @(all) all -+ @ (skip) +- skip -+ @ (some) | +- some -+ it | | +- TEXT @ (and) | | +- and @ (none) | | +- none -+ was | | | +- TEXT @ (end) | | | +- end @ (end) | | +- end a dark | +- TEXT @(end) *- end nesting can be indicated using whitespace between @ and the directive expression. Thus, the above is an @(all) query containing a @(skip) clause which applies to a @(some) that is followed by the the text line "a dark". The @(some) clause combines the text line "it", and a @(none) clause which contains just one clause consisting of the line "was". The semantics of the parallel directives is: .IP @(all) Each of the clauses is matched at the current position. If any of the clauses fails to match, the directive fails (and thus does not produce any variable bindings). Clauses following the failed directive are not evaluated. Bindings extracted by a successful clause are visible to the clauses which follow, and if the directive succeeds, all of the combined bindings emerge. .IP "@(some [ :resolve (vars ...) ])" Each of the clauses is matched at the current position. If any of the clauses succeed, the directive succeeds, retaining the bindings accumulated by the successully matching clauses. Evaluation does not stop on the first successful clause. Bindings extracted by a successful clause are visible to the clauses which follow. The :resolve parameter is for situations when the @(some) directive has multiple clauses that need to bind some common variables to different values: for instance, output parameters in functions. Resolve takes a list of variable name symbols as an argument. This is called the resolve set. If the clauses of @(some) bind variables in the resolve set, those bindings are not visible to later clauses. However, those bindings do emerge out of the @(some) directive as a whole. This creates a conflict: what if two or more clauses introduce non-matching bindings for a variable in the resolve set? This is why it is called the resolve set: conflicts for variables in the resolve set are automatically resolved in favor of later directives. Example: @(some :resolve (x)) @ (bind a "a") @ (bind x "x1") @(or) @ (bind b "b") @ (bind x "x2") @(end) Here, the two clauses both introduce a binding for x. Without the :resolve parameter, this would mean that the second clause fails, because x comes in with the value "x1", which does not bind with "x2". But because x is placed into the resolve set, the second clause does not see the "x1" binding. Both clauses establish their bindings independently creating a conflict over x. The conflict is resolved in favor of the second clause, and so the bindings which emerge from the directive are: a="a" b="b" x="x2" .IP @(none) Each of the clauses is matched at the current position. The directive succeeds only if all of the clauses fail. If any clause succeeds, the directive fails, and subsequent clauses are not evaluated. Thus, this directive never produces variable bindings, only matching success or failure. .IP @(maybe) Each of the clauses is matched at the current position. The directive always succeeds, even if all of the clauses fail. Whatever bindings are found in any of the clauses are retained. Bindings extracted by any successful clause are visible to the clauses which follow. .IP @(cases) Each of the clauses is matched at the current position. The clauses are matched, in order, at the current position. If any clause matches, the matching stops and the bindings collected from that clause are retained. Any remaining clauses after that one are not processed. If no clause matches, the directive fails, and produces no bindings. .IP "@(choose [ :longest | :shortest ])" Each of the clauses is matched at the current position in order. In this construct, bindings established by an earlier clause are not visible to later clauses. Although any or all of the clauses can potentially match, the clause which succeeds is the one which maximizes or minimizes the length of the text bound to the specified variable. The other clauses have no effect. For all of the parallel directives other than @(none) and @(choose), the query advances the input position by the greatest number of lines that match in any of the successfully matching subclauses that are evaluated. The @(none) directive does not advance the input position. For instance if there are two subclauses, and one of them matches three lines, but the other one matches five lines, then the overall clause is considered to have made a five line match at its position. If more directives follow, they begin matching five lines down from that position. .SS The Require Directive The syntax of @(require) is: @(require ) The require directive evaluates a TXR Lisp expression. (See TXR LISP far below.) If the expression yields a true value, then it succeeds, and matching continues with the directives which follow. Otherwise the directive fails. In the context of the @(require) directive, should not be delimited by an @. Example: @# require that 4 is greater than 3 @# This succeeds; therefore, @a is processed @(require (> (+ 2 2) 3)) @a .SS The If Directive The syntax of the directive can be exemplified as follows @(if ) . . . @(elif ) . . . @(elif ) . . . @(else) . . . @(end) The @(elif) and @(else) clauses are all optional. If @(else) is present, it must be last, before @(end), after any @(elif) clauses. Any of the clauses may be empty. See the TXR Lisp section about TXR Lisp expressions. In this directive, TXR Lisp expressions are not introduced by the @ symbol. For example: @(if (> (length str) 42)) foo: @a @b @(else) {@c} @(end) In this example, if the length of the variable str is greater than 42, then matching continues with "foo: @a b", otherwise it proceeds with {@c}. The if directive is actually a syntactic sugar which is translated to @(cases) and @(require). That is to say, the following pattern: @(cases) @(require ) A @(or) @(require ) B @(or) C @(end) corresponds to the somewhat shorter and clearer: @(if ) A @(elsif ) B @(else C @(end) .SS The Gather Directive Sometimes text is structured as items that can appear in an arbitrary order. When multiple matches need to be extracted, there is a combinatorial explosion of possible orders, making it impractical to write pattern matches for all the possible orders. The gather directive is for these situations. It specifies multiple clauses which all have to match somewhere in the data, but in any order. For further convenience, the lines of the first clause of the gather directive are implicitly treated as separate clauses. The syntax follows this pattern @(gather) one-line-query1 one-line-query2 . . . one-line-queryN @(and) multi line query1 . . . @(and) multi line query2 . . . @(end) Of course the multi-line clauses are optional. The gather directive takes keyword parameters, see below. Similarly to @(collect), @(gather) has an optional until/last clause: @(gather) ... @(until) ... @(end) How gather works is that the text is searched for matches for the single line and multi-line queries. The clauses are applied in the order in which they appear. Whenever one of the clauses matches, any bindings it produces are retained and it is removed from further consideration. Multiple clauses can match at the same text position. The position advances by the longest match from among the clauses which matched. If no clauses match, the position advances by one line. The search stops when all clauses are eliminated, and then the cumulative bindings are produced. If the data runs out, but unmatched clauses remain, the directive fails. Example: extract several environment variables, which do not appear in a particular order: @(next :env) @(gather) USER=@USER HOME=@HOME SHELL=@SHELL @(end) If the until or last clause is present and a match occurs, then the matches from the other clauses are discarded and the gather terminates. The difference between until and last is that any bindings bindings established in last are retained, and the input position is advanced past the matched material. The until/last clause has visibility to bindings established in the previous clauses in that same iteration, even though those bindings end up thrown away. .SS Gather Keyword Parameters The gather diretive accepts the keyword parameter :vars. The argument to vars is a list of required and optional variables. Optional variables are denoted by the specification of a default value. Example: @(gather :vars (a b c (d "foo"))) ... @(end) Here, a, b, c and e are required variables, and d is optional. Variable e is required because its default value is the empty list (), same as the symbol nil. The presence of vars changes the behavior in three ways. Firstly, even if all the clauses in the gather match successfully and are eliminated, the directive will fail if the required variables do not have bindings. It doesn't matter whether the bindings are existing, or whether they are established by the gather. Secondly, if some of the clauses of the gather did not match, but all of the required variables have bindings, then the directive succeeds. Without the presence of :vars, it would fail in this situation. Thirdly, if the the gather succeeds (all required variables have bindings), then all of the optional variables which do not have bindings are given bindings to their default values. .SS The Collect Directive The syntax of the collect directive is: @(collect) ... lines of subquery @(end) or with an until or last clause: @(collect) ... lines of subquery: main clause @(until) ... lines of subquery: until clause @(end) @(collect) ... lines of subquery: main clause @(last) ... lines of subquery: last clause @(end) The repeat symbol may be specified instead of collect, which changes the meaning, see below: @(repeat) ... lines of subquery @(end) The subquery is matched repeatedly, starting at the current line. If it fails to match, it is tried starting at the subsequent line. If it matches successfully, it is tried at the line following the entire extent of matched data, if there is one. Thus, the collected regions do not overlap. (Overlapping behavior can be obtained: see the @(trailer) directive). Unless certain keywords are specified, or unless the collect is explicitly failed with @(fail), it always succeeds, even if it collects nothing, and even if the until/last clause never finds a match. If no until/last clause is specified, and the collect is not limited using parameters, the collect is unbounded. It consumes the entire data file. If any query material follows such the collect clause, it will fail if it tries to match anything in the current file; but of course, it is possible to continue matching in another file by means of @(next). If an until/last clause is specified, the collection stops when that clause matches at the current position. If an until clause terminates collect, no bindings are collected at that position, even if the main clause matches at that position also. Moreover, the position is not advanced. The remainder of the query begins matching at that position. If a last clause terminates collect, the behavior is different. Any bindings captured by the main clause are thrown away, just like with the until clause. However, the bindings in the last clause itself survive, and the position is advanced to skip over that material. Example: Query: @(collect) @a @(until) 42 @b @(end) @c Data: 1 2 3 42 5 6 Output (-B): a[0]="1" a[1]="2" a[2]="3" c="42" The line 42 is not collected, even though it matches @a. Furthermore, the until does not advance the position, so variable c takes 42. If the @(until) is changed to @(last) the output will be different: Output (-B): a[0]="1" a[1]="2" a[2]="3" b=5 c=6 The 42 is not collected into the a list, just like before. But now the binding captured by @b emerges. Furthermore, the position advances so variable now takes 6. The binding variables within the clause of a collect are treated specially. The multiple matches for each variable are collected into lists, which then appear as array variables in the final output. Example: Query: @(collect) @a:@b:@c @(end) Data: John:Doe:101 Mary:Jane:202 Bob:Coder:313 Output (-B): a[0]="John" a[1]="Mary" a[2]="Bob" b[0]="Doe" b[1]="Jane" b[2]="Coder" c[0]="101" c[1]="202" c[2]="313" The query matches the data in three places, so each variable becomes a list of three elements, reported as an array. Variables with list bindings may be referenced in a query. They denote a multiple match. The -D command line option can establish a one-dimensional list binding. Collect clauses may be nested. Variable matches collated into lists in an inner collect, are again collated into nested lists in the outer collect. Thus an unbound variable wrapped in N nestings of @(collect) will be an N-dimensional list. A one dimensional list is a list of strings; a two dimensional list is a list of lists of strings, etc. It is important to note that the variables which are bound within the main clause of a collect - i.e. the variables which are subject to collection - appear, within the collect, as normal one-value bindings. The collation into lists happens outside of the collect. So for instance in the query: @(collect) @x=@x @(end) The left @x establishes a binding for some material preceding an equal sign. The right @x refers to that binding. The value of @x is different in each iteration, and these values are collected. What finally comes out of the collect clause is list variable called x which holds each value that was ever instantiated under that name within the collect clause. Also note that the until clause has visibility over the bindings established in the main clause. This is true even in the terminating case when the until clause matches, and the bindings of the main clause are discarded. .SS Collect Keyword Parameters By default, collect searches the rest of the input indefinitely, or until the @(until) clause matches. It skips arbitrary amounts of nonmatching material before the first match, and between matches. Within the @(collect) syntax, it is possible to specify some useful keyword parameters for additional control of the behavior. For instance @(collect :maxgap 5) means that the collect will terminate if it does not find a match within five lines of the starting position, or if more than five lines are skipped since any successful match. A :maxgap of 0 means that the collected regions must be adjacent. For instance: @(collect :maxgap 0) M @a @(end) means: from here, collect consecutive lines of the form "M ...". This will not search for the first such line, nor will it skip lines which do not match this form. Other keywords are :mingap, and :gap. The :mingap keyword specifies a minimum gap between matches, but has no effect on the distance to the first match. The :gap keyword specifies :mingap and :maxgap at the same time, and can only be used if these other two are not used. Thus: @(collect :gap 1) @a @(end) means collect every other line starting with the current line. Several other supported keywords are :times, :mintimes, :maxtimes and :lines. The shorthand :times N means the same thing as :mintimes N :maxtimes N. These specify how many matches should be collected. If there are fewer than mintimes matches, the collect fails. If maxtimes matches are collected, collect stops collecting immediately. Example: @(collect :times 3) @a @b @(end) This will collect a match for "@a @b" exactly three times. If three matches are not found, it will fail. The :lines parameter specifies the upper bound on how many lines should be scanned by collect, measuring from the starting position. The extent of the collect body is not counted. Example: @(collect :lines 2) foo: @a bar: @b baz: @c @(end) The above collect will look for a match only twice: at the current position, and one line down. There is one more keyword, :vars, discussed in the following section. .SS Specifying Variables in Collect Normally, any variable for which a new binding occurs in a collect is collected. A collect clause may be sloppy: it can neglect to collect some variables on some iterations, or bind some variables which are intended to behave like local temporaries, but end up collated into lists. Another issue is that the collect clause might not match anything at all, and then none of the variables are bound. The :vars keyword allows the query writer to add discipline the collect body. The argument to :vars is a list of variable specs. A variable spec is either a symbol, or a ( ) pair, where the expression specifies a default value. When a :vars list is specified, it means that only the given variables can emerge from the successful collect. Any newly introduced bindings for other variables do not propagate. Furthermore, for any variable which is not specified with a default value, the collect body, whenever it matches successfully, must bind that variable. If it neglects to bind the variable, an exception of type query_error is thrown. (If a collect body matches successfully, but produces no new bindings, then this error is suppressed.) For any variable which does have a default value, if the collect body neglects to bind that variable, the behavior is as if the collect did bind that variable to that default value. The default values are expressions, and so can be quasiliterals. Lastly, if in the event that the collect does not match anything, the variables specified in vars (whether or not they have a default value) are all bound to empty lists. (These bindings are established after the processing of the until/last clause, if present.) Example: @(collect :vars (a b (c "foo"))) @a @c @(end) Here, if the body "@a @c" matches, an error will be thrown because one of the mandatory variables is b, and the body neglects to produce a binding for b. Example: @(collect :vars (a (c "foo"))) @a @b @(end) Here, if "@a @b" matches, only a will be collected, but not b, because b is not in the variable list. Furthermore, because there is no binding for c in the body, a binding is created with the value "foo", exactly as if c matched such a piece of text. In the following example, the assumption is that THIS NEVER MATCHES is not found anywhere in the input but the line THIS DOES MATCH is found and has a successor which is bound to a. Because the body did not match, the :vars a and b should be bound to empty lists. But a is bound by the last clause to some text, so this takes precedence. Only b is bound to a an empty list. @(collect :vars (a b)) THIS NEVER MATCHES @(last) THIS DOES MATCH @a @(end) The following means: do not allow any variables to propagate out of any iteration of the collect and therefore collect nothing: @(collect :vars nil) ... @(end) Instead of writing @(collect :vars nil), it is possible to write @(repeat). @(repeat) takes all collect keywords, except for :vars. There is a repeat directive used in @(output) clauses; that is a different repeat directive. .SS The Coll Directive The coll directive is a kind of miniature version of the collect directive. Whereas the collect directive works with multi-line clauses on line-oriented material, coll works within a single line. With coll, it is possible to recognize repeating regularities within a line and collect lists. Regular-expression based Positive Match variables work well with coll. Example: collect a comma-separated list, terminated by a space. pattern: @(coll)@{A /[^, ]+/}@(until) @(end)@B data: foo,bar,xyzzy blorch result: A[0]="foo" A[1]="bar" A[2]="xyzzy" B=blorch Here, the variable A is bound to tokens which match the regular expression /[^, ]+/: non-empty sequence of characters other than commas or spaces. Like its big cousin, the coll directive searches for matches. If no match occurs at the current character position, it tries at the next character position. Whenever a match occurs, it continues at the character position which follows the last character of the match, if such a position exists. If not bounded by an until clause, it will exhaust the entire line. If the until clause matches, then the collection stops at that position, and any bindings from that iteration are discarded. Like collect, coll also supports a last clause, which propagates varaible bindings and advances the position. Coll clauses nest, and variables bound within a coll are available to clauses within the rest of the coll clause, including the until clause, and appear as single values. The final list aggregation is only visible after the coll clause. The behavior of coll is troublesome, when delimited variables are used, because in text file formats, the material which separates items is not repeated after the last item. For instance, a comma-separated list usually does not appear as "a,b,c," but rather "a,b,c". There might not be any explicit termination - the last item might be at the very end of the line. So for instance, the following result is not satisfactory: pattern: @(coll)@a @(end) data: 1 2 3 4 5 result: a[0]="1" a[1]="2" a[2]="3" a[3]="4" What happened to the 5? After matching "4 ", coll continues to look for matches. It tries "5", which does not match, because it is not followed by a space. Then the line is consumed. So in this sequence, a valid item is either followed by a space, or by nothing. So it is tempting to try this: pattern: @(coll)@a@/ ?/@(end) data: 1 2 3 4 5 result: a[0]="" a[1]="" a[2]="" a[3]="" a[4]="" a[5]="" a[6]="" a[7]="" a[8]="" however, the problem is that the regular expression / ?/ (match either a space or nothing), matches at any position. So when it is used as a variable delimiter, it matches at the current position, which binds the empty string to the variable, the extent of the match being zero. In this situation, the coll directive proceeds character by character. The solution is to use positive matching: specify the regular expression which matches the item, rather than a trying to match whatever follows. The collect directive will recognize all items which match the regular expression. pattern: @(coll)@{a /[^ ]+/}@(end) data: 1 2 3 4 5 result: a[0]="1" a[1]="2" a[2]="3" a[3]="4" a[4]="5" The until clause can specify a pattern which, when recognized, terminates the collection. So for instance, suppose that the list of items may or may not be terminated by a semicolon. We must exclude the semicolon from being a valid character inside an item, and add an until clause which recognizes a semicolon: pattern: @(coll)@{a /[^ ;]+/}@(until);@(end); data: 1 2 3 4 5; result: a[0]="1" a[1]="2" a[2]="3" a[3]="4" a[4]="5" Semicolon or not, the items are collected properly. Note that the @(end) is followed by a semicolon. That's because when the @(until) clause meets a match, the matching material is not consumed. Instead of regular expression hacks, this problem can be nicely solved with cases: pattern: @(coll)@(cases)@a @(or)@a@(end)@(end) data: 1 2 3 4 5 result: a[0]="1" a[1]="2" a[2]="3" a[3]="4" a[4]="5" .SS Coll Keyword Parameters The @(coll) directive takes most of the same parameters as @(collect). See the section Collect Keyword Parameters above. So for instance @(coll :gap 0) means that the collects must be consecutive, and @(coll :maxtimes 2) means that at most two matches will be collected. The :lines keyword does not exist, but there is an analogous :chars keyword. .SS The Flatten Directive. The flatten directive can be used to convert variables to one dimensional lists. Variables which have a scalar value are converted to lists containing that value. Variables which are multidimensional lists are flattened to one-dimensional lists. Example (without @(flatten)) pattern: @b @(collect) @(collect) @a @(end) @(end) data: 0 1 2 3 4 5 result: b="0" a_0[0]="1" a_1[0]="2" a_2[0]="3" a_3[0]="4" a_4[0]="5" Example (with flatten): pattern: @b @(collect) @(collect) @a @(end) @(end) @(flatten a b) data: 0 1 2 3 4 5 result: b[0]="0" a[0]="1" a[1]="2" a[2]="3" a[3]="4" a[4]="5" .SS The Merge Directive The merge directive provides a way of combining two or more variables in a somewhat complicated but very useful way. To understand what merge does we first have to define a property called depth. The depth of an atom such as a string is defined as 1. The depth of an empty list is 0. The depth of a nonempty list is one plus the depth of its deepest element. So for instance "foo" has depth 1, ("foo") has depth 2, and ("foo" ("bar")) has depth three. We can now define the binary (two argument) merge operation as follows. (merge A B) first normalizes the values A and B such that they have equal depth. 1. A value which has depth zero is put into a one element list. 2. If either value has a smaller depth than the other, it is wrapped in a list as many times as needed to give it equal depth. Finally, the values are appended together. Merge takes more than two arguments. These are merged by a left reduction. The leftmost two values are merged, and then this result is merged with the third value, and so on. Merge is useful for combining the results from collects at different levels of nesting such that elements are at the appropriate depth. .SS The Cat Directive The @(cat) directive converts a list variable into a single piece of text. The syntax is: @(cat VAR [ SEP ]) The SEP argument specifies a separating piece of text. If no separator is specified, then a single space is used. Example: pattern: @(coll)@{a /[^ ]+/}@(end) @(cat a ":") data: 1 2 3 4 5 result: a="1:2:3:4:5" .SS The Bind Directive The syntax of the @(bind) directive is: @(bind pattern expression { keyword value }*) The @(bind) directive is a kind of pattern match, which matches one or more variables on the left hand side pattern to the value of a variable on the right hand side. The right hand side variable must have a binding, or else the directive fails. Any variables on the left hand side which are unbound receive a matching piece of the right hand side value. Any variables on the left which are already bound must match their corresponding value, or the bind fails. Any variables which are already bound and which do match their corresponding value remain unchanged (the match can be inexact). The simplest bind is of one variable against itself, for instance bind A against A: @(bind A A) This will fail if A is not bound, (and complain loudly). If A is bound, it succeeds, since A matches A. The next simplest bind binds one variable to another: @(bind A B) Here, if A is unbound, it takes on the same value as B. If A is bound, it has to match B, or the bind fails. Matching means that either .IP - A and B are the same text .IP - A is text, B is a list, and A occurs within B. .IP - vice versa: B is text, A is a list, and B occurs within A. .IP - A and B are lists and are either identical, or one is found as substructure within the other. .PP The right hand side does not have to be a variable. It may be some other object, like a string, quasiliteral, regexp, or list of strings, et cetera. For instance @(bind A "ab\etc") will bind the string "ab\etc" (the letter a, b, a tab character, and c) to the variable A if A is unbound. If A is bound, this will fail unless A already contains an identical string. However, the right hand side of a bind cannot be an unbound variable, nor a complex expression that contains unbound variables. The left hand side of a bind can be a nested list pattern containing variables. The last item of a list at any nesting level can be preceded by a dot, which means that the variable matches the rest of the list from that position. Example: suppose that the list A contains ("now" "now" "brown" "cow"). Then the directive @(bind (H N . C) A), assuming that H, N and C are unbound variables, will bind H to "how", N to "now", and C to the remainder of the list ("brown" "cow"). Example: suppose that the list A is nested to two dimensions and contains (("how" "now") ("brown" "cow")). Then @(bind ((H N) (B C)) A) binds H to "how", N to "now", B to "brown" and C to "cow". The dot notation may be used at any nesting level. it must be preceded and followed by a symbol: the forms (.) (. X) and (X .) are invalid. The number of items in a left pattern match must match the number of items in the corresponding right side object. So the pattern () only matches an empty list. The notation () and nil means exactly the same thing. The symbols nil, t and keyword symbols may be used on either side. They represent themselves. For example @(bind :foo :bar) fails, but @(bind :foo :foo) succeeds since the two sides denote the same keyword symbol object. .SS Keywords in The Bind Directive The Bind directive accepts these keywords: .IP :lfilt The argument to :lfilt is a filter specification. When the left side pattern contains a binding which is therefore matched against its counterpart from the right side expression, the left side is filtered through the filter specified by :lfilt for the purposes of the comparison. For example: @(bind "a" "A" :lfilt :upcase) produces a match, since the left side is the same as the right after filtering through the :upcase filter. .IP :rfilt The argument to :rfilt is a filter specification. The specified filter is applied to the right hand side material prior to matching it against the left side. The filter is not applied if the left side is a variable with no binding. It is only applied to determine a match. Binding takes place the unmodified right hand side object. Example, the following produces a match: @(bind "A" "a" :rfilt :upcase) .IP :filter This keyword is a shorthand to specify both filters to the same value. So for instance :filter :upcase is equivalent to :lfilt :upcase :rfilt :upcase. For a description of filters, see Output Filtering below. Of course, compound filters like (:from_html :upcase) are supported with all these keywords. The filters apply across arbitrary patterns and nested data. Example: @(bind (a b c) ("A" "B" "C")) @(bind (a b c) (("z" "a") "b" "c") :rfilt :upcase) Here, the first bind establishes the values for a, b and c, and the second bind succeeds, because the value of a matches the second element of the list ("z" "a") if it is upcased, and likewise b matches "b" and c matches "c" if these are upcased. .SS The Set Directive The @(set) directive resembles bind, but is not a pattern match. It overwrites the previous values of variables with new values from the right hand side. Each variable that is assigned must have an existing binding. Examples follow. Store the value of A back into A, achieving nothing: @(set A A) Exchange the values of A and B: @(set (A B) (B A)) Store a string into A: @(set A "text") Store a list into A: @(set A ("line1" "line2")) Destructuring assignment. D assumed to contain the list @(bind D ("A" ("B1" "B2") "C1" "C2")) @(bind (A B C) (() () ())) @(set (A B . C) D) A ends up with "A", B ends up with ("B1" "B2") and C gets ("C1" and "C2"). .SS The Rebind Directive The @(rebind) directive resembles @(set) but it is not an assignment. It combines the semanticss of @(local), @(bind) and @(set). The expression on the right hand side is evaluated in the current environment. Then the variables in the pattern on the left are introduced as new bindings, whose values come from the pattern. Rebind makes it easy to create temporary bindings based on existing bindings. @(define pattern-function (arg)) @;; inside a pattern function: @(rebind recursion-level @(+ recursion-level 1)) @;; ... @(end) When the function terminates, the previous value of recursion-level is restored. The effect is like the following, but much easier to write and faster to execute: @(define pattern-function (arg)) @;; inside a pattern function: @(local temp) @(set temp recursion-level) @(local recursion-level) @(set recursion-level @(+ temp 1)) @;; ... @(end) .SS The Forget Directive The Forget Directive has two spellings @(forget) and @(local). The arguments are one or more symbols, for example: @(forget a) @(local a b c) this can be written @(local a) @(local a b c) Directives which follow the forget or local directive no longer see any bindings for the symbols mentioned in that directive, and can establish new bindings. It is not an error if the bindings do not exist. It is strongly recommended to use @(local) spelling in functions, because the forgetting action simulates local variables: for the given symbols, the machine forgets any earlier variables from outside of the function, and consequently, any new bindings for those variables belong to the function. (Furthermore, functions suppress the propagation of variables that are not in their parameter list, so these locals will be automatically forgotten when the function terminates.) .SH BLOCKS .SS Introduction Blocks are sections of a query which are denoted by a name. Blocks denoted by the name nil are understood as anonymous. The @(block NAME) directive introduces a named block, except when the name is the word nil. The @(block) directive introduces an unnamed block, equivalent to @(block nil). The @(skip) and @(collect) directives introduce implicit anonymous blocks, as do function bodies. Blocks are useful for terminating parts of a pattern matching search prematurely, and escaping to a higher level. This makes blocks not only useful for simplifying the semantics of certain pattern matches, but also an optimization tool. Judicious use of blocks and escapes can reduce or eliminate the amount of backtracking that TXR performs. .SS Block Scope The names of blocks are in a distinct namespace from the variable binding space. So @(block foo) has no interaction with the variable @foo. A block extends from the @(block ...) directive which introduces it, until the matching @(end), and may be empty. For instance: @(some) abc @(block foo) xyz @(end) @(end) Here, the block foo occurs in a @(some) clause, and so it extends to the @(end) which terminates the block. After that @(end), the name foo is not associated with a block (is not "in scope"). The second @(end) terminates the @(some) block. The implicit anonymous block introduced by @(skip) has the same scope as the @(skip): it extends over all of the material which follows the skip, to the end of the containing subquery. .SS Block Nesting Blocks may nest, and nested blocks may have the same names as blocks in which they are nested. For instance: @(block) @(block) ... @(end) @(end) is a nesting of two anonymous blocks, and @(block foo) @(block foo) @(end) @(end) is a nesting of two named blocks which happen to have the same name. When a nested block has the same name as an outer block, it creates a block scope in which the outer block is "shadowed"; that is to say, directives which refer to that block name within the nested block refer to the inner block, and not to the outer one. .SS Block Semantics A block normally does nothing. The query material in the block is evaluated normally. However, a block serves as a termination point for @(fail) and @(accept) directives which are in scope of that block and refer to it. The precise meaning of these directives is: .IP "@(fail NAME)" Immediately terminate the enclosing query block called NAME, as if that block failed to match anything. If more than one block by that name encloses the directive, the inner-most block is terminated. No bindings emerge from a failed block. .IP @(fail) Immediately terminate the innermost enclosing anonymous block, as if that block failed to match. If the implicit block introduced by @(skip) is terminated in this manner, this has the effect of causing the skip itself to fail. I.e. the behavior is as if skip search did not find a match for the trailing material, except that it takes place prematurely (before the end of the available data source is reached). If the implicit block associated with a @(collect) is terminated this way, then the entire collect fails. This is a special behavior, because a collect normally does not fail, even if it matches and collects nothing! To prematurely terminate a collect by means of its anonymous block, without failing it, use @(accept). .IP "@(accept NAME)" Immediately terminate the enclosing query block called NAME, as if that block successfully matched. If more than one block by that name encloses the directive, the inner-most block is terminated. Any bindings established within that block until this point emerge from that block. .IP @(accept) Immediately terminate the innermost enclosing anonymous block, as if that block successfully matched. Any bindings established within that block until this point emerge from that block. If the implicit block introduced by @(skip) is terminated in this manner, this has the effect of causing the skip itself to succeed, as if all of the trailing material successfully matched. If the implicit block associated with a @(collect) is terminated this way, then the collection stops. All bindings collected in the current iteration of the collect are discarded. Bindings collected in previous iterations are retained, and collated into lists in accordance with the semantics of collect. Example: alternative way to @(until) termination: @(collect) @ (maybe) --- @ (accept) @ (end) @LINE @(end) This query will collect entire lines into a list called LINE. However, if the line --- is matched (by the embedded @(maybe)), the collection is terminated. Only the lines up to, and not including the --- line, are collected. The effect is identical to: @(collect) @LINE @(until) --- @(end) The difference (not relevant in these examples) is that the until clause has visibility into the bindings set up by the main clause. However, the following example has a different meaning: @(collect) @LINE @ (maybe) --- @ (accept) @ (end) @(end) Now, lines are collected until the end of the data source, or until a line is found which is followed by a --- line. If such a line is found, the collection stops, and that line is not included in the collection! The @(accept) terminates the process of the collect body, and so the action of collecting the last @LINE binding into the list is not performed. .SS Data Extent of Terminated Blocks A query block may have matched some material prior to being terminated by accept. In that case, it is deemed to have only matched that material, and not any material which follows. This may matter, depending on the context in which the block occurs. Example: Query: @(some) @(block foo) @first @(accept foo) @ignored @(end) @second Data: 1 2 3 Output (-B): first="1" second="2" At the point where the accept occurs, the foo block has matched the first line, bound the text "1" to the variable @first. The block is then terminated. Not only does the @first binding emerge from this terminated block, but what also emerges is that the block advanced the data past the first line to the second line. So next, the @(some) directive ends, and propagates the bindings and position. Thus the @second which follows then matches the second line and takes the text "2". In the following query, the foo block occurs inside a maybe clause. Inside the foo block there is a @(some) clause. Its first subclause matches variable @first and then terminates block foo. Since block foo is outside of the @(some) directive, this has the effect of terminating the @(some) clause: Query: @(maybe) @(block foo) @ (some) @first @ (accept foo) @ (or) @one @two @three @four @ (end) @(end) @second Data: 1 2 3 4 5 Output (-B): first="1" second="2" The second clause of the @(some) directive, namely: @one @two @three @four is never processed. The reason is that subclauses are processed in top to bottom order, but the processing was aborted within the first clause the @(accept foo). The @(some) construct never had the opportunity to match four lines. If the @(accept foo) line is removed from the above query, the output is different: Query: @(maybe) @(block foo) @ (some) @first @# <-- @(accept foo) removed from here!!! @ (or) @one @two @three @four @ (end) @(end) @second Data: 1 2 3 4 5 Output (-B): first="1" one="1" two="2" three="3" four="4" second="5" Now, all clauses of the @(some) directive have the opportunity to match. The second clause grabs four lines, which is the longest match. And so, the next line of input available for matching is 5, which goes to the @second variable. .SS Interaction between Trailer and Accept Directives If one of the clauses which follow a @(trailer) request a successful termination to an outer block via @(accept), then @(trailer) intercepts the transfer and adjusts the data extent to the position that it was given. Example: Query: @(block) @(trailer) @line1 @line2 @(accept) @(end) @line3 Data: 1 2 3 Output (-B): line1="1" line2="2" line3="1" The variable line3 is bound to 1 because although the @(accept) yields a data position which is advanced to the third line, this is intercepted by @(trailer) and adjusted back to the first line. Directives other than @(trailer) have no such special interaction with accept. .SH FUNCTIONS .SS Introduction .B TXR functions allow a query to be structured to avoid repetition. On a theoretical note, because .B TXR functions support recursion, functions enable TXR to match some kinds of patterns which exhibit self-embedding, or nesting, and thus cannot be matched by a regular language. Functions in .B TXR are not exactly like functions in mathematics or functional languages, and are not like procedures in imperative programming languages. They are not exactly like macros either. What it means for a .B TXR function to take arguments and produce a result is different from the conventional notion of a function. A .B TXR function may have one or more parameters. When such a function is invoked, an argument must be specified for each parameter. However, a special behavior is at play here. Namely, some or all of the argument expressions may be unbound variables. In that case, the corresponding parameters behave like unbound variables also. Thus .B TXR function calls can transmit the "unbound" state from argument to parameter. It should be mentioned that functions have access to all bindings that are visible in the caller; functions may refer to variables which are not mentioned in their parameter list. With regard to returning, .B TXR functions are also unconventional. If the function fails, then the function call is considered to have failed. The function call behaves like a kind of match; if the function fails, then the call is like a failed match. When a function call succeeds, then the bindings emanating from that function are processed specially. Firstly, any bindings for variables which do not correspond to one of the function's parameters are thrown away. Functions may internally bind arbitrary variables in order to get their job done, but only those variables which are named in the function argument list may propagate out of the function call. Thus, a function with no arguments can only indicate matching success or failure, but not produce any bindings. Secondly, variables do not propagate out of the function directly, but undergo a renaming. For each parameter which went into the function as an unbound variable (because its corresponding argument was an unbound variable), if that parameter now has a value, that value is bound onto the corresponding argument. Example: @(define collect_words (list)) @(coll)@{list /[^ \et]+/}@(end) @(end) The above function "collect_words" contains a query which collects words from a line (sequences of characters other than space or tab), into the list variable called "list". This variable is named in the parameter list of the function, therefore, its value, if it has one, is permitted to escape from the function call. Suppose the input data is: Fine summer day and the function is called like this: @(collect_words wordlist) The result with txr -B is: wordlist[0]=Fine wordlist[1]=summer wordlist[1]=day How it works is that in the function call @(collect_words wordlist), "wordlist" is an unbound variable. The parameter corresponding to that unbound variable is the parameter "list". Therefore, that parameter is unbound over the body of the function. The function body collects the words of "Fine summer day" into the variable "list", and then yields the that binding. Then the function call completes by noticing that the function parameter "list" now has a binding, and that the corresponding argument "wordlist" has no binding. The binding is thus transferred to the "wordlist" variable. After that, the bindings produced by the function are thrown away. The only enduring effects are: .IP - the function matched and consumed some input; and .IP - the function succeeded; and .IP - the wordlist variable now has a binding. .PP Another way to understand the parameter behavior is that function parameters behave like proxies which represent their arguments. If an argument is an established value, such as a character string or bound variable, the parameter is a proxy for that value and behaves just like that value. If an argument is an unbound variable, the function parameter acts as a proxy representing that unbound variable. The effect of binding the proxy is that the variable becomes bound, an effect which is settled when the function goes out of scope. Within the function, both the original variable and the proxy are visible simultaneously, and are independent. What if a function binds both of them? Suppose a function has a parameter called P, which is called with an argument A, and then in the function @A and @P are bound. This is permitted, and they can even be bound to different values. However, when the function terminates, the local binding of A simply disappears (because, remember, the symbol A is not a member of the list of parameters). Only the value bound to P emerges, and is bound to A, which still appears unbound at that point. .SS Definition Syntax Function definition syntax comes in two flavors: vertical and horizontal. Horizontal definitions actually come in two forms, the distinction between which is hardly noticeable, and the need for which is made clear below. A function definition begins with a @(define ...) directive. For vertical functions, this is the only element in a line. The define symbol must be followed by a symbol, which is the name of the function being defined. After the symbol, there is a parenthesized optional argument list. If there is no such list, or if the list is specified as () or the symbol "nil" then the function has no parameters. Examples of valid define syntax are: @(define foo) @(define bar ()) @(define match (a b c)) If the define directive is followed by more material on the same line, then it defines a horizontal function: @(define match_x)x@(end) If the define is the sole element in a line, then it is a vertical function, and the function definition continues below: @(define match_x) x @(end) The difference between the two is that a horizontal function matches characters within a line, whereas a vertical function matches lines within a stream. The former match_x matches the character x, advancing to the next character position. The latter match_x matches a line consisting of the character x, advancing to the next line. Material between @(define) and @(end) is the function body. The define directive may be followed directly by the @(end) directive, in which case the function has an empty body. Functions may be nested within function bodies. Such local functions have dynamic scope. They are visible in the function body in which they are defined, and in any functions invoked from that body. The body of a function is an anonymous block. (See BLOCKS above). .SS Two Forms of The Horizontal Function If a horizontal function is defined as the only element of a line, it may not be followed by additional material. The following construct is erroneous: @(define horiz (x))@foo:@bar@(end)lalala This kind of definition is actually considered to be in the vertical context, and like other directives that have special effects and that do not match anything, it does not consume a line of input. If the above syntax were allowed, it would mean that the line would not only define a function but also match "lalala". This would, in turn, would mean that the @(define)...@(end) is actually in horizontal mode, and so it matches a span of zero characters within a line (which means that is would require a line of input to match: a nasty thing for a non-matching directive to do!) A horizontal function can be defined in an actual horizontal context. This occurs if its is in a line where it is preceded by other material. For instance: X@(define fun)...@(end)Y This is a query line which must match the text XY. It also defines the function fun. The main use of this form is for nested horizontal functions: @(define fun)@(define local_fun)...@(end)@(end) .SS Vertical-Horizontal Overloading A function of the same name may be defined as both vertical and horizontal. Both functions are available at the same time. Which one is used by a call is resolved by context. See the section Vertical Versus Horizontal Calls below. .SS Call Syntax A function is invoked by compound directive whose first symbol is the name of that function. Additional elements in the directive are the arguments. Arguments may be symbols, or other objects like string and character literals, quasiliterals ore regular expressions. Example: Query: @(define pair (a b)) @a @b @(end) @(pair first second) @(pair "ice" cream) Data: one two ice milk Output (-B) first="one" second="two" cream="milk" The first call to the function takes the line "one two". The parameter "a" takes "one" and parameter b takes "two". These are rebound to the arguments first and second. The second call to the function binds the a parameter to the word "ice", and the b is unbound, because the corresponding argument "cream" is unbound. Thus inside the function, @a is forced to match "ice". Then a space is matched and @b collects the text "milk". When the function returns, the unbound "cream" variable gets this value. If a symbol occurs multiple times in the argument list, it constrains both parameters to bind to the same value. That is to say, all parameters which, in the body of the function, bind a value, and which are all derived from the same argument symbol must bind to the same value. This is settled when the function terminates, not while it is matching. Example: Query: @(define pair (a b)) @a @b @(end) @(pair same same) Data: one two Output: [query fails, prints "false"] .SS Vertical Versus Horizontal Calls A function call which is the only element of the query line in which it occurs is ambiguous. It can go either to a vertical function or to the horizontal one. If both are defined, then it goes to the vertical one. Example: Query: @(define which (x))@(bind x "horizontal")@(end) @(define which (x)) @(bind x "vertical") @(end) @(which fun) Output (-B): fun="vertical" Not only does this call go to the vertical function, but it is in a vertical context. If only a horizontal function is defined, then that is the one which is called, even if the call is the only element in the line. This takes place in a horizontal character-matching context, which requires a line of input which can be traversed: Example: Query: @(define which (x))@(bind x "horizontal")@(end) @(which fun) Data: ABC Output (-B): false The query failed. Why? Because since @(which fun) is in horizontal mode, it matches characters in a line. Since the function body consists of @(bind ...) which doesn't match any characters, the function call requires an empty line to match. The line ABC is not empty, and so there is a matching failure. The following example corrects this: Example: Query: @(define which (x))@(bind x "horizontal")@(end) @(which fun) Data: Output (-B): fun="horizontal" A call made in a clearly horizontal context will prefer the horizontal function, and only fall back on the vertical one if the horizontal one doesn't exist. (In this fall-back case, the vertical function is called with empty data; it is useful for calling vertical functions which process arguments and produce values.) In the next example, the call is followed by trailing material, placing it in a horizontal context. Leading material will do the same thing: Example: Query: @(define which (x))@(bind x "horizontal")@(end) @(define which (x)) @(bind x "vertical") @(end) @(which fun)B Data: B Output (-B): fun="horizontal" .SS Local Variables As described earlier, variables bound in a function body which are not parameters of the function are discarded when the function returns. However, that, by itself, doesn't make these variables local, because pattern functions have visibility to all variables in their calling environment. If a variable x exists already when a function is called, then an attempt to bind it inside a function may result in a failure. The @(local) directive must be used in a pattern function to list which variables are local. Example: @(define path (path))@\e @(local x y)@\e @(cases)@\e (@(path x))@(path y)@(bind path `(@x)@y`)@\e @(or)@\e @{x /[.,;'!?][^ \et\ef\ev]/}@(path y)@(bind path `@x@y`)@\e @(or)@\e @{x /[^ .,;'!?()\et\ef\ev]/}@(path y)@(bind path `@x@y`)@\e @(or)@\e @(bind path "")@\e @(end)@\e @(end) This is a horizontal function which matches a path, which lands into four recursive cases. A path can be parenthesized path followed by a path; it can be a certain character followed by a path, or it can be empty This function ensures that the variables it uses internally, x and y, do not have anything to do with any inherited bindings for x and y. Note that the function is recursive, which cannot work without x and y being local, even if no such bindings exist prior to the top-level invocation of the function. The invocation @(path x) causes x to be bound, which is visible inside the invocation @(path y), but that invocation needs to have its own binding of x for local use. .SS Nested Functions Function definitions may appear in a function. Such definitions are visible in all functions which are invoked from the body (and not necessarily enclosed in the body). In other words, the scope is dynamic, not lexical. Inner definitions shadow outer definitions. This means that a caller can redirect the function calls that take place in a callee, by defining local functions which capture the references. Example: Query: @(define which) @ (fun) @(end) @(define fun) @ (output) toplevel fun! @ (end) @(end) @(define callee) @ (define fun) @ (output) local fun! @ (end) @ (end) @ (which) @(end) @(callee) @(which) Output: local fun! toplevel fun! Here, the function "which" is defined which calls "fun". A toplevel definition of "fun" is introduced which outputs "toplevel fun!". The function "callee" provides its own local definition of "fun" which outputs "local fun!" before calling "which". When callee is invoked, it calls @(which), whose @(fun) call is routed to callee's local definition. When @(which) is called directly from the top level, its @(fun) call goes to the toplevel definition. .SH MODULARIZATION .SS The Load Directive The syntx of the load directive is: @(load EXPR) Where EXPR evaluates to a string giving the path of the file to load. Unless the path is absolute, it is interpreted relative to the directory of the source file from which the @(load) form was read. If there was no such source file (for instance, the script was read from standard input), then it is resolved relative to the current working directory. If the file cannot be opened, then the .txr suffix is added and another attempt is made. Thus load expressions need not refer to the suffix. In the future, additional suffixes may be searched (compiled versions of a file). Loading is performed at evaluation time; it is not a source file inclusion mechanism. A TXR script is read from beginning to end and parsed prior to being evaluated. See also: the *self-path* variable in TXR Lisp. .SH OUTPUT .SS Introduction A .B TXR query may perform custom output. Output is performed by @(output) clauses, which may be embedded anywhere in the query, or placed at the end. Output occurs as a side effect of producing a part of a query which contains an @(output) directive, and is executed even if that part of the query ultimately fails to find a match. Thus output can be useful for debugging. An output clause specifies that its output goes to a file, pipe, or (by default) standard output. If any output clause is executed whose destination is standard output, .B TXR makes a note of this, and later, just prior to termination, suppresses the usual printing of the variable bindings or the word false. .SS The Output Directive The syntax of the @(output) directive is: @(output [ ] { bool-keyword | keyword value }* ) . . one or more output directives or lines . @(end) The optional destination is a filename, the special name, - which redirects to standard output, or a shell command preceded by the ! symbol. In the first form, the destination may be specified as a variable which holds text, a string literal or a quasiliteral The keyword list consists of a mixture of boolean keywords which do not have an argument, or keywords with arguments. The following boolean keywords are supported: .IP :nothrow The output directive throws an exception if the output destination cannot be opened, unless the :nothrow keyword is present, in which case the situation is treated as a match failure. Note that since command pipes are processes that report errors asynchronously, a failing command will not throw an immediate exception that can be suppressed with :nothrow. This is for synchronous errors, like trying to open a destination file, but not having permissions, etc. .IP :append This keyword is meaningful for files, specifying append mode: the output is to be added to the end of the file rather than overwriting the file. The following value keywords are supported: .IP :filter The argument can be a symbol, which specifies a filter to be applied to the variable substitutions occuring within the output clause. The argument can also be a list of filter symbols, which specifies that multiple filters are to be applied, in left to right order. See the later sections Output Filtering below, and The Deffilter Directive. .IP :into The argument of :into is a symbol which denotes a variable. The output will go into that variable. If the variable is unbound, it will be created. Otherwise, its contents are overwritten unless the :append keyword is used. If :append is used, then the new content will be appened to the previous content of the variable, after flattening the content to a list, as if by the @(flatten) directive. .IP :named The argument of :named is a symbol which denotes a variable. The file or pipe stream which is opened for the output is stored in this variable, and is not closed at the end of the output block. This allows a subsequent output block to continue output on the same stream, which is possible using the next two keywords, :continue or :finish. A new binding is established for the variable, even if it already has an existing binding. .IP :continue A destination should not be specified if :continue is used. The argument of :continue is an expression, such as a variable name, that must evaluates to a stream object. That stream object is used for the output block. At the end of the output block, the stream is flushed, but not closed. A usage example is given in the documentation for the Close Directive below. .IP :finish A destination should not be specified if :finish is used. The argument of :finish is an expression, such as a variable name, that must evaluates to a stream object. That stream object is used for the output block. At the end of the output block, the stream is closed. An example is given in the documentation for the Close Directive below. .SS Output Text Text in an output clause is not matched against anything, but is output verbatim to the destination file, device or command pipe. .SS Output Variables Variables occurring in an output clause do not match anything, but instead their contents are output. A variable being output can be any object. If it is of a type other than a string, it will be converted to a string as if by the tostring function in TXR Lisp. A list is converted to a string in a special way: the elements are individually converted to a string and then they are catenated together. The default separator string is a single space: an alternate separation can be specified as an argument in the brace substitution syntax. Lists may be output within @(repeat) or @(rep) clauses. Each nesting of these constructs removes one level of nesting from the list variables that it contains. In an output clause, the @{NAME NUMBER} variable syntax generates fixed-width field, which contains the variable's text. The absolute value of the number specifies the field width. For instance -20 and 20 both specify a field width of twenty. If the text is longer than the field, then it overflows the field. If the text is shorter than the field, then it is left-adjusted within that field, if the width is specified as a positive number, and right-adjusted if the width is specified as negative. An output variable may specify a filter which overrides any filter established for the output clause. The syntax for this is @(NAME :filter }. The filter specification syntax is the same as in the output clause. See Output Filtering below. .SS Output Variables: Indexing Additional syntax is supported in output variables that does not appear in pattern matching variables. A square bracket index notation may be used to extract elements or ranges from a variable, which works with strings, vectors and lists. Elements are indexed from zero. This notation is only available in brace-enclosed syntax, and looks like this: .IP @{NAME[expr]} Extract the element at the position given by expr. .IP @{NAME[expr1..expr2]} Extract a range of elements from the position given by expr1, up to one position less than the position given by expr2. If the variable is a list, it is treated as a list substitution, exactly as if it were the value of an unsubscripted list variable. The elements of the list are converted to strings and catenated together wit ha separator string between them, the default one being a single space. An alternate character may be given as a string argument in the brace notation. .TP Example: @(bind a ("a" "b" "c" "d")) @(output) @{a[1..3] "," 10} @(end) The above produces the text "b,c" in a field 10 spaces wide. The [1..3] argument extracts a range of a; the "," argument specifies an alternate separator string, and 10 specifies the field width. .SS Output Substitutions The brace syntax has another syntactic and semantic extension. In place of the symbol, an expression may appear. The value of that expression is substituted. Example: @(bind a "foo") @(output) @{`@a:` -10} Here, the quasiliteral expression `@a:` is evaluated, producing the string "foo:". This string is printed right-adjusted in a 10 character field. .SS The Repeat Directive The repeat directive generates repeated text from a ``boilerplate'', by taking successive elements from lists. The syntax of repeat is like this: @(repeat) . . main clause material, required . . special clauses, optional . . @(end) Repeat has four types of special clauses, any of which may be specified with empty contents, or omitted entirely. They are described below. Repeat takes arguments, also described below. All of the material in the main clause and optional clauses is examined for the presence of variables. If none of the variables hold lists which contain at least one item, then no output is performed, (unless the repeat specifies an @(empty) clause, see below). Otherwise, among those variables which contain non-empty lists, repeat finds the length of the longest list. This length of this list determines the number of repetitions, R. If the repeat contains only a main clause, then the lines of this clause is output R times. Over the first repetition, all of the variables which, outside of the repeat, contain lists are locally rebound to just their first item. Over the second repetition, all of the list variables are bound to their second item, and so forth. Any variables which hold shorter lists than the longest list eventually end up with empty values over some repetitions. Example: if the list A holds "1", "2" and "3"; the list B holds "A", "B"; and the variable C holds "X", then @(repeat) >> @C >> @A @B @(end) will produce three repetitions (since there are two lists, the longest of which has three items). The output is: >> X >> 1 A >> X >> 2 B >> X >> 3 The last line has a trailing space, since it is produced by "@A @B", where @B has an empty value. Since C is not a list variable, it produces the same value in each repetition. The special clauses are: .IP @(single) If the repeat produces exactly one repetition, then the contents of this clause are processed for that one and only repetition, instead of the main clause or any other clause which would otherwise be processed. .IP @(first) The body of this clause specifies an alternative body to be used for the first repetition, instead of the material from the main clause. .IP @(last) The body of this clause is used instead of the main clause for the last repetition. .IP @(empty) If the repeat produces no repetitions, then the body of this clause is output. If this clause is absent or empty, the repeat produces no output. .IP "@(mod n m)" The forms n and m are expressions that evaluate to integers. The value of m should be nonzero. The clause denoted this way is active if the repetition modulo m is equal to n. The first repetition is numbered zero. For instance the clause headed by @(mod 0 2) will be used on repetitions 0, 2, 4, 6, ... and @(mod 1 2) will be used on repetitions 1, 3, 5, 7, ... .IP "@(modlast n m)" The meaning of n and m is the same as in @(mod n m), but one more condition is imposed. This clause is used if the repetition modulo m is equal to n, and if it is the last repetition. .PP The precedence among the clauses which take an iteration is: single > first > mod > modlast > last > main. That is if two or more of these clauses can apply to a repetition, then the leftmost one in this precedence list applies. For instance, if there is just a single repetition, then any of these special clause types can apply to that repetition, since it is the only repetition, as well as the first and last one. In this situation, if there is a @(single) clause present, then the repetition is processed using that clause. Otherwise, if there is a @(first) clause present, that clause is used. Failing that, @(mod) is used if there is such a clause and its numeric conditions are satisfied. If not then @(modlast) clauses are considered, and if there are none, or none of them activate, then @(last) is considered. If none of those clauses are present or apply, then the repetition is processed using the main clause. Repeat supports arguments. @(repeat [:counter ] [:vars (*)]) The :counter argumnt designates a symbol which will behave as an integer variable over the scope of the clauses inside the repeat. The variable provides access to the reptition count, starting at zero, incrementing with each repetition. The :vars argument specifies a list of variables. The repeat directive will pick out from this list those variables which have bindings. It will assume that all these variables occur in the repeat block and are to be iterated. This syntax is needed for situations in which @(repeat) is not able to deduce the existence of a variable in the block. It does not dig very deeply to discover variables, and does not "see" variables that are referenced via embedded TXR Lisp expressions. For instance, the following produces no output: @(bind list ("a" "b" "c")) @(output) @(repeat) @(format nil "<~a>" list) @(end) @(end) Although the list variable appears in the repeat block, it is embedded in a TXR Lisp construct. That construct will never be evaluated because no repetitions take place: the repeat construct doesn't find any variables and so doesn't iterate. The remedy is to provide a little help via the :vars parameter: @(bind list ("a" "b" "c")) @(output) @(repeat :vars (list)) @(format nil "<~a>" list) @(end) @(end) Now the repeat block iterates over list and the output is: .SS Nested Repeats If a repeat clause encloses variables which hold multidimensional lists, those lists require additional nesting levels of repeat (or rep). It is an error to attempt to output a list variable which has not been decimated into primary elements via a repeat construct. Suppose that a variable X is two-dimensional (contains a list of lists). X must be twice nested in a repeat. The outer repeat will walk over the lists contained in X. The inner repeat will walk over the elements of each of these lists. A nested repeat may be embedded in any of the clauses of a repeat, not only the main clause. .SS The Rep Directive The @(rep) directive is similar to @(repeat), but whereas @(repeat) is line oriented, @(rep) generates material within a line. It has all the same clauses, but everything is specified within one line: @(rep)... main material ... .... special clauses ...@(end) More than one @(rep) can occur within a line, mixed with other material. A @(rep) can be nested within a @(repeat) or within another @(rep). Also, @(rep) accepts the same :counter and :vars arguments. .SS Repeat and Rep Examples Example 1: show the list L in parentheses, with spaces between the elements, or the symbol NIL if the list is empty: @(output) @(rep)@L @(single)(@L)@(first)(@L @(last)@L)@(empty)NIL@(end) @(end) Here, the @(empty) clause specifies NIL. So if there are no repetitions, the text NIL is produced. If there is a single item in the list L, then @(single)(@L) produces that item between parentheses. Otherwise if there are two or more items, the first item is produced with a leading parenthesis followed by a space by @(first)(@L , and the last item is produced with a closing parenthesis: @(last)@L). All items in between are emitted with a trailing space by the main clause: @(rep)@L . Example 2: show the list L like Example 1 above, but the empty list is (). @(output) (@(rep)@L @(last)@L@(end)) @(end) This is simpler. The parentheses are part of the text which surrounds the @(rep) construct, produced unconditionally. If the list L is empty, then @(rep) produces no output, resulting in (). If the list L has one or more items, then they are produced with spaces each one, except the last which has no space. If the list has exactly one item, then the @(last) applies to it instead of the main clause: it is produced with no trailing space. .SS The Close Directive The syntax of the @(close) directive is: @(close ) Where evaluates to a stream. The close directive can be used to explicitly close streams created using @(output ... :named ) syntax, as an alternative to the @(output :finish ) Examples: Write two lines to "foo.txt" over two output blocks using a single stream: @(output "foo.txt" :named foo) Hello, @(end) @(output :continue foo) world! @(end) @(close foo) The same as above, using :finish rather than :continue so that the stream is closed at the end of the second block: @(output "foo.txt" :named foo) Hello, @(end) @(output :finish foo) world! @(end) .SS Output Filtering Often it is necessary to transform the output to preserve its meaning under the convention of a given data format. For instance, if a piece of text contains the characters < or >, then if that text is being substituted into HTML, these should be replaced by < and >. This is what filtering is for. Filtering is applied to the contents of output variables, not to any template text. .B TXR implements named filters. Built-in filters are named by keywords, given below. User-defined filters are possible, however. See notes on the deffilter directive below. Instead of a filter name, the syntax (fun NAME) can be used. This denotes that the function called NAME is to be used as a filter. This is discussed in the next section Function Filters below. Built-in filters named by keywords: .IP :to_html Filter text to HTML, representing special characters using HTML ampersand sequences. For instance '>' is replaced by '>'. .IP :from_html Filter text with HTML codes into text in which the codes are replaced by the corresponding characters. For instance '>' is replaced by '>'. .IP :upcase Convert the 26 lower case letters of the English alphabet to upper case. .IP :downcase Convert the 26 upper case letters of the English alphabet to lower case. .IP :frompercent Decode percent-encoded text. Character triplets consisting of the % character followed by a pair of hexadecimal digits (case insensitive) are are converted to bytes having the value represented by the hexadecimal digits (most significant nybble first). Sequences of one or more such bytes are treated as UTF-8 data and decoded to characters. .IP :topercent Convert to percent encoding according to RFC 3986. The text is first converted to UTF-8 bytes. The bytes are then converted back to text as follows. Bytes in the range 0 to 32, and 127 to 255 (note: including the ASCII DEL), bytes whose values correspond to ASCII characters which are listed by RFC 3986 as being in the "reserved set", and the byte value corresponding to the ASCII % character are encoded as a three-character sequence consisting of the % character followed by two hexadecimal digits derived from the byte value (most significant nybble first, upper case). All other bytes are converted directly to characters of the same value without any such encoding. .IP :fromurl Decode from URL encoding, which is like percent encoding, except that if the unencoded + character occurs, it is decoded to a space character. Of course %20 still decodes to space, and %2B to the + character. .IP :tourl Encode to URL encoding, which is like percent encoding except that a space maps to + rather than %20. The + character, being in the reserved set, encodes to %2B. .IP :tonumber Converts strings to numbers. Strings that contain a period, e or E are converted to floating point as if by the function flo-str. Otherwise they are converted to integer as if using int-str with a radix of 10. Non-numeric junk results in the object nil. .IP :tointeger Converts strings to integers as if using int-str with a radix of 10. Non-numeric junk results in the object nil. .IP :tofloat Converts strings to floating-point values as if using the function flo-str. Non-numeric junk results in the object nil. .IP :hextoint Converts strings to integers as if using int-str with a radix of 16. Non-numeric junk results in the object nil. .PP .TP Examples To escape HTML characters in all variable substitutions occuring in an output clause, specify :filter :to_html in the directive: @(output :filter :to_html) ... @(end) To filter an individual variable, add the syntax to the variable spec: @(output) @{x :filter :to_html} @(end) Multiple filters can be applied at the same time. For instance: @(output) @{x :filter (:upcase :to_html)} @(end) This will fold the contents of x to upper case, and then encode any special characters into HTML. Beware of combinations that do not make sense. For instance, suppose the original text is HTML, containing codes like '"'. The compound filter (:upcase :from_html) will not work because '"' will turn to '"' which no longer be recognized by the :from_html filter, because the entity names in HTML codes are case-sensitive. Capture some numeric variables and convert to numbers: @date @time @temperature @pressure @(filter :tofloat temperature pressure) @;; temperature and pressure can now be used in calculations .SS Function Filters A function can be used as a filter. For this to be possible, the function must conform to certain rules: .IP 1. The function must take two special arguments, which may be followed by additional arguments. .IP 2. When the function is called, the first argument will be bound to a string, and the second argument will be unbound. The function must produce a value by binding it to the second argument. If the filter is to be used as the final filter in a chain, it must produce a string. For instance, the following is a valid filter function: @(define foo_to_bar (in out)) @ (next :string in) @ (cases) foo @ (bind out "bar") @ (or) @ (bind out in) @ (end) @(end) This function binds the out parameter to "bar" if the in parameter is "foo", otherwise it binds the out parameter to a copy of the in parameter. This is a simple filter. To use the filter, use the syntax (:fun foo_to_bar) in place of a filter name. For instance in the bind directive: @(bind "foo" "bar" :lfilt (:fun foo_to_bar)) The above should succeed since the left side is filtered from "foo" to "bar", so that there is a match. Of course, function filters can be used in a chain: @(output :filter (:downcase (:fun foo_to_bar) :upcase)) ... @(end) Here is a split function which takes an extra argument. @(define split (in out sep)) @ (next :list in) @ (coll)@(maybe)@token@sep@(or)@token@(end)@(end) @ (bind out token) @(end) Furthermore, note that it produces a list rather than a string. This function separates the argument in into tokens according to the separator text sep. Here is another function, join, which catenates a list: @(define join (in out sep)) @ (output :into out) @ (rep)@in@sep@(last)@in@(end) @ (end) @(end) Now here is these two being used in a chain: @(bind text "how,are,you") @(output :filter (:fun split ",") (:fun join "-")) @text @(end) Output: how-are-you When the filter invokes a function, it generates the first two arguments internally to pass in the input value and capture the output. The remaining arguments from the (:fun ...) construct are also passed to the function. Thus the "," and "-" are passed as the sep argument to split and join. Note that split puts out a list, which join accepts. So the overall filter chain operates on a string: a string goes into split, and a string comes out of join. .SS The Deffilter Directive The deffilter directive allows a query to define a custom filter, which can then be used in @(output) clauses to transform substituted data. This directive's syntax is illustrated in this example: Query: @(deffilter rot13 ("a" "n") ("b" "o") ("c" "p") ("d" "q") ("e" "r") ("f" "s") ("g" "t") ("h" "u") ("i" "v") ("j" "w") ("k" "x") ("l" "y") ("m" "z") ("n" "a") ("o" "b") ("p" "c") ("q" "d") ("r" "e") ("s" "f") ("t" "g") ("u" "h") ("v" "i") ("w" "j") ("x" "k") ("y" "l") ("z" "m")) @(collect) @line @(end) @(output :filter rot13) @(repeat) @line @(end) @(end) Input: hey there! Output: url gurer! The deffilter symbol must be followed by the name of the filter to be defined, followed by forms which evaluate to lists of strings. Each list must be at least two elements long and specifies one or more texts which are mapped to a replacement text. For instance, the following specifies a telephone keypad mapping from upper case letters to digits. @(deffilter alpha_to_phone ("E" "0") ("J" "N" "Q" "1") ("R" "W" "X" "2") ("D" "S" "Y" "3") ("F" "T" "4") ("A" "M" "5") ("C" "I" "V" "6") ("B" "K" "U" "7") ("L" "O" "P" "8") ("G" "H" "Z" "9")) @(deffilter foo (`@a` `@b`) ("c" `->@d`)) @(bind x ("from" "to")) @(bind y ("---" "+++")) @(deffilter sub x y) The last deffilter above equivalent to @(deffilter sub ("from" "to") ("---" "+++")). Filtering works using a longest match algorithm. The input is scanned from left to right, and the longest piece of text is identified at every character position which matches a string on the left hand side, and that text is replaced with its associated replacement text. The scanning then continues at the first character after the matched text. If none of the strings matches at a given character position, then that character is passed through the filter untranslated, and the scan continues at the next character in the input. Filtering is not in-place but rather instantiates a new text, and so replacement text is not re-scanned for more replacements. If a filter definition accidentally contains two or more repetitions of the same left hand string with different right hand translations, the later ones take precedence. No warning is issued. .SS The Filter Directive The syntax of the filter directive is: @(filter FILTER { VAR }+ ) A filter is specified, followed by one or more variables whose values are filtered and stored back into each variable. Example: convert a, b, and c to upper case and HTML encode: @(filter (:upcase :to_html) a b c) .SH EXCEPTIONS .SS Introduction The exceptions mechanism in .B TXR is another disciplined form of non-local transfer, in addition to the blocks mechanism (see BLOCKS above). Like blocks, exceptions provide a construct which serves as the target for a dynamic exit. Both blocks and exceptions can be used to bail out of deep nesting when some condition occurs. However, exceptions provide more complexity. Exceptions are useful for error handling, and TXR in fact maps certain error situations to exception control transfers. However, exceptions are not inherently an error-handling mechanism; they are a structured dynamic control transfer mechanism, one of whose applications is error handling. An exception control transfer (simply called an exception) is always identified by a symbol, which is its type. Types are organized in a subtype-supertype hierarchy. For instance, the file_error exception type is a subtype of the error type. This means that a file error is a kind of error. An exception handling block which catches exceptions of type error will catch exceptions of type file_error, but a block which catches file_error will not catch all exceptions of type error. A query_error is a kind of error, but not a kind of file_error. The symbol t is the supertype of every type: every exception type is considered to be a kind of t. (Mnemonic: t stands for type, as in any type). Exceptions are handled using @(catch) clauses within a @(try) directive. In addition to being useful for exception handling, the @(try) directive also provides unwind protection by means of a @(finally) clause, which specifies query material to be executed unconditionally when the try clause terminates, no matter how it terminates. .SS The Try Directive The general syntax of the try directive is @(try) ... main clause, required ... ... optional catch clauses ... ... optional finally clause @(end) A catch clause looks like: @(catch TYPE) . . . and also the this form, equivalent to @(catch (t)): @(catch) . . . which catches all exceptions. A finally clause looks like: @(finally) ... . . The main clause may not be empty, but the catch and finally may be. A try clause is surrounded by an implicit anonymous block (see BLOCKS section above). So for instance, the following is a no-op (an operation with no effect, other than successful execution): @(try) @(accept) @(end) The @(accept) causes a successful termination of the implicit anonymous block. Execution resumes with query lines or directives which follow, if any. Try clauses and blocks interact. For instance, a block accept from within a try clause invokes a finally. Query: @(block foo) @ (try) @ (accept foo) @ (finally) @ (output) bye! @ (end) @ (end) Output: bye! How this works: the try block's main clause is @(accept foo). This causes the enclosing block named foo to terminate, as a successful match. Since the try is nested within this block, it too must terminate in order for the block to terminate. But the try has a finally clause, which executes unconditionally, no matter how the try block terminates. The finally clause performs some output, which is seen. .SS The Finally Clause A try directive can terminate in one of three ways. The main clause may match successfully, and possibly yield some new variable bindings. The main clause may fail to match. Or the main clause may be terminated by a non-local control transfer, like an exception being thrown or a block return (like the block foo example in the previous section). No matter how the try clause terminates, the finally clause is processed. Now, the finally clause is itself a query which binds variables, which leads to the question: what happens to such variables? What if the finally block fails as a query? Another question is: what if a finally clause itself initiates a control transfer? Answers follow. Firstly, a finally clause will contribute variable bindings only if the main clause terminates normally (either as a successful or failed match). If the main clause successfully matches, then the finally block continues matching at the next position in the data, and contributes bindings. If the main clause fails, then the finally block matches at the same position. The overall try directive succeeds as a match if either the main clause or the finally clause succeed. If both fail, then the try directive is a failed match. The subquery in which it is located fails, et cetera. Example: Query: @(try) @a @(finally) @b @(end) @c Data: 1 2 3 Output (-B): a="1" b="2" c="3" In this example, the main clause of the try captures line "1" of the data as variable a, then the finally clause captures "2" as b, and then the query continues with the @c variable after try block, and captures "3". Example: Query: @(try) hello @a @(finally) @b @(end) @c Data: 1 2 Output (-B): b=1 c=2 In this example, the main clause of the try fails to match, because the input is not prefixed with "hello ". However, the finally clause matches, binding b to "1". This means that the try block is a successful match, and so processing continues with @c which captures "2". When finally clauses are processed during a non-local return, they have no externally visible effect if they do not bind variables. However, their execution makes itself known if they perform side effects, such as output. A finally clause guards only the main clause and the catch clauses. It does not guard itself. Once the finally clause is executing, the try block is no longer guarded. This means if a nonlocal transfer, such as a block accept or exception, is initiated within the finally clause, it will not re-execute the finally clause. The finally clause is simply abandoned. The disestablishment of blocks and try clauses is properly interleaved with the execution of finally clauses. This means that all surrounding exit points are visible in a finally clause, even if the finally clause is being invoked as part of a transfer to a distant exit point. The finally clause can make a control transfer to an exit point which is more near than the original one, thereby "hijacking" the control transfer. Also, the anonymous block established by the try directive is visible in the finally clause. Example: @(try) @ (try) @ (next "nonexistent-file") @ (finally) @ (accept) @ (end) @(catch file_error) @ (output) file error caught @ (end) @(end) In this example, the @(next) directive throws an exception of type file_error, because the given file does not exist. The exit point for this exception is the @(catch file_error) clause in the outer-most try block. The inner block is not eligible because it contains no catch clauses at all. However, the inner try block has a finally clause, and so during the processing of this exception which is headed for the @(catch file_error), the finally clause performs an anonymous accept. The exit point for the accept is the anonymous block surrounding the inner try. So the original transfer to the catch clause is forgotten. The inner try terminates successfully, and since it constitutes the main clause of the outer try, that also terminates successfully. The "file error caught" message is never printed. .SS Catch Clauses Catch clauses establish a try block as a potential exit point for an exception-induced control transfer (called a ``throw''). A catch clause specifies an optional list of symbols which represent the exception types which it catches. The catch clause will catch exceptions which are a subtype of any one of those exception types. If a try block has more than one catch clause which can match a given exception, the first one will be invoked. When a catch is invoked, it is of course understood that the main clause did not terminate normally, and so the main clause could not have produced any bindings. Catches are processed prior to finally. If a catch clause itself throws an exception, that exception cannot be caught by that same clause or its siblings in the same try block. The catches of that block are no longer visible at that point. Nevertheless, the catch clauses are still protected by the finally block. If a catch clause throws, the finally block is still processed. If a finally block throws an exception, then it is simply aborted; the remaining directives in that block are not processed. So the success or failure of the try block depends on the behavior of the catch clause or the finally, if there is one. If either of them succeed, then the try block is considered a successful match. Example: Query: @(try) @ (next "nonexistent-file") @ x @ (catch file_error) @a @(finally) @b @(end) @c Data: 1 2 3 Output (-B): a="1" b="2" c="3" Here, the try block's main clause is terminated abruptly by a file_error exception from the @(next) directive. This is handled by the catch clause, which binds variable a to the input line "1". Then the finally clause executes, binding b to "2". The try block then terminates successfully, and so @c takes "3". .SS Catch Clauses with Parameters A catch may have parameters following the type name, like this: @(catch pair (a b)) To write a catch-all with parameters, explicitly write the master supertype t: @(catch t (arg ...)) Parameters are useful in conjunction with throw. The built-in error exceptions generate one argument, which is a string containing the error message. Using throw, arbitrary parameters can be passed from the throw site to the catches. .SS The Throw Directive The throw directive generates an exception. A type must be specified, followed by optional arguments. For example, @(throw pair "a" `@file.txt`) throws an exception of type pair, with two arguments, being "a" and the expansion of the quasiliteral `@file.txt`. The selection of the target catch is performed purely using the type name; the parameters are not involved in the selection. Binding takes place between the arguments given in throw, and the target catch. If any catch parameter, for which a throw argument is given, is a bound variable, it has to be identical to the argument, otherwise the catch fails. (Control still passes to the catch, but the catch is a failed match). Query: @(bind a "apple") @(try) @(throw e "banana") @(catch e (a)) @(end) Output: false If any argument is an unbound variable, the corresponding parameter in the catch is left alone: if it is an unbound variable, it remains unbound, and if it is bound, it stays as is. Query: @(try) @(trow e "honda" unbound) @(catch e (car1 car2)) @car1 @car2 @(end) Data: honda toyota Output (-B): car1="honda" car2="toyota" If a catch has fewer parameters than there are throw arguments, the excess arguments are ignored. Query: @(try) @(throw e "banana" "apple" "pear") @(catch e (fruit)) @(end) Output (-B): fruit="banana" If a catch has more parameters than there are throw arguments, the excess parameters are left alone. They may be bound or unbound variables. Query: @(try) @(trow e "honda") @(catch e (car1 car2)) @car1 @car2 @(end) Data: honda toyota Output (-B): car1="honda" car2="toyota" A throw argument passing a value to a catch parameter which is unbound causes that parameter to be bound to that value. Throw arguments are evaluated in the context of the throw, and the bindings which are available there. Consideration of what parameters are bound is done in the context of the catch. Query: @(bind c "c") @(try) @(forget c) @(bind (a c) ("a" "lc")) @(throw e a c) @(catch e (b a)) @(end) Output (-B): c="c" b="a" a="lc" In the above example, c has a toplevel binding to the string "c", but is then unbound within the try construct, and rebound to the value "c". Since the try construct is terminated by a throw, these modifications of the binding environment are discarded. Hence, at the end of the query, variable c ends up bound to the original value "c". The throw still takes place within the scope of the bindings set up by the try clause, so the values of a and c that are thrown are "a" and "lc". However, at the catch site, variable a does not have a binding. At that point, the binding to "a" established in the try has disappeared already. Being unbound, the catch parameter a can take whatever value the corresponding throw argument provides, so it ends up with "lc". .SS The Defex Directive The defex directive allows the query writer to invent custom exception types, which are arranged in a type hierarchy (meaning that some exception types are considered subtypes of other types). Subtyping means that if an exception type B is a subtype of A, then every exception of type B is also considered to be of type A. So a catch for type A will also catch exceptions of type B. Every type is a supertype of itself: an A is a kind of A. This of course implies that ever type is a subtype of itself also. Furthermore, every type is a subtype of the type t, which has no supertype other than itself. Type nil is is a subtype of every type, including itself. The subtyping relationship is transitive also. If A is a subtype of B, and B is a subtype of C, then A is a subtype of C. Defex may be invoked with no arguments, in which case it does nothing: @(defex) It may be invoked with one argument, which must be a symbol. This introduces a new exception type. Strictly speaking, such an introduction is not necessary; any symbol may be used as an exception type without being introduced by @(defex): @(defex a) Therefore, this also does nothing, other than document the intent to use a as an exception. If two or more argument symbols are given, the symbols are all introduced as types, engaged in a subtype-supertype relationship from left to right. That is to say, the first (leftmost) symbol is a subtype of the next one, which is a subtype of the next one and so on. The last symbol, if it had not been already defined as a subtype of some type, becomes a direct subtype of the master supertype t. Example: @(defex d e) @(defex a b c d) The fist directive defines d as a subtype of e, and e as a subtype of t. The second defines a as a subtype of b, b as a subtype of c, and c as a subtype of d, which is already defined as a subtype of e. Thus a is now a subtype of e. It should be obvious that the above could be condensed to: @(defex a b c d e) Example: Query: @(defex gorilla ape primate) @(defex monkey primate) @(defex human primate) @(collect) @(try) @(skip) @(cases) gorilla @name @(throw gorilla name) @(or) monkey @name @(throw monkey name) @(or) human @name @(throw human name) @(end)@#cases @(catch primate (name)) @kind @name @(output) we have a primate @name of kind @kind @(end)@#output @(end)@#try @(end)@#collect Input: gorilla joe human bob monkey alice Output: we have a primate joe of kind gorilla we have a primate bob of kind human we have a primate alice of kind monkey Exception types have a pervasive scope. Once a type relationship is introduced, it is visible everywhere. Moreover, the defex directive is destructive, meaning that the supertype of a type can be redefined. This is necessary so that something like the following works right. @(defex gorilla ape) @(defex ape primate) These directives are evaluated in sequence. So after the first one, the ape type has the type t as its immediate supertype. But in the second directive, ape appears again, and is assigned the primate supertype, while retaining gorilla as a subtype. This situation could instead be diagnosed as an error, forcing the programmer to reorder the statements, but instead TXR obliges. However, there are limitations. It is an error to define a subtype-supertype relationship between two types if they are already connected by such a relationship, directly or transitively. So the following definitions are in error: @(defex a b) @(defex b c) @(defex a c)@# error: a is already a subtype of c, through b @(defex x y) @(defex y x)@# error: circularity; y is already a supertype of x. .SS The Assert directive The assert directive requires the remaining query or sub-query which follows it to match. If the remainder fails to match, the assert directive throws an exception. If the directive is simply @(assert) Then it throws an assertion of type assert, which is a subtype of error. The assert directive also takes arguments similar to the throw directive. The following assert directive, if it triggers, will throw an exception of type foo, with arguments 1 and "2". @(assert foo 1 "2") The throw directive generates an exception. A type must be specified, followed by optional arguments. Example: @(collect) Important Header ---------------- @(assert) Foo: @a, @b @(end) Without the assertion in places, if the "Foo: @a, @b" part does not match, then the entire interior of the @(collect) clause fails, and the collect continues searching for another match. With the assertion in place, if the "Important Header" and its underline match, then the remainder of the collect body must match, otherwise an exception is thrown. Now the program will not silently skip over any Important Header sections due to a problem in its matching logic. This is particularly useful when the matching is varied with numerous cases, and they must all be handled. There is a horizontal directive also. For instance: abc@(assert)d@x asserts that if the prefix "abc" is matched, then it must be followed by a successful match for "d@x", or else an exception is thrown. .SH TXR LISP The TXR language contains an embedded Lisp dialect called TXR Lisp. This language is exposed in TXR in several ways. Firstly, in any situation that calls for an expression, a Lisp expression can be used, if it is preceded by the @ symbol. The Lisp expression is evaluated and its value becomes the value of that expression. Thus, TXR directives are embedded in literal text using @, and Lisp expressions are embedded in directives using @ also. Secondly, the @(do) directive can be used for evaluating one or more Lisp forms, such that their value is thrown away. This is useful for evaluating some Lisp code for the sake of its side effect, such as defining a variable, updating a hash table, et cetera. Thirdly, the @(require) directive can be used to evaluate Lisp expressions as part of the matching logic of the TXR pattern language. The return value of the rightmost expression is examined. If it is nil, then the @(require) directive triggers a match failure. Otherwise, matching proceeds. Lastly, TXR Lisp expressions can be evaluated via the txr command line, using the -e and -p options. Examples: Bind variable a to the integer 4: @(bind a @(+ 2 2)) Bind variable b to the standard input stream: @(bind a @*stdin*) Define several Lisp functions using @(do): @(do (defun add (x y) (+ x y)) (defun occurs (item list) (cond ((null list) nil) ((atom list) (eql item list)) (t (or (eq (first list) item) (occurs item (rest list))))))) Trigger a failure unless previously bound variable "answer" is greater than 42: @(require (> (str-int answer) 42) .SS Overview TXR Lisp is a small and simple dialect, like Scheme, but much more similar to Common Lisp than Scheme. It has separate value and function binding namespaces, like Common Lisp (and thus is a Lisp-2 type dialect), and represents boolean true and false with the symbols t and nil (but note the case sensitivity of identifiers denoting symbols!) Furthermore, the symbol nil is also the empty list, which terminates nonempty lists. Function and variable bindings are dynamically scoped in TXR Lisp. However, closures do capture variables. .SS Additional Syntax Much of the TXR Lisp syntax has been introduced in the previous sections of the manual, since directive forms are based on it. There is some additional syntax that is useful in TXR Lisp programming. .SS Consing Dot Unlike other major Lisp dialects, TXR Lisp allows a consing dot with no forms preceding it. This construct simply denotes the form which follows the dot. That is to say, the parser implements the following transformation: (. expr) -> expr This is convenient in writing function argument lists that only take variable arguments. Instead of the syntax: (defun fun args ...) the following syntax can be used: (defun fun (. args) ...) When a lambda form is printed, it is printed in the following style. (lambda nil ...) -> (lambda () ...) (lambda sym ...) -> (lambda (. sym) ...) (lambda (sym) ...) -> (lambda (sym) ...) In no other circumstances is nil printed as (), or a symbol as (. sym). symbol is followed by a symbol. .SS Quote and Quasiquote .IP ' The quote character in front of an expression is used for suppressing evaluation, which is useful for forms that evaluate to something other than themselves. For instance if '(+ 2 2) is evaluated, the value is the three-element list (+ 2 2), wheras if (+ 2 2) is evaluated, the value is 4. Similarly, the value of 'a is the symbol a itself, whereas the value of a is the value of the variable a. .IP ^ The caret in front of an expression is a quasiquote. A quasiquote is like a quote, but with the possibility of substitution of material. Under a quasiquote, form is considered to be a quasiquote template. The template is considered to be a literal structure, except that it may contain the notations , and ,* which denote non-constant parts. A quasiquote gets translated into code which, when evaluated, constructs the structure implied by , taking into account the unquotes and splices. A quasiquote also processes nested quasiquotes specially. If does not contain any unquotes or splices (which match its level of nesting), or is simply an atom, then ^ is equivalent to ': in other words, it is like an ordinary quote. For instance ^(a b ^(c ,d)) is equivalent to '(a b ^(c ,d)). Although there is an unquote ,d it belongs to the inner quasiquote ^(c ,d), and the outer quasiquote does not have any unquotes of its own, making it equivalent to a quote. Dialect note: in Common Lisp and Scheme, ^form is written `form, and quasiquotes are also informally known as backquotes. In TXR, the backquote is used for quasi string literals already. Moreover, the caret is more visually distinct from the backquote, making it somewhat easier to tell apart generated code from its surroundings. .IP , The comma character is used within a to denote an unquote. Whereas the quote suppresses evaluation, the comma introduces an exception: an element of a form which is evaluated. For example, list ^(a b c ,(+ 2 2) (+ 2 2)) is the list (a b c 4 (+ 2 2)). Everything in the quasiquote stands for itself, except for the ,(+ 2 2) which is evaluated. Note: if a variable is called *x*, then the syntax ,*x* means ,* x*: splice the value of x*. In this situation, whitespace between the comma and the variable name should be used: , *x*. .IP ,* The comma-star operator is used within quasiquote list to denote a splicing unquote. The form which follows ,* must evaluate to a list. That list is spliced into the structure which the quasiquote denotes. For example: '(a b c ,*(list (+ 3 3) (+ 4 4) d)) evaluates to (a b c 6 8 d). The expression (list (+ 3 3) (+ 4 4)) is evaluated to produce the list (6 8), and this list is spliced into the quoted template. Dialect note: in other Lisp dialects, the equivalent syntax is usually ,@ (comma at). The @ character already has an assigned meaning, so * is used. .SS Quasiquoting non-List Objects Quasiquoting is supported over hash table and vector literals (see Vectors and Hashes below). A hash table or vector literal can be quoted, like any object, for instance: '#(1 2 3) The #(1 2 3) literal is turned into a vector atom right in the TXR parser, and this atom is being quoted: this is (quote ) syntactically, which evaluates to . When a vector is quasi-quoted, this is a case of ^, which evaluates to . ^#(1 2 3) If unquotes occur in the quasi-quoted vector, it is a quasivector. (let ((a 42)) ^#(1 ,a 3)) ; value is #(1 42 3) In this situation, the ^#(...) notation produces code which constructs a vector. The vector in the following example is also a quasivector. It contains unquotes, and is though the quote is not directly applied to it, it is surrounded in a quote. (let ((a 42)) ^(a b c #(d ,a))) ; value is (a b c #(d 42)) Hash table literals have two parts: the list of hash construction arguments and the key-value pairs. For instance: #H((:equal-based) (a 1) (b 2)) where (:equal-based) is the list of arguments and the pairs are (a 1) and (b 2). Hash literals may be quasiquoted. In quasiquoting, the arguments and pairs are treated as separate syntax; it is not one big list. So the following is not a possible way to express the above hash: ;; not supported: splicing across the entire syntax (let ((hash-syntax '((:equal-based) (a 1) (b 2)))) ^#H(,*hash-syntax)) This is correct: ;; fine: splicing hash arguments and contents separatly (let ((hash-args '(:equal-based)) (hash-contents '((a 1) (b 2)))) ^#H(,hash-args ,*hash-contents)) .SS Vectors .IP "#(...)" A hash token followed by a list denotes a vector. For example #(1 2 a) is a three-element vector containing the numbers 1 and 2, and the symbol a. .SS Hashes .IP "#H((*) ( )*)" The notation #H followed by a nested list syntax denotes a hash table literal. The first item in the syntax is a list of keywords. These are the same keywords as are used when calling the function hash to construct a hash table. Allowed keywords are: :equal-based, :weak-keys, :weak-values. An empty list can be specified as nil or (), which defaults to a hash table basd on the eq function, with no weak semantics. .SS The .. notation In TXR Lisp, there is a special "dotdot" notation consiting of a pair of dots. This can be written between successive atoms or compound expressions, and is a shorthand for cons. That is to say, A .. B translates to (cons A B), and so for instance (a b .. (c d) e .. f . g) means (a (cons b (c d)) (cons e f) . g). This is a syntactic sugar useful in certain situations in which a cons is used to represent a pair of numbers or other objects. For instance, if L is a list, then [L 1 .. 3] computes a sublist of L consisting of elements 1 through 2 (counting from zero). .TP Restrictions: The notation must be enclosed in a list. For instance a..b is not an expression, but (a..b) is. This is important if Lisp data is being parsed from a string or stream using the read function. If the data "a..b" is parsed, the symbol "a" will be extracted, leaving "..a", which, if parsed, produces a syntax error since it consists of a "dotdot" token followed by a symbol, which is not valid syntax, akin to something like ")a" or ".a". The notation cannot occur in the dot position; that is, the syntax (a . b .. c) is invalid. The dotdot operator can only be used between the non-dot-position elements of a list. .SS The DWIM Brackets TXR Lisp has a square bracket notation. The syntax [...] is a shorthand way of writing (dwim ...). The [] syntax is useful for situations where the expressive style of a Lisp-1 dialect is useful. For instance if foo is a variable which holds a function object, then [foo 3] can be used to call it, instead of (call foo 3). If foo is a vector, then [foo 3] retrieves the fourth element, like (vecref foo 3). Indexing over lists, strings and hash tables is possible, and the notation is assignable. Furthermore, any arguments enclosed in [] which are symbols are treated according to a modified namespace lookup rule. More details are given in the documentation for the dwim operator. .SS Compound Forms In TXR Lisp, there are two types of compound forms: the Lisp-2 style compound forms, denoted by ordinary lists that are expressed with parentheses. There are Lisp-1 style compound forms denoted by the DWIM Brackets, discussed in the previous section. The first position of an ordinary Lisp-2 style compound form, is expected to have a function or operator name. Then arguments follow. Finally, there may also be an expression in the dotted position, if the form is a function call. If the form is a function call then the arguments are evaluated. If any of the arguments are symbols, they are treated according to Lisp-2 namespacing rules. Additionally, if there is an expression in the dotted position, it is also evaluated. It should evaluate to a sequence: a list, vector or string. The elements of the sequence generate additional arguments for the function call. Note, however, that a compound form cannot be used in the dot position, for obvious reasons, namely that (a b c . (foo z)) does not mean that there is a compound form in the dot position, but a different spelling for (a b c foo z), where foo behaves as a variable. The DWIM brackets are similar, except that the first position is an arbitrary expression which is evaluated according to the same rules as the remaining positions. The first expression must evaluate to a function, or else to some other object for which the DWIM syntax is defined, such as a vector, string, list or hash. Operators are not supported. The dotted syntax for application of additional arguments from a list or vector is supported in the DWIM brackets just like in the parentheses. .TP Examples: ;; a contains 3 ;; b contains 4 ;; c contains #(5 6 7) ;; s contains "xyz" (foo a b . c) ;; calls (foo 3 4 5 6 7) (foo a) ;; calls (foo 3) (foo . s) ;; calls (foo #\ex #\ey #\ez) [foo a b . c] ;; calls (foo 3 4 5 6 7) [c 1] ;; indexes into vector #(5 6 7) to yield 6 .TP Dialect Note: In some other Lisp dialects, the improper list syntax is not supported; a function called apply (or similar) must be used for application even if the expression which gives the trailing arguments is a symbol. Moreover, applying sequences other than lists is not supported. .SS Regular Expressions In TXR Lisp, the / character can occur in symbol names, and the / token is a symbol. Therefore the /regex/ syntax is absent, replaced with the #/regex/ syntax. .SS Generalization of List Accessors car and cdr In ancient Lisp in the 1960's, it was not possible to apply the operations car and cdr to the nil symbol (empty list), because it is not a cons cell. In the InterLisp dialect, this restriction was lifted: these operations were extended to accept nil (and return nil). The convention was adopted in other Lisp dialects such as MacLisp and eventually in Common Lisp. Thus there exists an object which is not a cons, yet which takes car and cdr. In TXR Lisp, this relaxation is extended further. For the sake of convenience, the operations car and cdr, are extended to work with strings and vectors: (cdr "") -> nil (car "") -> nil (car "abc") -> #\ea (cdr "abc") -> "bc" (cdr #(1 2 3)) -> #(2 3) (car #(1 2 3)) -> 1 The ldiff function is also extended in a special way. When the right parameter is a string or vector, then it uses the equal equality test rather than eq for detecting the tail of the list. (ldiff "abcd" "cd") -> (#\ea #\eb) The ldiff operation starts with "abcd" and repeatedly applies cdr to produce "bcd" and "cd", until the suffix is equal to the second argument: (equal "cd" "cd") yields true. Operations based on car, cdr and ldiff, such as keep-if and remq extend to strings and vectors. Derived list processing operations such as remq or mapcar obey the following rule: the returned object follows the type of the leftmost input list object. For instance, if one or more sequences are processed by mapcar, and the leftmost one is a character string, the function is expected to return characters, which are converted to a character string. However, in the event that the objects produced cannot be assembled into that type of sequence, a list is returned instead. For example [mapcar list "ab" "12"] returns ((#\ea #\eb) (#\e1 #\e2)), because a string cannot hold lists of characters. However [mappend list "ab" "12"] returns "a1b2". The lazy versions of these functions such as mapcar* do not have this behavior; they produce lazy lists. .SS Callable Objects In TXR Lisp, sequences (strings, vectors and lists) and hashes can be used as functions everywhere, not just with the DWIM brackets. Sequences work as one or two-argument functions. With a single argument, an element is selected by position and returned. With two arguments, a range is extracted and returned. Hashes also work as one or two argument functions, corresponding to the arguments of the gethash function. Moreover, when a sequence is used as a function of one argument, and the argument is a cons cell rather than an integer, then the call becomes a two-argument call in which the car and cdr of the cell are passed as separate arguments. This allows for syntax like (call "abc" 0..1). .TP Example 1: (mapcar "abc" '(2 0 1)) -> (#\ec #\ea #\eb) Here, mapcar treats the string "abc" as a function of one argument (since there is one list argument). This function maps the indices 0, 1 and 2 to the corresponding characters of string "abc". Through this function, the list of integer indices (2 0 1) is taken to the list of characters (#\ec #\ea #\eb). .TP Example 2: (call '(1 2 3 4) 1..3) -> (2 3) Here, the shorthand 1 .. 3 denotes (cons 1 3). A cons cell as an argument to a sequence performs range extraction: taking a slice starting at index 1, up to and not including index 3, as if by the call (sub '(1 2 3 4) 1 3). .TP Example 3: (call '(1 2 3 4) '(0 2)) -> (1 2) A list of indices applied to a sequence is equivalent to using the select function, as if (select '(1 2 3 4) '(0 2)) were called. .SS Special Variables Similarly to Common Lisp, TXR Lisp is lexically scoped by default, but also has dynamically scoped (a.k.a "special") variables. When a variable is defined with defvar, it is introduced as a global (global) binding, regardless of where in the scope the defvar form occurs. Furthermore, at the time the defvar form is evaluated, the symbol which names the variable is tagged as special. When a symbol is tagged as special, it behaves differently when it is used in a lexical binding construct like let, and all other such constructs such as function parameter lists. Such a binding is not the usual lexical binding, but a "rebinding" of the global variable. Over the dynamic scope of the form, the global variable takes on the value given to it by the rebinding. When the form terminates, the prior value of the variable is restored. (This is true no matter how the form terminates; even if by an exception.) Because of this "pervasive special" behavior of a symbol that has been used as the name of a global variable, a good practice is to make global variables have visually distinct names via the "earmuffs" convention: beginning and ending the name with an asterisk. Certain variables in TXR's library break this convention; however, they at least have distinct prefixes, examples being example s-ifmt, log-emerg and sig-hup. .TP Example: (defvar *x* 42) ;; *x* has a value of 42 (defun print-x () (format t "~a\en" *x*)) (let ((*x* "abc")) ;; this overrides *x* (print-x)) ;; *x* is now "abc" and so that is printed (print-x) ;; *x* is 42 again and so "42" is printed .SH OPERATOR AND FUNCTION LIBRARY A compound expression with a symbol as its first element, if intended to be evaluated, denotes either an operator invocation or a function call. This depends on whether the symbol names an operator or a function. When the form is an operator invocation, the interpretation of the meaning of that form is under the complete control of that operator. Functions calls do not control the evaluation of the function call forms which invoke them; function call forms are evaluated according to very regular rules that apply to all function calls. Some operators are macros. There exist predefined macros in the library, and macro operators can also be user-defined using the macro-defining operator defmacro. Operators that are not macros are called special operators. Macro operators work as functions which are given the source code of the form. They analyze the form, and translate it to another form which is substituted in their place. This happens during a code walking phase called the expansion phase, which is applied to Lisp code prior to evaluation. All macros are expanded in the expansion phase, resulting in code which contains only function calls and the executable forms of the operators. (Special operators can also perform code transformations during the expansion phase, but that is not considered macroexpansion, but rather an adjustment of the representation of the operator into an required executable form.) The following sections list all of the special operators, macros and functions in TXR Lisp. In these sections Syntax is indicated using these conventions: .TP A symbol in angle brackets denotes some syntactic unit: it may be a symbol or compound form. The syntactic unit is explained in the Description section. .TP {syntax}* * This indicates a repetition of zero or more of the given syntax enclosed in the braces or syntactic unit. .TP {syntax}+ + This indicates a repetition of one or more of the given syntax enclosed in the braces or syntactic unit. .TP [syntax] [] Square brackets indicate optional syntax. .TP alternative1 | alternative2 | ... | alternativeN Multiple syntactic variations allowed in one place are indicated as bar-separated items. .SH CONTROL FLOW AND SEQUENCING .SS Operators progn and prog1 .TP Syntax: (progn
*) (prog1 *) .TP Description The progn operator evaluates forms in order, and returns the value of the last form. The return value of (progn) is nil. The prog1 operator evaluates forms in order, and returns the value of the first form. The return value of (prog1) is nil. Various other operators such as let also arrange for the evaluation of a body of forms, the value of the last of which is returned. These operators are said to feature an "implicit progn". .SS Operator cond .TP Syntax: (cond {( {form}*)}*) .TP Description: The cond operator provides a multi-branching conditional evaluation of forms. Enclosed in the cond form are groups of forms expressed as lists. Each group must be a list of at least one form. The forms are processed from left to right as follows: the first form, , in each group is evaluated. If it evaluates true, then the remaining forms in that group, if any, are also evaluated. Processing then terminates and the result of the last form in the group is taken as the result of cond. If is the only form in the group, then result of is taken as the result of cond. If the first form of a group yields nil, then processing continues with the next group, if any. If all form groups yield nil, then the cond form yields nil. This holds in the case that the syntax is empty: (cond) yields nil. .SS Macros when and unless .TP Syntax: (when *) (unless *) .TP Description: The when macro operator evaluates . If yields true, and there are additional forms, then each is evaluated. The value of the last form is becomes the result value of the when form. If there are no forms, then the result is nil. The unless operator is similar to when, except that it reverses the logic of the test. The forms, if any, are evaluated if, and only if is false. .SS Macros while and until .TP Syntax: (while *) (until *) .TP Description: The while macro operator provides a looping construct. It evaluates . If yields nil, then the evaluation of the while form terminates, producing the value nil. Otherwise, if there are additional forms, then each is evaluated. Next, evaluation returns to , repeating all of the previous steps. The until macro operator is similar to while, except that the until form terminates when evaluates true, rather than false. These operators arrange for the evaluation of all their enclosed forms in an anonymous block. Any of the -s, or , may use the return operator to terminate the loop, and optionally to specify a result value for the form. The only way these forms can yield a value other than nil is if the return operator is used to terminate the implicit anonymous block, and is given an argument, which becomes the result value. .SS Operator/function if .TP Syntax: (if []) [if []] .TP Description: There exist both an if operator and an if function. A list form with the symbol if in the fist position is interpreted as an invocation of the if operator. The function can be accessed using the DWIM bracket notation and in other ways. The if operator provides a simple two-way-selective evaluation control. The form is evaluated. If it yields true then is evaluated, and that form's return value becomes the return value of the if. If yields false, then is evaluated and its return value is taken to be that of if. If is omitted, then the behavior is as if were specified as nil. The if function provides no evaluation control. All of arguments are evaluated from left to right. If the argument is true, then it returns the argument, otherwise it returns the value of the argument, if present, otherwise it returns nil. .SS Operator/function and .TP Syntax: (and {}*) [and {]*) .TP Description: There exist both an and operator and an and function. A list form with the symbol and in the fist position is interpreted as an invocation of the operator. The function can be accessed using the DWIM bracket notation and in other ways. The and operator provides three functionalities in one. It computes the logical "and" function over several forms. It controls evaluation (a.k.a. "short-circuiting"). It also allows the convenient substitution of an arbitrary true value in the true case. The and operator evaluates as follows. First, a return value is established and initialized to the value t. The forms, if any, are evaluated from left to right. The return value is overwritten with the result of each form. Evaluation stops when all forms are exhausted, or when any of them yields nil. When evaluation stops, the operator yields the return value. The and function provides no evaluation control; it receives all of its arguments fully evaluated. If it is given no arguments, it returns t. If it is given one or more arguments, and any of them are nil, it returns nil. Otherwise it returns the value of the last argument. .TP Examples: (and) -> t (and (> 10 5) (stringp "foo")) -> t (and 1 2 3) -> 3 .SS Operator/function or .TP Syntax: (or {}*) [or {}*] .TP Description: There exist both an or operator and an or function. A list form with the symbol or in the fist position is interpreted as an invocation of the operator. The function can be accessed using the DWIM bracket notation and in other ways. The or operator provides three functionalities in one. It computes the logical "or" function over several forms. It controls evaluation (a.k.a. "short-circuiting"). The behavior of or also provides for a simplified selection of the first non-nil value from a sequence of forms. The or operator evaluates as follows. First, a return value is established and initialized to the value nil. The forms, if any, are evaluated from left to right. The return value is overwritten with the result of each form. Evaluation stops when all forms are exhausted, or when a form yields a true value. When evaluation stops, the operator yields the return value. The and function provides no evaluation control; it receives all of its arguments fully evaluated. If it is given no arguments, it returns nil. If all of its arguments are nil, it also returns nil. Otherwise, it returns the value of the first non-nil argument. .TP Examples: (or) -> nil (or 1 2) -> 1 (or nil 2) -> 2 (or (> 10 20) (stringp "foo")) -> t .SS Operator unwind-protect .TP Syntax: (unwind-protect *) .TP Description: The unwind-protect operator evaluates in such a way that no matter how the execution of terminates, the -s will be executed. The cleanup forms, however, are not protected. If a cleanup form terminates via some non-local jump, the subsequent cleanup forms are not evaluated. Cleanup forms themselves can "hijack" a non-local control transfer such as an exception. If a cleanup form is evaluated during the processing of a dynamic control transfer such as an exception, and that cleanup form initiats its own dynamic control transfer, the original control transfer is aborted and replaced with the new one. .TP Example: (block foo (unwind-protect (progn (return-from foo 42) (format t "not reached!\en")) (format t "cleanup!\en"))) In this example, the protected progn form terminates by returning from block foo. Therefore the form does not complete and so the output "not reached!" is not produced. However, the cleanup form excecutes, producing the output "cleanup!". .SS Operator block .TP Syntax: (block *) .TP Description: The block operator introduces a named block around the execution of some forms. The argument must be a symbol. Since a block name is not a variable binding, keyword symbols are permitted, and so are the symbols t and nil. A block named by the symbol nil is slighlty special: it is understood to be an anonymous block. Blocks in TXR Lisp have dynamic scope. This means that the following situation is allowed: (defun func () (return-from foo 42)) (block foo (func)) The function can return from the foo block even though the foo block does not lexically surround foo. Thus blocks in TXR Lisp provide dynamic non-local returns, as well as returns out of lexical nesting. .TP Dialect Note: In Common Lisp, blocks are lexical. A separate mechanism consisting of catch and throw operators performs non-local transfer based on symbols. The TXR Lisp example: (defun func () (return-from foo 42)) (block foo (func)) is not allowed in Common Lisp, but can be transliterated to: (defun func () (throw 'foo 42)) (catch 'foo (func)) Note that foo is quoted in CL. This underscores the dynamic nature of the construct. THROW itself is a function and not an operator. .SS Operators return, return-from .TP Syntax: (return []) (return-from []) .TP Description: The return operator must be dynamically enclosed within an anonymous block (a block named by the symbol nil). It immediately terminates the evaluation of the innermost anonyous block which encloses it, causing it to return the specified value. If the value is omitted, the anonymous block returns nil. The return-from operator must be dynamically enclosed within a named block whose name matches the argument. It immediately terminates the evaluation of the innermost such block, causing it to return the specified value. If the value is omitted, that block returns nil. .TP Example: (block foo (let ((a "abc\en") (b "def\en")) (pprint a *stdout*) (return-from foo 42) (pprint b *stdout*))) Here, the output produced is "abc". The value of b is not printed because the return-from terminates block foo, and so the second pprint form is not evaluated. .SH EVALUATION .SS Operator dwim .TP Syntax: (dwim *) [*] .TP Description: The dwim operator's name is an acronym: DWIM may be taken to mean "Do What I Mean", or alternatively, "Dispatch, in a Way that is Intelligent and Meaningful". The notation [...] is a shorthand equivalent to (dwim ...) and is the preferred way for writing dwim expressions. The dwim operator takes a variable number of arguments, which are all evaluated in the same way: the first argument is not evaluated differently from the remaining arguments. This means that the first argument isn't a function name, but an ordinary expression which can simply compute a function object (or, more generally, a callable object). Furthermore, for those arguments of dwim which are symbols (after all macro-expansion is performed on the arguments), the evaluation rules are altered. For the purposes of resolving symbols to values, the function and variable binding namespaces are considered to be merged into a single space, creating a situation that is very similar to a Lisp-1 style dialect. This special Lisp-1 evaluation is not recursively applied. All arguments of dwim which, after macro expansion, are not symbols are evaluated using the normal Lisp-2 evaluation rules. Thus, the DWIM operator must be used in every expression where the Lisp-1 rules for reducing symbols to values are desired. After macro expansion, the first argument of dwim may not be an operator such as let, or the name of a macro. Prior to macroexpansion, any argument of dwim may be a symbol macro. If a symbol has bindings both in the variable and function namespace in scope, and is referenced by a dwim argument, this constitues a conflict which is resolved according to two rules. When nested scopes are concerned, then an inner binding shadows an outer binding, regardless of their kind. An inner variable binding for a symbol X shadows an outer or global function binding, and vice versa. If a symbol is bound to both a function and variable in the global namespace, then the variable binding is favored. Macros do not participate in the special scope conflation. This is apparent in two ways. Firstly, the space of symbol macros is not folded together with the space of regular macros. An argument of dwim that is a symbol might be symbol macro, variable or function, but it cannot be interpreted as the name of a regular macro. Secondly, function bindings cannot shadow symbol macros. If a function is defined in an inner scope relative to a symbol macro for the same symbol, that symbol macro still hides the function when it appears in a dwim form. Symbol macros are expanded without any consideration for the dwim operator, which is not treated specially by the expansion phase. (Interaction between dwim and macros may become more sophisticated in the future.) How many arguments are required by the dwim operator depends on the type of object to which the first argument expression evaluates. The possibilities are: .IP "[ *]" Call the given the function object to the given arguments. .IP "[ *]" If the first expression evaluates to a symbol, that symbol is resolved in the function namespace, and then the resulting function, if found, is called with the given arguments. .IP "[ ]" Retrieve the specified element from the specified list. Index zero refers to the first element. Indexed list access does not throw exceptions. Negative indices yield nil, and indices beyond the end of a list yield nil. (However assignment to a nonexistent list element throws.) .IP "[ ..]" Retrieve the specified range of elements, exactly as if using (sub-list ). The range of elements is specified in the car and cdr fields of a cons cell, for which the .. (dotdot) syntactic sugar is useful. See the section on Indexing below. .IP "[ ]" Retrieve the specified element of a vector. This is equivalent to (vecref ). .IP "[ ..]" Retrieve the specified range of elements, exactly as if using (sub-vec ). The range of elements is specified in the car and cdr fields of a cons cell, for which the .. (dotdot) syntactic sugar is useful. See the section on Range Indexing below. .IP "[ ]" Retrieve the specified element of a string. This is equivalent to (chr-str ). .IP "[ ..]" Retrieve the specified range of characters from the string, exactly as if using (sub-str ). The range of elements is specified in the car and cdr fields of a cons cell, for which the .. (dotdot) syntactic sugar is useful. See the section on Indexing below. .IP [ ] This is equivalent to (select ). Elements specified by are extracted from and returned as a sequence of the same kind as . See the description of the sequence function for the exact semantics. .IP "[ ]" Retrieve a value from the hash table corresponding to , or if there is no such entry. The first argument may not be an operator such as let, only a function. The places denoted by the dwim operator can be assigned. There are some restrictions. List, string and vector ranges can only be replaced using the set operator. The other operators like push do not apply. Characters in a string can only be assigned with set or incremented with inc and dec. The source of a range assignment can be a string, vector or list, regardless of whether the target is a string, vector or list. If the target is a string, the replacement sequence must be a string, or a list or vector of characters. .TP Range Indexing Vector and list range indexing is based from zero. The first element element zero. Furthermore, the value -1 refers to the last element of the vector or list, and -2 to the second last and so forth. So the range 1 .. -2 means "everything except for the first element and the last two". The symbol t represents the position one past the end of the vector, string or list, so 0 .. t denotes the entire list or vector, and the range t .. t represents the empty range just beyond the last element. It is possible to assign to t .. t. For instance: (defvar list '(1 2 3)) (set [list t .. t] '(4)) ;; list is now (1 2 3 4) Either end of the range can also be specified as nil. If the start is specified as nil, it means zero. If the end is specified as nil, it means one element past the end. Thus nil .. nil spans all of the elements. The value zero has a "floating" behavior when used as the end of a range. If the start of the range is a negative value, and the end of the range is zero, the zero is interpreted as being the position past the end of the sequence, rather than the first element. For instance the range -1..0 means the same thing as -1..t or -1..nil. Zero at the start of a range always means the first element, so that 0..-1 refers to all the elements except for the last one. .TP Notes: The dwim operator allows for a Lisp-1 flavor of programming in TXR Lisp, which is normally Lisp-2, with some useful extensions. A Lisp-1 dialect is one in which an expression like (a b) treats both a and b as expressions with the same evaluation rules. The symbols a and b are looked up in a variable namespace. A function call occurs if the value of variable a is a function object. Thus in a Lisp-1, named functions do not exist as such: they are just variable bindings. In a Lisp-1 (car 1 2) means that there is a variable called car, which holds a function. In a Lisp-2 (car 1 2) means that there is a function called car, and so (car car car) is possible, because there can be also a variable called car. The Lisp-1 design has certain disadvantages, which are avoided in TXR Lisp by confining the Lisp-1 expressivity inside the [...] notation, in which operators are not allowed. When round parentheses are used, the normal Lisp-2 rules apply. A "best of both worlds" situation is achieved. The square brackets are just as convenient as parentheses and at the same time visually distinct, making it clear that different rules apply. The Lisp-1 is useful for functional programming, because it eliminates occurences of the call and fun operators. For instance: ;; regular notation (call foo (fun second) '((1 a) (2 b))) ;; [] notation [foo second '((1 a) (2 b))] Lisp-1 dialects can also provide useful extensions by giving a meaning to objects other than functions in the first position of a form, and the dwim/[...] syntax does exactly this. However, unlike Lisp-1 dialects, the [] syntax does not allow operators. It .B is an operator: (dwim ...). .SS Function identity .TP Syntax: (identity ) .TP Description: The identity function returns its argument. .TP Notes: The identify function is useful as a functional argument, when a transformation function is required, but no transformation is actually desired. .SS Function eval .TP Syntax: (eval ) .TP Description: The eval function treats the object as a Lisp expression, which is evaluated. The side effects implied by the form are performed, and the value which it produces is returned. The object specifies an environment for resolving the function and variable references encountered in the expression. The object nil can be specified as an environment, in which case the evaluation takes place in the global environment. See also: the make-env function. .SS Function make-env .TP Syntax: (make-env [ [ []]]) .TP Description: The make-env function creates an environment object suitable as the parameter. The and parameters, if specified, should be association lists, mapping symbols to objects. The objects in should be functions, or objects callable as functions. The argument, if specified, should be an environment. Note: bindings can also be added to an environment usign the env-vbind and env-fbind functions. .SS Functions env-vbind and env-fbind .TP Syntax: (env-vbind ) (env-fbind ) .TP Description: These functions bind a symbol to a value in either the function or variable space of environment . Values established in the function space should be functions or objects that can be used as functions such as lists, strings, arrays or hashes. If already exists in the environment, in the given space, then its value is updated with . .SH MUTATION .SS Operators inc, dec, set, push, pop, flip and del .TP Syntax: (inc []) (dec []) (set ) (push ) (pop ) (flip ) (del ) .TP Description: These destructive operators update the value of a place. A place is a storage location which is denoted by a form. Place forms are identical to value accessing forms. That is to say, any form recognized as a place by these operators can be evaluated by itself to retrieve the value of the storage location. However, the converse is false: not all forms which access storage location are recognized as places. With are exceptions noted below, it is an error if a place does not exist. For instance, a variable being assigned must exist. Literal objects which are directly specified in the source code are considered part of the program body. Modifying parts of these objects therefore gives rise to self-modifying code. The behavior of self-modifying code is not specified. The inc and dec update the place by adding or subtracting, respectively, a displacement to or from that number. If the expression is specified, then it is evaluated and its value is used as the increment. Otherwise, a default increment of 1 is used. The prior value of the place and the delta must be suitable operands for the + and - functions. (inc x) is equivalent to (set x (+ 1 x)), except that expression x is evaluated only once to determine the storage location. The inc and dec operators return the new value that was stored. The set operator overwrites the previous value of a place with a new value, and also returns that value. The push and pop operators operate on a place which holds a list. The push operator updates the list by replacing it with a new list which has a new item at the front, followed by the previous list. The item is returned. The pop operator performs the reverse operation: it removes the first item from the list and returns it. (push y x) is similar to (let ((temp y)) (set x (cons temp x)) temp) except that x is evaluated only once to determine the storage place, and no such temporary variable is visible to the program. Similarly, (pop x) is much like (let ((temp (car x))) (set x (cdr x)) temp) except that x is evaluated only once, and no such temporary variable is visible to the program. The flip operator toggles a place between true and false. If the place contains a value other than nil, then its value is replaced with nil. If it contains nil, it is replaced with t. The del operator does not modify the value of a place, but rather deletes the place itself. Index values and ranges of lists denoted using the dwim operator indexing notation can be subject to a deletion, as can hash table entries denoted using dwim or gethash. It is an error to try to delete other kinds of places such as simple variables. The del operator returns the value of the place that was deleted. Deleting from a sequence means removing the element or elements. Deleting a hash place means removing the corresponding entry from the hash table. Currently, these forms are recognized as places: (car ) (cdr ) (gethash ) (vecref ) (dwim ...) [ ...] ;; equivalent to (dwim ...) A place denotes a variable. If the variable does not exist, it is an error. The (car ) and (cdr ) places denote the corresponding slots of a cons cell. The form must be an expression which evaluates to a cons. The gethash place denotes a value stored in a hash table. The form must evaluate to a hash table. If the place does not exist in the hash table under the given key, then the destructive operation will create it. In that case, the form is evaluated to determine the initial value of the place. Otherwise it is ignored. The vecref place denotes a vector element, allowing vector elements to be treated as assignment places. The dwim/[] place denotes a vector element, list element, string, or hash table, depending on the type of obj. .SH BINDING AND ITERATION .SS Operator defvar .TP Syntax: (defvar ) .TP Description: The defvar operator binds a variable in the global environment. If the variable named already exists in the global environment, the form has no effect; the form is not evaluated, and the value of the variable is unchanged. If the variable does not exist, then it is introduced, with a value given by evaluating the form. The form is evaluated in the environment in which the defvar form occurs, not necessarily in the global environment. The symbols t and nil may not be used as variables, and neither can be keyword symbols: symbols denoted by a leading colon. In addition to creating a binding, the defvar operator also marks as the name of a special variable. This changes what it means to bind that symbol in a lexical binding construct such as the let operator, or a function parameter list. See the section "Special Variables" far above. .SS Operators let and let* .TP Syntax: (let ({ | ( )}*) *) (let* ({ | ( )}*) *) .TP Description: The let and let* operators introduce a new scope with variables and evaluate forms in that scope. The operator symbol, either let or let*, is followed by a list which can contain any mixture of variable name symbols, or ( ) pairs. A symbol denotes the name of variable to be instantiated and initialized to the value nil. A symbol specified with an init-form denotes a variable which is intialized from the value of the init-form. The symbols t and nil may not be used as variables, and neither can be keyword symbols: symbols denoted by a leading colon. The difference between let and let* is that in let*, later init-forms have visibility over the variables established by earlier variables in the same let* construct. In plain let, the variables are not visible to any of the init-forms. When the variables are established, then the body forms are evaluated in order. The value of the last form becomes the return value of the let. If the forms are omitted, then the return value nil is produced. The variable list may be empty. .TP Examples: (let ((a 1) (b 2)) (list a b)) -> (1 2) (let* ((a 1) (b (+ a 1))) (list a b (+ a b))) -> (1 2 3) (let ()) -> nil (let (:a nil)) -> error, :a and nil can't be used as variables .SS Operators for and for* .TP Syntax: ({for | for*} ({ | ( )}*) ([ *]) (*) *) .TP Description: The for and for* operators combine variable binding with loop iteration. The first argument is a list of variables with optional initializers, exactly the same as in the let and let* operators. Furthermore, the difference between for and for* is like that between let and let* with regard to this list of variables. The for operators execute these steps: 1. Establish bindings for the specified variables similarly to let and let*. The variable bindings are visible over the , each , each and each . 2. Establish an anonymous block over the remaining forms, allowing the return operator to be used to terminate the loop. 3. Evaluate . If yields nil, then the loop terminates. Each is evaluated, and the value of the last of these forms is is the result value of the for loop. If there are no -s then the result value is nil. If the is omitted, then the the test is taken to be true, and the loop does not terminate. 4. Otherwise, if yields non-nil, then each is evaluated in turn. Then, each is evaluated in turn and processing resumes at step 2. Furthermore, the for operators establish an anonymous block, allowing the return operator to be used to terminate at any point. .SS Operators each, each*, collect-each, collect-each*, append-each and append-each* .TP Syntax: (each ({( )}*) *) (each* ({( )}*) *) (collect-each ({( )}*) *) (collect-each* ({( )}*) *) (append-each ({( )}*) *) (append-each* ({( )}*) *) .TP Description: These operator establish a loop for iterating over the elements of one or more lists. Each must evaluate to a list. The lists are then iterated in parallel over repeated evaluations of the -s, which each variable being assigned to successive elements of its list. The shortest list determines the number of iterations, so if any of the -s evaluate to an empty list, the body is not executed. The body forms are enclosed in an anonymous block, allowing the return operator to terminate the looop prematurely and optionally specify the return value. The collect-each and collect-each* variants are like each and each*, except that for each iteration, the resulting value of the body is collected into a list. When the iteration terminates, the return value of the collect-each or collect-each* operator is this collection. The append-each and append-each* variants are like each and each*, except that for each iteration other than the last, the resulting value of the body must be a list. The last iteration may produce either an atom or a list. The objects produced by the iterations are combined together as if they were arguments to the append function, and the resulting value is the value of the append-each or append-each* operator. The alternate forms denoted by the adorned symbols each*, collect-each* and append-each*, variants differ from each, collect-each and append-each* in the following way. The plain forms evaluate the -s in an environment in which none of the variables are yet visible. By contrast, the alternate forms evaluate each in an environment in which bindings for the previous variables are visible. In this phase of evaluation, variables are list-valued: one by one they are each bound to the list object emanating from their corresponding . Just before the first loop iteration, however, the variables are assigned the first item from each of their lists. .TP Examples: ;; print numbers from 1 to 10 and whether they are even or odd (each* ((n (range 1 10)) (even (collect-each ((n m)) (evenp m)))) ;; n is a list here (format t "~s is ~s\en" n (if even "even" "odd"))) ;; n is an item here Output: 1 is odd 2 is even 3 is odd 4 is even 5 is odd 6 is even 7 is odd 8 is even 9 is odd 10 is even .SH FUNCTION OBJECTS AND NAMED FUNCTIONS .SS Operator defun .TP Syntax: (defun (* [: *] [. ]) *) Description: The defun operator introduces a new function in the global function namespace. The function is similar to a lambda, and has the same parameter syntax and semantics as the lambda operator. Unlike in lambda, the -s of a defun are surrounded by a block. The name of this block is the same as the name of the function, making it possible to terminate the function and return a value using (return-from ). For more information, see the definition of the block operator. A function may call itself by name, allowing for recursion. .SS Operator lambda .TP Syntax: (lambda (* [: *] [. ]) {}*) (lambda {}*) .TP Description: The lambda operator produces a value which is a function. Like in most other Lisps, functions are objects in TXR Lisp. They can be passed to functions as arguments, returned from functions, aggregated into lists, stored in variables, et cetera. The first argument of lambda is the list of parameters for the function. It may be empty, and it may also be an improper list (dot notation) where the terminating atom is a symbol other than nil. It can also be a single symbol. The second and subsequent arguments are the forms making up the function body. The body may be empty. When a function is called, the parameters are instantiated as variables that are visible to the body forms. The variables are initialized from the values of the argument expressions appearing in the function call. The dotted notation can be used to write a function that accepts a variable number of arguments. To write a function that accepts variable arguments only, with no required arguments, use a single symbol. The keyword symbol : (colon) can appear in the parameter list. This is the symbol in the keyword package whose name is the empty string. This symbol is treated specially: it serves as a separator between required parameters and optional parameters. Furthermore, the : symbol has a role to play in function calls: it can be specified as an argument value to an optional parameter by which the caller indicates that the optional argument is not being specified. It will be processed exactly that way. An optional parameter can also be written in the form ( []). In this situation, if the call does not specify a value for the parameter (or specifies a value as the keyword : (colon)) then the parameter takes on the value of the expression . If is specified, then will be introduced as an additional binding with a boolean value which indicates whether or not the optional parameter had been specified by the caller. The initializer expressions are evaluated an environment in which all of the previous parameters are visible, in addition to the surrounding environment of the lambda. For instance: (let ((default 0)) (lambda (str : (end (length str)) (counter default)) (list str end counter))) In this lambda, the initializing expression for the optional parameter end is (length str), and the str variable it refers to is the previous argument. The initializer for the optional variable counter is the expression default, and it refers to the binding established by the surrounding let. This reference is captured as part of the lambda's lexical closure. .TP Examples: Counting function. This function, which takes no arguments, captures the variable "counter". Whenever this object is called, it increments the counter by 1 and returns the incremented value. (let ((counter 0)) (lambda () (inc counter))) Function that takes two or more arguments. The third and subsequent arguments are aggregated into a list passed as the single parameter z: (lambda (x y . z) (list 'my-arguments-are x y z)) Variadic funcion: (lambda args (list 'my-list-of-arguments args)) Optional arguments: [(lambda (x : y) (list x y)) 1] -> (1 nil) [(lambda (x : y) (list x y)) 1 2] -> (1 2) .SS Function call .TP Syntax: (call *) .TP Description: The call function invokes , passing it the given arguments, if any. .TP Examples: Apply arguments 1 2 to a lambda which adds them to produce 3: (call (lambda (a b) (+ a b)) 1 2) -> 3 Useless use of call on a named function; equivalent to (list 1 2): (call (fun list) 1 2) -> (1 2) .SS Operator fun .TP Syntax: (fun ) .TP Description: The fun operator retrieves the function object corresponding to a named function in the current lexical environment. The is a symbol denoting a named function: a built in function, or one defined by defun. Note: the fun operator does not see macro bindings. It is possible to retrieve a global macro expander using symbol-function. .TP Dialect Note: A lambda expression is not a function name in TXR Lisp. The syntax (fun (lambda ...)) is invalid. .SS Functions symbol-function and symbol-value .TP Syntax: (symbol-function ) (symbol-value ) .TP Description: The symbol-function retrieves the value of the global function binding of the given symbol if it has one: that is, the function object tied to the symbol. If the symbol has no global function binding, then the value of the global macro binding is returned. If that doesn't exist, then the value of a global special operator binding is returned, and if that doesn't exist, then nil is returned. The symbol-value retrives the value of a global variable, if it exists, otherwise nil. Note: a function binding is a function, but a macro or special operator binding binding isn't. The value of a macro binding is a list of the following form: (# *) The value of a special operator binding is a "C pointer" object, whose printed representation looks like: # These details may change in future version of TXR. .TP Dialect note: Forms which call symbol-function or symbol-value are currently not an assignable place. Only the defun operator defines functions, and the set operator modifies variables. .SS Functions boundp and fboundp .TP Syntax: (boundp ) (fboundp ) .TP Description: boundp returns t if the symbol has a variable binding in the global environment, otherwise nil. Foundp returns t if the symbol has a function or macro binding in the global environment, or if it is an operator, otherwise nil. .SS Function func-get-form .TP Syntax: (func-get-form ) .TP Description: The func-get-form function retrieves a source code form of , which must be an interpreted function. The source code form has the syntax ( {}*). .SS Function func-get-env .TP Syntax: (func-get-env ) .TP Description: The func-get-env function retrieves the environment object associated with function . The environment object holds the captured bindings of a lexical closure. .SS Function functionp .TP Syntax: (functionp ) .TP Description: The functionp function returns t if is a function, otherwise it returns nil. .SS Function interp-fun-p .TP Syntax: (interp-fun-p ) .TP Description: The interp-fun-p function returns t if is an interpreted function, otherwise it returns nil. .SH OBJECT TYPE AND EQUIVALENCE .SS Function typeof .TP Syntax: (typeof ) .TP Description The typeof function returns a symbol representing the type of . .RS The core types are identified by the following symbols. .IP cons A cons cell. .IP str String. .IP lit A literal string embedded in the TXR executable image. .IP chr Character. .IP fixnum Fixnum integer. An integer that fits into the value word, not having to be heap allocated. .IP sym Symbol. .IP pkg Symbol package. .IP fun Function. .IP vec Vector. .IP lcons Lazy cons. .IP lstr Lazy string. .IP env Function/variable binding environment. .IP bignum A bignum integer: arbitrary precision integer that is heap-allocated. .PP There are additional kinds of objects, such as streams. .SS Functions null, not, false .TP Syntax: (null ) (not ) (false ) .TP Description: The null, not and false functions are synonyms. They tests whether is the object nil. They return t if this is the case, nil otherwise. .TP Examples: (null '()) -> t (null nil) -> t (null ()) -> t (false t) -> nil (if (null x) (format t "x is nil!")) (let ((list '(b c d))) (if (not (memq 'a list)) (format t "list ~s does not contain the symbol a\en"))) .SS Function true .TP Syntax: (true ) .TP Description: The true function is the complement of the null, not and false functions. It return t if the is any object other than nil. If is nil, it returns nil. Note: programs should avoid explicitly testing values with true. For instance (if x ...) should be favored over (if (true x) ...) .TP Example: ;; Compute indices where the list '(1 nil 2 nil 3) has true values: ;; "Where is (1 nil 2 nil 3) true?" [where '(1 nil 2 nil 3) true] -> (1 3) .SS Functions eq, eql and equal .TP Syntax: (eq ) (eql ) (equal ) .TP Description: The principal equality test functions eq, eql and equal test whether two objects are equivalent, using different criteria. They return t if the objects are equivalent, and nil otherwise. The eq function uses the strictest equivalence test, called implementation equality. The eq function returns t if, and only if, and are actually the same object. The eq test is is implemented by comparing the raw bit pattern of the value, whether or not it is an immediate value or a pointer to a heaped object, including its type tags. Consequently, two objects of different type are never equal, two character values are eq if they are the same character, and two fixnum integers are eq if they have the same value. All other objects kinds are actually represented as pointers, and are eq if they point to the same object in memory. So two bignum integers might not be eq even if they have the same numeric value, two lists might not be eq even if all their corresponding elements are eq, two strings might not be eq even if they hold identical text, etc. The eql function is slightly less strict than eq. The difference between eql and eq is that if and are bignums which have the same numeric value, eql returns t, even if they are different objects. For all other objects, eql behaves like eq. The equal function is less strict than eql. In general, it recurses into some kinds of aggregate objects to perform a structural equivalence. If and are eql then they are also equal. If the two objects are both cons cells, then they are equal if their "car" fields are equal and their "cdr" fields are equal. If two objects are vectors, they are equal if they have the same length, and their corresponding elements are equal. If two objects are strings, they are equal if they are textually identical. If two objects are functions, they are equal if they have equal environments, and if they have equal functions. Two compiled functions are the same if they are the same function. Two interpreted functions are equal if their list structure is equal. Two hashes are equal if they use the same equality (both are equal-based, or both are the default eql-based), if their user-data elements are equal, if their sets of keys are identical, and if the data items associated with corresponding keys from each respective hash are equal objects. For some aggregate objects, there is no special semantics. Two arguments which are symbols, packages, or streams are equal if and only if they are the same object. Certain object types have a custom equal function. .SH BASIC LIST LIBRARY When the first element of a compound form is a symbol denoting a function, the evaluation takes place as follows. The remaining forms, if any, denote the arguments to the function. They are evaluated in left to right order to produce the argument values, and passed to the function. An exception is thrown if there are not enough arguments, or too many. Programs can define named functions with the defun operator The following are Lisp functions and variables built-in to TXR. .SS Function cons .TP Syntax: (cons ) .TP Description: The cons function allocates, intializes and returns a single cons cell. A cons has two fields called "car" and "cdr", which are accessed by functions of the same name, or by the functions "first" and "rest", which are alternative spellings. Lists are made up of conses. A (proper) list is either the symbol nil denoting an empty list, or a cons cell which holds the first item of the list in its "car", and the list of the remaining items in "cdr". (cons 1 nil) allocates a one element list denoted (1). The "cdr" is nil, so there are no additional items. A cons cell with a "cdr" other than nil is printed with the dotted pair notation. For example (cons 1 2) yields (1 . 2). The notation (1 . nil) is valid as input into the machine, but is printed as (1). A list terminated by an atom other than nil is called an improper list, and the dot notation is extended to cover improper lists. For instance (1 2 . 3) is an improper list of two elements, terminated by 3, and can be constructed using (cons 1 (cons 2 3)). Another notation for this list is (1 . (2 . 3)) The list (1 2) is (1 . (2 . nil)). .SS Function atom .TP Syntax: (atom ) .TP Description: The atom function tests whether is an atom. It returns t if this is the case, nil otherwise. All values which are not cons cells are atoms. (atom x) is equivalent to (not (consp x)). .TP Examples: (atom 3) -> t (atom (cons 1 2)) -> nil (atom "abc") -> t (atom '(3)) -> nil .SS Function consp .TP Syntax: (consp ) .TP Description: The atom function tests whether is a cons. It returns t if this is the case, nil otherwise. (consp x) is equivalent to (not (atom x)). Non-empty lists test positive under consp because a list is represented as a reference to the first cons in a chain of one or more conses. .TP Examples: (consp 3) -> nil (consp (cons 1 2)) -> t (consp "abc") -> nil (consp '(3)) -> t .SS Functions car and first .TP Syntax: (car ) (first ) .TP Description: The functions car and first are synonyms. They retrieve the "car" field of a cons cell. (car (cons 1 2)) yields 1. For programming convenience, (car nil) is allowed, and returns nil, even though nil isn't a cons and doesn't have a "car" field. .SS Functions cdr and rest .TP Syntax: (cdr ) (rest ) .TP Description: The functions cdr and rest are synonyms. They retrieve the "cdr" field of a cons cell. (cdr (cons 1 2)) yields 2. For programming convenience, (cdr nil) is allowed, and returns nil, even though nil isn't a cons and doesn't have a "cdr" field. .TP Example: Walk every element of the list (1 2 3): (for ((i '(1 2 3))) (i) ((set i (cdr i))) (print (car i) *stdout*) (print #\enewline *stdout*)) The variable i marches over the cons cells which make up the "backbone" of the list. The elements are retrieved using the car function. Advancing to the next cell is achieved using (cdr i). If i is the last cell in a (proper) list, (cdr i) yields nil. The guard expression i fails and the loop terminates. .SS Functions rplaca and rplacd .TP Syntax: (rplaca ) (rplacd ) .TP Description: The rplaca and rplacd functions assign new values into the "car" and "cdr" fields of the cell . Note that (rplaca x y) is the same as the more generic (set (car x) y), and likewise (rplacd x y) can be written as (set (cdr x) y). It is an error if is not a cons or lazy cons. In particular, whereas (car nil) is correct, (rplaca nil ...) is erroneous. The rplaca and rplacd functions return . Note: TXR versions 89 and earlier, these functions returned the new value. The behavior was undocumented. .SS Functions second, third, fourth, fifth and sixth .TP Syntax: (first ) (second ) (third ) (fourth ) (fifth ) (sixth ) .TP Description: These functions access the elements of a proper list by position. If the list is shorter than implied, these functions return nil. .TP Examples: (third '(1 2)) -> nil (second '(1 2)) -> 2 (third '(1 2 . 3)) -> **error** .SS Functions append and append* .TP Syntax: (append [* ]) (append* [* ]) .TP Description: The append function creates a new list which is a catenation of the arguments. All arguments are optional, such that (append) produces the empty list. If a single argument is specified, then append simply returns the value of that argument. It may be any kind of object. If N arguments are specified, where N > 1, then the first N-1 arguments must be proper lists. Copies of these lists are catenated together. The last argument N, shown in the above syntax as , may be any kind of object. It is installed into the cdr field of the last cons cell of the resulting list. Thus, if argument N is also a list, it is catenated onto the resulting list, but without being copied. Argument N may be an atom other than nil; in that case append produces an improper list. The append* function works like append, but returns a lazy list which produces the catenation of the lists on demand. If some of the arguments are themselves lazy lists which are infinite, then append* can return immediately, whereas append will get caught in an infinite loop trying to produce a catenation and eventually exhaust available memory. (However, the last argument to append may be an infinite lazy list, because append does not traverse the last argument.) .TP Examples: ;; An atom is returned. (append 3) -> 3 ;; A list is also just returned: no copying takes place. ;; The eq function can verify that the same object emerges ;; from append that went in. (let ((list '(1 2 3))) (eq (append list) list)) -> t (append '(1 2 3) '(4 5 6) 7) -> '(1 2 3 4 5 6 . 7)) ;; the (4 5 6) tail of the resulting list is the original ;; (4 5 6) object, shared with that list. (append '(1 2 3) '(4 5 6)) -> '(1 2 3 4 5 6) (append nil) -> nil ;; (1 2 3) is copied: it is not the last argument (append '(1 2 3) nil) -> (1 2 3) ;; empty lists disappear (append nil '(1 2 3) nil '(4 5 6)) -> (1 2 3 4 5 6) (append nil nil nil) -> nil ;; atoms and improper lists other than in the last position ;; are erroneous (append '(a . b) 3 '(1 2 3)) -> **error** .SS Function list .TP Syntax: (list *) .TP Description: The list function creates a new list, whose elements are the argument values. .TP Examples: (list) -> nil (list 1) -> (1) (list 'a 'b) -> (a b) .SS Function list* .TP Syntax: (list* *) .TP Description: The list* function is a generalization of cons. If called with exactly two arguments, it behaves exactly like cons: (list* x y) is identical to (cons x y). If three or more arguments are specified, the leading arguments specify additional atoms to be consed to the front of the list. So for instance (list* 1 2 3) is the same as (cons 1 (cons 2 3)) and produces the improper list (1 2 . 3). Generalizing in the other direction, list* can be called with just one argument, in which case it returns that argument, and can also be called with no arguments in which case it returns nil. .TP Examples: (list*) -> nil (list* 1) -> 1 (list* 'a 'b) -> (a . b) (list* 'a 'b 'c) -> (a b . c) .TP Dialect Note: Note that unlike in some other Lisp dialects, the effect of (list* 1 2 x) can also be obtained using (list 1 2 . x). However, (list* 1 2 (func 3)) cannot be rewritten as (list 1 2 . (func 3)) because the latter is equivalent to (list 1 2 func 3). .SS Function sub-list .TP Syntax: (sub-list [ []]) .TP Description: The sub-list function extracts a sublist from . It is exactly like the more generic function sub, except that it operates only on lists. For a description of the arguments and semantics, refer to the sub function. .SS Function replace-list .TP Syntax: (replace-list [ []]) .TP Description: The replace-list function replaces a subrange of with items from the item-sequence argument, which may be any kind of sequence (list, vector or string). It is like the replace function, except that the first argument must be a list. For a description of the arguments and semantics, refer to the replace function. .SS Functions listp and proper-listp .TP Syntax: (listp ) (proper-listp ) .TP Description: The listp and proper-listp functions test, respectively, whether is a list, or a proper list, and return t or nil accordingly. The listp test is weaker, and executes without having to traverse the object. (listp x) is equivalent to (or (null x) (consp x)). The empty list is a list, and a cons cell is a list. The proper-listp function returns t only for proper lists. A proper list is either nil, or a cons whose cdr is a proper list. proper-listp traverses the list, and its execution will not terminate if the list is circular. .SS Function length-list .TP Syntax: (length-list ) .TP Description: The length-list function returns the length of , which may be a proper or improper list. The length of a list is the number of conses in that list. .SS Function copy-list .TP Syntax: (copy-list ) .TP Description: The copy-list function which returns a list similar to , but with a newly allocated cons cell structure. If is an atom, it is simply returned. Otherwise, is a cons cell, and copy-list returns (cons (car ) (copy-list (cdr ))) except that recursion is not necessarily used. Note that the object (car ) is not deeply copied, but only propagated by reference into the new list. copy-list produces a new list structure out of the same items that are in . .TP Dialect Note: Common Lisp does not allow the argument to be an atom, except for the empty list nil. .SS Function copy-cons .TP Syntax: (copy-cons ) .TP Description: This function creates a fresh cons cell, whose car and cdr fields are copied from . .SS Functions reverse, nreverse .TP Syntax: (reverse ) (nreverse ) .TP Description: The functions reverse and nreverse produce an object which contains the same items as proper list , but in reverse order. If is nil, then both functions return nil. The reverse function is non-destructive: it creates a new list. The nreverse function creates the structure of the reversed list out of the cons cells of the input list, thereby destructively altering it (if it contains more than one element). How nreverse uses the material from the original list is unspecified. It may rearrange the cons cells into a reverse order, or it may keep the structure intact, but transfer the "car" values among cons cells into reverse order. Other approaches are possible. .SS Function ldiff .TP Syntax: (ldiff ) .TP Description: The values and are proper lists. The ldiff function determines whether is a structural suffix of (meaning that it actually is a suffix, and is not merely equal to one). This is true if and are the same object, or else, recursively, if is a suffix of (cdr ). The object nil is the sublist of every list, including itself. The ldiff function returns a new list consisting of the elements of the prefix of which come before the suffix. The elements are in the same order as in . If is not a suffix of , then a copy of is returned. .TP Examples: ;;; unspecified: the compiler could make '(2 3) a suffix of '(1 2 3), ;;; or they could be separate objects. (ldiff '(1 2 3) '(2 3)) -> either (1) or (1 2 3) ;; b is the (1 2) suffix of a, so the ldiff is (1) (let ((a '(1 2 3)) (b (cdr a))) (ldiff a b)) -> (1) .SS Function last .TP Syntax: (last ) .TP Description: If is a nonempty proper or improper list, the last function returns the last cons cell in the list: that cons cell whose cdr field is a terminating atom. If is nil, then nil is returned. .SS Functions flatten, flatten* .TP Syntax: (flatten ) (flatten* ) .TP Description: The flatten function produces a list whose elements are all of the non-nil atoms contained in the structure of . The flatten* function works like flatten except that flatten creates and returns a complete flattened list, whereas flatten* produces a lazy list which is instantiated on demand. This is particularly useful when the input structure is itself lazy. .TP Examples: (flatten '(1 2 () (3 4))) -> (1 2 3 4) ;; precisely equivalent to previous example! nil is the same thing as () (flatten '(1 2 nil (3 4))) -> (1 2 3 4) (flatten nil) -> nil (flatten '(((()) ()))) -> nil .SS Functions memq, memql and memqual .TP Syntax: (memq ) (memql ) (memqual ) .TP Description: The memq, memql and memqual functions search the for a member which is, respectively, eq, eql or equal to . (See the eq, eql and equal functions below.) If no such element found, nil is returned. Otherwise, that tail of the list is returned whose first element is the matching object. .SS Functions member and member-if .TP Syntax: (member [ []]) (member-if []) .TP Description: The find and find-if functions search through for an item which matches a key, or satisfies a predicate function, respectively. The keyfun argument specifies a function which is applied to the elements of the sequence to produce the comparison key. If this argument is omitted, then the untransformed elements of the sequence themselves are examined. The member function's testfun argument specifies the test function which is used to compare the comparison keys taken from the sequence to the search key. If this argument is omitted, then the equal function is used. If member does not find a matching element, it returns nil. Otherwise it returns the suffix of which begins with the matching element. The member-if function's predfun argument specifies a predicate function which is applied to the successive comparison keys pulled from the sequence by applying the key function to successive elements. If no match is found, then nil is returned, otherwise the suffix of which begins with the matching element. .SS Functions remq, remql and remqual .TP Syntax: (remq ) (remql ) (remqual ) .TP Description The remq, remql and remqual functions produce a new list based on , removing the the items which are eq, eql or equal to . The input is unmodified, but the returned list may share substructure with it. If no items are removed, it is possible that the return value is itself. .SS Functions remq*, remql* and remqual* .TP Syntax: (remq* ) (remql* ) (remqual* ) .TP Description: The remq*, remql* and remqual* functions are lazy versions of remq, remql and remqual. Rather than computing the entire new list prior to returning, these functions return a lazy list. Caution: these functions can still get into infinite looping behavior. For instance, in (remql* 0 (repeat '(0))), remql will keep consuming the 0 values coming out of the infinite list, looking for the first item that does not have to be deleted, in order to instantiate the first lazy value. .TP Examples: ;; Return a list of all the natural numbers, excluding 13, ;; then take the first 100 of these. ;; If remql is used, it will loop until memory is exhausted, ;; because (range 1) is an infinite list. [(remql* 13 (range 1)) 0..100] .SS Functions countqual, countql and countq .TP Syntax: (countq ) (countql ) (countqual ) .TP Description The countq, countql and countqual functions count the number of objects in which are eq, eql or equal to , and return the count. .SH APPLICATIVE LIST PROCESSING .SS Functions remove-if, keep-if, remove-if* and keep-if* .TP Syntax: (remove-if []) (keep-if []) (remove-if* []) (keep-if* []) .TP Description The remove-if function produces a list whose contents are those of but with those elements removed which satisfy . Those elements which are not removed appear in the same order. The result list may share substructure with the input list, and may even be the same list object if no items are removed. The optional specifies how each element from the is transformed to an argument to . If this argument is omitted then the predicate function is applied to the elements directly, a behavior which is identical to being (fun identity). The keep-if function is exactly like remove-if, except the sense of the predicate is inverted. The function keep-if retains those items which remove-if will delete, and removes those that remove-if will preserve. The remove-if* and keep-if* are like remove-if and keep-if, but produce lazy lists. .TP Examples: ;; remove any element numerically equal to 3. (remove-if (op = 3) '(1 2 3 4 3.0 5)) -> (1 2 4 5) ;; remove those pairs whose first element begins with "abc" [remove-if (op equal [@1 0..3] "abc") '(("abcd" 4) ("defg" 5)) car] -> (("defg 5)) ;; equivalent, without test function (remove-if (op equal [(car @1) 0..3] "abc") '(("abcd" 4) ("defg" 5))) -> (("defg 5)) .SS Function count-if .TP Syntax: (count-if []) .TP Description: The count-if function counts the numer of elements of which satisfy and returns the count. The optional specifies how each element from the is transformed to an argument to . If this argument is omitted then the predicate function is applied to the elements directly, a behavior which is identical to being (fun identity). .SS Functions posqual, posql and posq .TP Syntax: (posq ) (posql ) (posqual ) .TP Description The posq, posql and posqual functions return the zero-based position of the first item in which is, respectively, eq, eql or equal to . .SS Functions posqual, posql and posq .TP Syntax: (posq ) (posql ) (posqual ) .TP Description The posq, posql and posqual functions return the zero-based position of the first item in which is, respectively, eq, eql or equal to . .SS Functions pos and pos-if .TP Syntax: (pos [ []]) (pos-if []) .TP Description: The pos and pos-if functions search through a list for an item which matches a key, or satisfies a predicate function, respectively. They return the zero based position of the matching item. The keyfun argument specifies a function which is applied to the elements of the list to produce the comparison key. If this argument is omitted, then the untransformed elements of the list themselves are examined. The pos function's testfun argument specifies the test function which is used to compare the comparison keys from the list to the search key. If this argument is omitted, then the equal function is used. The position of the first element from the list whose comparison key (as retrieved by the key function) matches the search (under the test function) is returned. If no such element is found, nil is returned. The pos-if function's predfun argument specifies a predicate function which is applied to the successive comparison keys pulled from the list by applying the key function to successive elements. The position of the first element for which the predicate function yields true is returned. If no such element is found, nil is returned. .SS Functions pos-max and pos-min .TP Syntax: (pos-max [ []]) (pos-min [ []]) .TP Description: The pos-min and pos-max function implement exactly the same algorithm; they differ only in their defaulting behavior with regard to the argument. If is not given, then the pos-max function defaults it to the > function, whereas pos-min defaults it to the < function. If is empty, both functions return nil. Without a argument, the pos-max function finds the zero-based position index of the numerically maximum value occurring in , whereas pos-min without a argument finds the index of the minimum value. If a argument is given, the two functions are equivalent. The function must be callable with two arguments. If behaves like a greater-than comparison, then pos-max and pos-min return the index of the maximum element. If behaves like a less-than comparison, then the functions return the index of the minimum element. The argument defaults to the identity function. Each element from sequence is passed through this one-argument function, and the resulting value is used in its place. .SS Function where .TP Syntax: (where ) .TP Description: The where function searches for elements which satisfy , and returns a list of the numeric indices of those elements within , in order of increasing index. must be a function that can be called with one argument. For each element of , is called with that element as an argument. If a non-nil value is returned, then the zero-based index of that element is added to a list. Finally, the list is returned. .SS Function select .TP Syntax: (select ) .TP Description: The select function returns a sequence, of the same kind as , which consists of those elements of sequence which are identified by the numeric indices in . The select function stops processing upon encountering an index inside which is out of range. If is a list, then must contain montonically increasing numeric values, even if no value is out of range, since the ) .TP Description: The match-fun function invokes a txr pattern function whose name is given by the , which must be a symbol. The argument is a list of expressions. The expressions may be symbols which will be interpreted as pattern variables, and may be bound or unbound. If they are not symbols, then they are treated as expressions (of the pattern language, not TXR Lisp) and evaluated accordingly. The argument is a list of strings, which may be lazy. It represents the lines of the text stream to be processed. The argument is a list of filename specifications, which follow the same conventions as files given on the TXR command line. If the pattern function uses the @(next) directive, it can process these additional files. The match-fun function's return value falls into three cases. If there is a match failure, it returns nil. Otherwise it returns a cons cell. The car field of the cons cell holds the list of captured bindings. The cdr of the cons cell is one of two values. If the entire input was processed, the cdr field holds the symbol t. Otherwise it holds another cons cell whose car is the remainder of the list of lines which were not matched, and the cdr is the line number. .TP Example: @(define foo (x y)) @x:@y @line @(end) @(do (format t "~s\en" (match-fun 'foo '(a b) '("alpha:beta" "gamma" "omega") nil))) Output: (((a . "alpha") (b . "beta")) ("omega") . 3) In the above example, the pattern function foo is called with arguments (a b). These are unbound variables, so they correspond to parameters x and y of the function. If x and y get bound, those values propagate to a and b. The data being matched consists of the lines "alpha:beta", "gamma" and "omega". Inside foo, x and y bind to "alpha" and "beta", and then the line variable binds t "gamma". The input stream is left with "omega". Hence, the return value consists of the bindings of x and y transferred to a and b, and the second cons cell giving information about the rest of the stream: it is the part starting at "omega", which is line 3. Note that the binding for for line variable does not propagate out of the pattern function foo; it is local inside it. .SH QUOTE/QUASIQUOTE OPERATOR SYNTAX The quasiquote read syntax has a target language made up of operators. The TXR parser rewrites the syntax into the appropriate combination of these operators, and automatically selects whether a form is based on the quote operator or the qquote operator. The operators are described below. .SS Operator quote .TP Syntax: (quote ) .TP Description: The quote operator, when evaluated, suppresses the evaluation of , and instead returns itself as an object. For example, if is a symbol, then is not evaluated to the symbol's value; rather the symbol itself is returned. Note: when the quote syntax ' is used, then if is not a list structure which contains unquotes or splices, it is translated to (quote ). .TP Example: (quote a) ;; yields a (quote (+ 2 2)) ;; yields (+ 2 2), not 4. .SS Macro qquote .TP Syntax: (qquote ) .TP Description: The qquote (quasi-quote) macro operator implements a notation for convenient list construction. If is an atom, or a list structure which does not contain any unquote or splice operators, then (qquote ) is equivalent to (quote ). If , however, is a list structure which contains unquote or splice operators, then the substitutions implied by those operators are performed on , and the qquote operator returns the resulting structure. Note: how the qquote operator actually works is that it is compiled into code: a Lisp expression which computes the resulting structure when evaluated. A qquote can contain another qquote. If an unquote or splice operator occurs within a nested qquote, it belongs to that qquote, and not to the outer one. However, an unquote operator which occurs inside another one belongs one level higher. For instance in (qquote (qquote (unquote (unquote x)))), the leftmost qquote belongs with the rightmost unquote, and the inner qquote and unquote belong together. When the outer qquote is evaluated, it will insert the x, resulting in the value (qquote (unquote )). where represents the object which is the value of variable x. If this resulting qquote value is evaluated again as Lisp syntax, then it will yield , where denotes the value of when is treated as a Lisp expression and evaluated. .TP Examples: (qquote a) -> a (qquote (a b c)) -> (a b c) (qquote (1 2 3 (unquote (+ 2 2) (+ 2 3)))) -> (1 2 3 4 (+ 2 3)) (qquote (unquote (+ 2 2))) -> 4 In the second-to-last example, the 1 2 3 and the (+ 2 3) were taken verbatim. But the (unquote (+ 2 2)) operator caused the evaluation of (+ 2 2) and the substitution of the resulting value. The last example shows that can itself be an unquote operator. However, note: (quote (splice )) is not valid. Note: a way to understand the nesting behavior is a model of quasi-quote expansion which recursively compiles any nested quasi quotes first, and then treats the result of their expansion. For instance, in the processing of (qquote (qquote (unquote (unquote x)))), the quote operator finds the internal (qquote ...) and compiles it to code. During that recursive compilation, the syntax (unquote (unquote x)) is encountered. The inner quote processes the outer unquote which belongs to it, and the (unquote x) becomes material embedded in the compilation, which will then be found when the outer quasiquote takes the inner compilation and processes its interior. .TP Dialect note: In Lisp dialects which have a published syntax, there is the expectation that the quasiquote read syntax corresponds to it. That is to say, that for instance ^(a b ,c) is translated to (qquote b (unquote c)). In TXR Lisp, this is not true! Although ^(b b ,c) is translated to a quasiquoting macro, it is an internal one, not based on the public qquote, unquote and splice symbols. This idea exists for hygiene. The quasiquote read syntax is not confused by the presence of the symbols qquote, unquote or splice in the template, since it doesn't treat them specially. This also allows programmers to use the quasiquote read syntax to construct quasiquote macros. For instance ^(qquote (unquote ,x)) does not mean ^^,x. To the quasiquote reader, the qquote and unquote symbols mean nothing special, and so this syntax simply means that if the value of x is foo, the result will be (qquote (unquote foo)). The form's expansion is actually this: (sys:qquote (qquote (unquote (sys:unquote x)))) the sys:qquote macro recognizes sys:unquote embedded in the form, and the other symbols not in the sys: package are just static template material. .SS Operator unquote .TP Syntax: (qquote (... (unquote ) ...)) (qquote (unquote )) .TP Description: The unquote operator is not an operator per se. The unquote symbol has no binding in the global environment. It is a special syntax that is recognized within a qquote form, to indicate forms within the quasiquote which are to be evaluated and insertd into the resulting structure. The variant (qquote (unquote )) is equivalent to : the qquote and unquote "cancel out". Nesting of qquotes and unquotes is explained in the qquote operator. .SS Operator splice .TP Syntax: (qquote (... (splice ) ...)) .TP Description: The splice operator is not an operator per se. The splice symbol has no binding in the global environment. It is a special syntax that is recognized within a qquote form, to indicate forms within the quasiquote which are to be evaluated and inserted into the resulting structure. The variant (qquote (unquote )) is not permitted and raises an exception if evaluated. The splice syntax must occur within a list, and not in the dotted position. The splice form differs from unquote in that (splice ) requires that must evaluate to a list. That list is integrated into the surrounding list. Nesting of qquotes and unquotes is explained in the qquote operator. .SH MACROS TXR Lisp supports structural macros. TXR's model of macroexpansion is that TXR Lisp code is processed in two phases: the expansion phase and the evaluation phase. The expansion phase is invoked on Lisp code early during the processing of source code. For instance when a TXR File containing a @(do ...) directive is loaded, expansion of the Lisp forms are its arguments takes place during the parsing of the entire source file, and is complete before any of the code in that file is executed. If and when the @(do ...) form is later executed, the expanded forms are evaluated. TXR Lisp also supports symbol macros, which are symbolic forms that stand for forms, with which they are replaced at macro expansion time. When Lisp data is processed as code by the eval function, it is first expanded, and so processed in its entirety in the expansion phase. Then it is processed in the evaluation phase. .SS Macro parameter lists. TXR macros support destructuring, similarly to Common Lisp macros. This means that macro parameter lists are like function argument lists, but support nesting. A macro parameter list can specify a nested parameter list in every place where a function argument list allows only a parameter name. For instance, consider this macro parameter list: ((a (b c)) : (c frm) ((d e) frm2 de-p) . g) The top level of this list has four elements: the mandatory parameter (a (b c)), the optional parameter c (with default init form frm), the optional parameter (d e) with default init form frm2 and presence-indicating variable de-p, and the parameter g which captures trailing arguments. Note that some of the parameters are compounds: (a (b c)) and (d e). These compounds express nested macro parameter lists. Macro parameter lists match a similar tree structure to their own. For instance a mandatory parameter (a (b c)) matches a structure like (1 (2 3)), such that the parameters a, b and c will end up bound to 1, 2, and 3, respectively. The binding strictness is relaxed for optional parameters. If (a (b c)) is optional, and the argument is, say, (1), then a gets 1, and b and c receive nil. Macro parameter lists also support two special keywords, namely :env and :whole. The parameter list (:whole x :env y) will bind parameter x to the entire macro form, and y will bind parameter y to the macro environment. (Note: Macro environments currently do not do anything in TXR Lisp; this functionality is for future expansion.) The :whole and :env notation can occur anywhere in a macro parameter list. .SS Operator macro-time .TP Syntax: (macro-time {}*) .TP Description: The macro-time operator has a syntax similar to a progn form. The forms are evaluated from left to right, and the resulting value is that of the last form. The special behavior of macro-time is that the evaluation takes place during the expansion phase, rather than during the evaluation phase. During the expansion phase, all macro-time expressions which occur in a context that calls for evaluation are evaluated, and replaced by their quoted values. For instance (macro-time (list 1 2 3)) evaluates (list 1 2 3) to the object (1 2 3) and the entire macro-time form is replaced by the quoted list '(1 2 3). If the form is evaluated again at evaluation-time, the resulting value will be that of the quote. macro-time forms do not see the surrounding lexical environment; the see only global function and variable bindings and macros. Note 1: macro-time is intended for defining helper functions and variables that are used by macros. A macro cannot "see" a defun function or defvar variable because defun and defvar forms are evaluated at evaluation time, which occurs after expansion time. The macro-time operator hoists the evaluation of these forms to macro-expansion time. Note 2: defmacro forms are implicitly in macro-time; they do not have to be wrapped with the macro-time operator. The macro-time operator doesn't have to be used in programs whose macros do not make references to variables or functions. .SS Operator defmacro .TP Syntax: (defmacro (* [: *] [. ]) *) .TP Description: The defmacro operator is evaluated at expansion time. It defines a macro-expander function under the name , effectively creating a new operator. Note that the parameter list is a macro parameter list, and not a function parameter list. This means that each can have not only the syntax but it can itself be a parameter list. The corresponding argument is then treated as an argument list. Similarly, each symbol in the syntax can be an argument list. This nesting can be carried to an arbitrary depth. A macro is called like any other operator, and resembles a function. Unlike in a function call, the macro receives the argument expressions themselves, rather than their values. Therefore it operates on syntax rather than on values. Also, unlike a function call, a macro call occurs in the expansion phase, rather than the evaluation phase. The return value of the macro is is the macro expansion. It is substituted in place of the entire macro call form. That form is then expanded again; it may itself be another macro call, or contain more macro calls. .TP Examples: ;; A macro to swap two places, such as variables. ;; (swap x y) will now exchange the contents of x and y. (defmacro swap (a b) (let ((temp (gensym))) ^(let ((,temp ,a)) (set ,a ,b) (set ,b ,temp)))) ;; dolist macro similar to Common Lisp's: ;; ;; The following will print 1, 2 and 3 on separate lines: ;; and return 42. ;; ;; (dolist (x '(1 2 3) 42) ;; (format t "~s\en")) (defmacro dolist ((var list : result) . body) (let ((i (my-gensym))) ^(for ((i ,list)) (i ,result) ((set i (cdr i))) (let ((,var (car i))) ,*body)))) .SS Operator macrolet .TP Syntax: (macrolet ({( *)}*) *) .TP Description: The macrolet binding operator extends the macro-time lexical environment by making zero or more new local macros visible. The operator symbol is followed by a list of macro definitions. Each definition is a form which begins with a name, followed by a macro parameter list, and zero or more forms. These macro definitions are similar to those globally defined by the defmacro operator, except that they are in a local environment, and do not capture any regular lexical scope. The macro definitions are followed by optional forms. The macros thus defined are visible to the the -s. Forms inside the macro definitions such as the -s, and initializer forms appearing in are subject to macro-expansion in a scope in which none of the new macros being defined are yet visible. Once the macro definitions are themselves macro-expanded, they are placed into a new macro environment, which is then used for macro expanding the -s. A macrolet form is fully processed in the expansion phase of a form, and is effectively replaced by progn form which contains expanded versions of -s, only. This expanded structure shows no evidence that any macrolet forms ever existed in it. Therefore, it is impossible for the code evaluated in the bodies and parameter lists of macrolet macros to have any visibility to any surrounding lexical variable bindings, which are only instantiated in the evaluation phase, after expansion is done. .SS Function macro-form-p .TP Syntax: (macro-form-p []) .TP Description: The macro-form-p function returns t if represents the syntax of a form which is a macro form: either a compound macro or a symbol macro. Otherwise it returns nil. A macro form will transform under macroexpand-1 or macroexpand; an object which isn't a macro form will not undergo expansion. The optional parameter is a macroexpansion environment. A macroexpansion environment is passed down to macros and can be received via their special :env parameter. A macro can use this environment to determine whether a piece of syntax is a macro form in that environment. .TP Example: ;; macro which translates form to the form 'yes if it is a macro ;; form, otherwise to the form 'no. (defmacro global (:env menv form) (if (macro-form-p form menv) ''yes ''no)) (macrolet ((local ())) (global (local))) ;; yields yes (global (local)) ;; yields no During macroexpansion, the global macro is handed the environment which it receives via :env menv. When the macro is invoked within the macrolet, this environment includes the macro-time lexical scope in which the local macro is defined. So when global checks whether the argument form (local) is a macro, the conclusion is yes: the (local) form is a macro call in that environment: macro-form-p yields t. When (global (local)) is invoked outside of the macrolet, no local macro is visible, and so macro-form-p yields nil. .SS Functions macroexpand-1 and macroexpand .TP Syntax: (macroexpand-1 []) (macroexpand []) .TP Description: If is a macro form (an object for which macro-form-p returns t), these functions expand the macro form and return the expanded form. Otherwise, they return . macroexpand-1 performs a single expansion, expanding just the macro that is referenced by the symbol in the first position of , and returns the expansion. That expansion may itself be a macro form. macroexpand performs an expansion, like macroexpand-1. If the result is a macro form, then it expands that form, and keeps repeating this process until the expansion yields a non-macro-form. That non-macro-form is then returned. The optional parameter is a macroexpansion environment. A macroexpansion environment is passed down to macros and can be received via their special :env parameter. The environment they receive is their lexically apparent macro-time environment in which local macros may be visible. A macro can use this environment to "manually" expand some form in the context of that environment. .TP Example: ;; (foo x) expands x, and if x begins with a number, it removes ;; the number and returns the resulting form. Otherwise, it ;; returns the entire form. (defmacro rem-num (:env menv some-form) (let ((expanded (macroexpand some-form menv))) (if (numberp (car expanded)) (cdr expanded) some-form))) ;; The following yields (42 a). (macrolet ((foo () '(1 list 42)) (bar () '(list 'a))) (list (rem-num (foo)) (rem-num (bar))))) The rem-num macro is able to expand the (foo) and (bar) forms it receives as the some-form argument, using the local macro that is lexically apparent where those forms are passed, and is correctly able to work with the expansions (1 list 42) and (list 'a) to produce (list 42) and (list 'a) which evaluate to 42 and a respectively. .SS Operator defsymacro .TP Syntax: (defsymacro ) .TP Description: The defsymacro operator binds a symbol macro in the global environment. A defsymacro form is implicitly executed at expansion time, and thus need not be wrapped in a macro-time form. The visibility of a symbol macro binding for specifies that occurrences of in program code which are to be evaluated, or which denote places which are the targets of assignments, are subject to a replacement by . The replacement is then subject to further expansion, if possible. It may be a compound form which is a macro call, or another symbol that has a symbol macro binding. Note: if a symbol macro expands to itself directly, expansion stops. However, if a symbol macro expands to itself through a chain of expansions, an infinite expansion time loop results. .SS Operator symacrolet .TP Syntax: (symacrolet ({( )}*) *) .TP Description: The symacrolet operator binds local, lexically scoped macros that are similar to the global symbol macros introduced by defsymacro. Each in the bindings list is bound to its corresponding form, creating a new extension of the expansion-time lexical macro environment. Each is subequently macro-expanded in this new environment in which the new symbol macros are visible. Note: ordinary lexical bindings such as those introduced by let or by function parameters lists shadow symbol macros. If a symbol X is bound by both a macrolet and a let, then a body which is enclosed by both constructs will see whichever of the two bindings is innermost, even though the bindings are active in completely separate phases of processing. .SS Operator tree-bind .TP Syntax: (tree-bind *) .TP Description: The tree-bind operator evaluates , and then uses the resulting value as a counterpart to a macro-style parameter list. If the value has a tree structure which matches the parameters, then those parameters are established as bindings, and the -s, if any, are evaluated in the scope of those bindings. The value of the last is returned. If there are no forms, nil is returned. Note: this operator throws an exception if there is a mismatch between the parameters and the value of . .SS Operator tree-case .TP Syntax: (tree-case { ( *) }*) .TP Description: The tree case operator evaluates and matches it against a succession of zero or more cases. Each case defines a pattern match, expressed as a macro style parameter list . If the object produced by matches , then the parameters are bound, becoming local variables, and the -s, if any, are evaluated in order in the environment in which those variables are visible. If there are forms, the value of the last becomes the result value of the case, otherwise the result value of the case is nil. If the result value of a case is the object : (the colon symbol), then processing continues with the next case. Otherwise the evaluation of tree-case terminates, returning the result value. If the value of does not match the parameter list, then the usual exception is thrown; instead, processing continues with the next case. If no cases match, then tree-case terminates, returning nil. .SS Example: ;; reverse function implemented using tree-case (defun tb-reverse (obj) (tree-case obj (() ()) ;; the empty list is just returned ((a) obj) ;; one-element list just returned (unnecessary case) ((a . b) ^(,*(tb-reverse b) ,a)) ;; car/cdr recursion (a a))) ;; atom is just returned Note that in this example, the atom case is placed last, because an argument list which consists of a symbol is a "catch all" match that matches any object. We know that it matches an atom, because the previous (a . b) case matches conses. In general, the order of the cases in tree-case is important. Also note that the one-element case can be removed. .SH DEBUGGING FUNCTIONS .SS Functions source-loc and source-loc-str .TP Syntax: (source-loc ) (source-loc-str ) .TP Description: These functions map an expression in a txr program to the file name and line number of the source code where that form came from.o The source-loc function returns the raw information as a cons cell whose car/cdr consist of the line number, and file name. The source-loc-str function formats the information as a string. If is not a piece of the program source code that was constructed by the TXR parser, then source-loc returns nil, and source-loc-str returns a string whose text says that source location is not available. .SS Function rlcp .TP Syntax: (rlcp ) The rlcp function copies the source code location info (rl = read location) from the object to the object. These objects are pieces of list-based syntax. Note: the function is intended to be used in macros. If a macro transforms to , this function can be used to propagate the source code location info also, so that when the TXR Lisp evaluator encounters errors in transformed code, it can give diagnostics which refer to the original untransformed source code. .SH PROFILING .SS Operator prof .TP Syntax: (prof *) .TP Description: The prof operator evaluates the enclosed forms from left to right similarly to progn, while determining the memory allocation requests and time consumed by the evaluation of the forms. If there are no forms, the prof operator measures the smallest measurable operation of evaluating nothing and producing nil. If the evaluation terminates normally (not abruptly by a non-local control transfer), then prof yields a list consisting of: ( ) where is the value returned by the rightmost , or nil if there are no forms, is the total number of bytes of all malloc requests (or at least those known to the TXR runtime, such as those of all internal objects), is the total number of bytes drawn from the garbage-collected heaps, and is the total processor time consumed over the execution of those forms. Notes: The bytes allocated by the garbage collector from malloc to create heap areas are not counted as . includes storage such as the space used for dynamic strings, vectors and bignums (in addition to their gc-heap allocated nodes), and the various structures used by the cobj type objects such as streams and hashes. Objects in external libraries that use un-instrumented allocators are not counted: for instance the C FILE * streams. .SS Macro pprof .TP Syntax: (pprof *) .TP Description: The pprof (pretty-printing prof) macro is similar to progn. It evaluates forms, and returns the rightmost one, or nil if there are no forms. Over the evaluation of the forms, it counts memory allocations, and measures CPU time. If the forms terminate normally, then just prior to returning, pprof prints these statistics are printed in a concise report on the *std-output* stream, prior to returning the value or nil. The pprof macro relies on the prof operator. .SH MODULARIZATION .SS Special variable *self-path* This variable holds the invocation path name of the TXR program. .SH DEBUGGER .B TXR has a simple, crude, built-in debugger. The debugger is invoked by adding the the -d command line option to an invocation of txr. In this debugger it is possible to step through code, set breakpoints, and examine the variable binding environment. Prior to executing any code, the debugger waits at the txr> prompt, allowing for the opportunity to set breakpoints. Help can be obtained with the h or ? command. Whenever the program stosp at the debugger, it prints the Lisp-ified piece of syntax tree that is about to be interpreted. It also shows the context of the input being matched. The s command can be used to step into a form; n to step over. Sometimes the behavior seems counter-intuitive. For instance stepping over a @(next) directive actually means skipping everything which follows it. This is because the query material after a @(next) is actually child nodes in the abstract syntax tree node of the next directive. .SS Sample Session Here is an example of the debugger beign applied to a web scraping program which connects to a US NAVY clock server to retrieve a dynamically-generated web page, from which the current time is extracted, in various time zones. The handling of the web request is done by the wget command; the txr query opens a wget command as and scans the body of the HTTP response containing HTML. This is the code, saved in a file called navytime.txr: @(next `!wget -c http://tycho.usno.navy.mil/cgi-bin/timer.pl -O - 2> /dev/null`) What time is it?

US Naval Observatory Master Clock Time

  @(collect :vars (MO DD HH MM SS (PM "  ") TZ TZNAME))
  
@MO. @DD, @HH:@MM:@SS @(maybe)@{PM /PM/} @(end)@TZ@/\et+/@TZNAME
  @  (until)
@/.*/ @(end)

US Naval Observatory @(output) @ (repeat) @MO-@DD @HH:@MM:@SS @PM @TZ @ (end) @(end) This is the debug session: $ txr -d navytime.txr stopped at line 1 of navytime.txr form: (next (sys:quasi "!wget -c http://tycho.usno.navy.mil/cgi-bin/timer.pl -O - 2> /dev/null")) depth: 1 data (nil): nil The user types s to step into the (next ...) form. txr> s stopped at line 2 of navytime.txr form: (sys:text " 1+ #\espace) "HTML" (# 1+ #\espace) "PUBLIC" (# 1+ #\espace) "\e"-//W3C//DTD" (# 1+ #\espace) "HTML" (# 1+ #\espace) "3.2" (# 1+ #\espace) "Final\e"//EN>") depth: 2 data (1): "" txr> s The current form now is a syt:text form which is an internal representation of a block of horizontal material. The pattern matching is in vertical mode at this point, and so the line of data is printed without an indication of character position. stopped at line 2 of navytime.txr form: (sys:text " 1+ #\espace) "HTML" (# 1+ #\espace) "PUBLIC" (# 1+ #\espace) "\e"-//W3C//DTD" (# 1+ #\espace) "HTML" (# 1+ #\espace) "3.2" (# 1+ #\espace) "Final\e"//EN>") depth: 3 data (1:0): "" . "" The user types s to step in. txr> s stopped at line 2 of navytime.txr form: "" Now, the form about to be processed is the first item of the (sys:text ...), a the string " s stopped at line 2 of navytime.txr form: (# 1+ #\espace) depth: 4 data (1:9): "" Control has now passed to the second element of the (sys:text ...), a regular expression which matches one or more spaces, generated by a single space in the source code according to the language rules. The input context shows taht " s stopped at line 2 of navytime.txr form: "HTML" depth: 4 data (1:10): "" Now, the regular expression has matched and moved the psoition past the space; the facing input is now "HTML ...". The programmer then repeats the s command by hitting Enter. txr> stopped at line 2 of navytime.txr form: (# 1+ #\espace) depth: 4 data (1:14): "" txr> stopped at line 2 of navytime.txr form: "PUBLIC" depth: 4 data (1:15): "" txr> stopped at line 2 of navytime.txr form: (# 1+ #\espace) depth: 4 data (1:21): "" txr> stopped at line 2 of navytime.txr form: "\e"-//W3C//DTD" depth: 4 data (1:22): "" txr> stopped at line 2 of navytime.txr form: (# 1+ #\espace) depth: 4 data (1:34): "" It is not evident from the session transcript, but during interactive use, the input context appears to be animated. Whenever the programmer hits Enter, the new context is printed and the dot appears to advance. Eventually the programmer becomes bored and place a breakpoint on line 15, where the @(output) block begins, and invokes the c command to continue the execution: txr> b 15 txr> c stopped at line 15 of navytime.txr form: (output (((repeat nil (((sys:var MO nil nil) "-" (sys:var DD nil nil) " " (sys:var HH nil nil) ":" (sys:var MM nil nil) ":" (sys:var SS nil nil) " " (sys:var PM nil nil) " " (sys:var TZ nil nil))) nil nil nil nil nil nil)))) depth: 2 data (16): "" The programmer issues a v command to take a look at the variable bindings, which indicate that the @(collect) has produced some lists. txr> v bindings: 0: ((PM " " "PM" "PM" "PM" "PM" "PM" "PM") (TZNAME "Universal Time" "Eastern Time" "Central Time" "Mountain Time" "Pacific Time" "Alaska Time" "Hawaii-Aleutian Time") (TZ "UTC" "EDT" "CDT" "MDT" "PDT" "AKDT" "HAST") (SS "35" "35" "35" "35" "35" "35" "35") (MM "32" "32" "32" "32" "32" "32" "32") (HH "23" "07" "06" "05" "04" "03" "01") (DD "30" "30" "30" "30" "30" "30" "30") (MO "Mar" "Mar" "Mar" "Mar" "Mar" "Mar" "Mar")) Then a continue command, which finishes the program, whose output appears: txr> c Mar-30 23:22:52 UTC Mar-30 07:22:52 PM EDT Mar-30 06:22:52 PM CDT Mar-30 05:22:52 PM MDT Mar-30 04:22:52 PM PDT Mar-30 03:22:52 PM AKDT Mar-30 01:22:52 PM HAST .SH APPENDIX A: NOTES ON EXOTIC REGULAR EXPRESSIONS Users familiar with regular expressions may not be familiar with the complement and intersection operators, which are often absent from text processing tools that support regular expressions. The following remarks are offered in hope that they are of some use. .SS Equivalence to Sets Regexp intersection is not essential; it may be obtained from complement and union as follows, since De Morgan's law applies to regular expression algebra: (R1)&(R2) = ~(~(R1)|~(R2)). (The complement of the union of the complements of R1 and R2 constitutes the intersection.) This law works because the regular expression operators denote set operations in a straightforward way. A regular expression denotes a set of strings (a potentially infinite one) in a condensed way. The union of two regular expressions R1 | R2 denotes the union of the set of texts denoted by R1 and that denoted by R2. Similarly R1 & R2 denotes a set intersection, and ~R denotes a set complement. Thus algebraic laws that apply to set operations apply to regular expressions. It's useful to keep in mind this relationship between regular expressions and sets in understanding intersection and complement. Given a finite set of strings, like the set { "abc", "def" }, which corresponds to the regular expression (abc|def), the complement is the set which contains an infinite number of strings: it consists of all possible strings except "abc" and "def". It includes the empty string, all strings of length 1, all strings of length 2, all strings of length 3 other than "abc" and "def", all strings of length 4, etc. This means that a "harmless looking" expression like ~(abc|def) can actually match arbitrarily long inputs. .SS Set Difference How about matching only three-character-long strings other than "abc" or "def"? To express this, regex intersection can be used: these strings are the intersection of the set of all three-character strings, and the set of all strings which are not "abc" or "def". The straightforward set-based reasoning leads us to this: ...&~(abc|def). This A&~B idiom is also called set difference, sometimes notated with a minus sign: A-B (which is not supported in .B TXR regular expression syntax). Elements which are in the set A, but not B, are those elements which are in the intersection of A with the complement of B. This is similar to the arithmetic rule A - B = A + -B: subtraction is equivalent to addition of the additive inverse. Set difference is a useful tool: it enables us to write a positive match which captures a more general set than what is intended (but one whose regular expression is far simpler than a positive match for the exact set we want), then we can intersect this over-generalized set with the complemented set of another regular expression which matches the particulars that we wish excluded. .SS Expressivity versus Power It turns out that regular expressions which do not make use of the complement or intersection operators are just as powerful as expressions that do. That is to say, with or without these operators, regular expressions can match the same sets of strings (all regular languages). This means that for a given regular expression which uses intersection and complement, it is possible to find a regular expression which doesn't use these operators, yet matches the same set of strings. But, though they exist, such equivalent regular expressions are often much more complicated, which makes them difficult to design. Such expressions do not necessarily . B express what it is they match; they merely capture the equivalent set. They perform a job, without making it obvious what it is they do. The use of complement and intersection leads to natural ways of expressing many kinds of matching sets, which not only demonstrate the power to carry out an operation, but also easily express the concept. .SS Example: Matching C Language Comments For instance, using complement, we can write a straightforward regular expression which matches C language comments. A C language comment is the digraph /*, followed by any string which does not contain the closing sequence */, followed by that closing sequence. Examples of valid comments are /**/, /* abc */ or /***/. But C comments do not nest (cannot contain comments), so that /* /* nested */ */ actually consists of the comment /* /* nested */, which is followed by the trailing junk */. Our simple characterization of interior part of a C comment as a string which does not contain the terminating digraph makes use of the complement, and can be expressed using the complemented regular expression like this: (~.*[*][/].*). That is to say, strings which contain */ are matched by the expression .*[*][/].*: zero or more arbitrary characters, followed by */, followed by zero or more arbitrary characters. Therefore, the complement of this expression matches all other strings: those which do not contain */. These strings up the inside of a C comment between the /* and */. The equivalent simple regex is quite a bit more complicated. Without complement, we must somehow write a positive match for all strings such that we avoid matching */. Obviously, sequences of characters other than * are included: [^*]*. Occurrences of * are also allowed, but only if followed by something other than a slash, so let's include this via union: ([^*]|[*][^/])*. Alas, already, we have a bug in this expression. The subexpression [*][^/] can match "**", since a * is not a /. If the next character in the input is /, we missed a comment close. To fix the problem we revise to this: ([^*]|[*][^*/])* (The interior of a C language comment is a any mixture of zero or more non-asterisks, or digraphs consisting of an asterisk followed by something other than a slash or another asterisk). Oops, now we have a problem again. What if two asterisks occur in a comment? They are not matched by [^*], and they are not matched by [*][^*/]. Actually, our regex must not simply match asterisk-non-asterisk digraphs, but rather sequences of one or more asterisks followed by a non-asterisk: ([^*]|[*]*[^*/])* This is still not right, because, for instance, it fails to match the interior of a comment which is terminated by asterisks, including the simple test cases where the comment interior is nothing but asterisks. We have no provision in our expression for this case; the expression requires all runs of asterisks to be followed by something which is not a slash or asterisk. The way to fix this is to add on a subexpression which optionally matches a run of zero or more interior asterisks before the comment close: ([^*]|[*]*[^*/])*[*]* Thus our the semi-final regular expression is [/][*]([^*]|[*]*[^*/])*[*]*[*][/] (A C comment is an interior string enclosed in /* */, where this interior part consists of a mixture of non-asterisk characters, as well as runs of asterisk characters which are terminated by a character other than a slash, except for possibly one rightmost run of asterisks which extends to the end of the interior, touching the comment close. Phew!) One final simplification is possible: the tail part [*]*[*][/] can be reduced to [*]+[/] such that the final run of asterisks is regarded as part of an extended comment terminator which consists of one or more asterisks followed by a slash. The regular expression works, but it's cryptic; to someone who has not developed it, it isn't obvious what it is intended to match. Working out complemented matching without complement support from the language is not impossible, but it may be difficult and error-prone, possibly requiring multiple iterations of trial-and-error development involving numerous test cases, resulting in an expression that doesn't have a straightforward relationship to the original idea. .SS The Non-Greedy Operator The non-greedy operator % is actually defined in terms of a set difference, which is in turn based on intersection and complement. The uninteresting case (R%) where the right operand is empty reduces to (R*): if there is no trailing context, the non-greedy operator matches R as far as possible, possibly to the end of the input, exactly like the greedy Kleene. The interesting case (R%T) is defined as a "syntactic sugar" for the equivalent expression ((R*)&(~.*(T&.+).*))T which means: match the longest string which is matched by R*, but which does not contain a non-empty match for T; then, match T. This is a useful and expressive notation. With it, we can write the regular expression for matching C language comments simply like this: [/][*].%[*][/] (match the opening sequence /*, then match a sequence of zero or more characters non-greedily, and then the closing sequence */. With the non-greedy operator, we don't have to think about the interior of the comment as set of strings which excludes */. Though the non-greedy operator appears expressive, its apparent simplicity may be deceptive. It looks as if it works "magically" by itself; "somehow" this .% "knows" only to consume enough characters so that it doesn't swallow an occurrence of the trailing context. Care must be taken that the trailing context passed to the operator really is the correct text that should be excluded by the non-greedy match. For instance, take the expression .%abc. If you intend the trailing context to be merely a, you must be careful to write (.%a)bc. Otherwise the trailing context is abc, and this means that the .% match will consume the longest string that does not contain "abc", when in fact what was intended was to consume the longest string that does not contain a. The change in behavior of the % operator upon modifying the trailing context is not as intuitive as that of the * operator, because the trailing context is deeply involved in its logic. For single-character trailing contexts, it may be a good idea to use a complemented character class instead. That is to say, rather than (.%a)bc, consider [^a]*bc. The set of strings which don't contain the character a is adequately expressed by [^a]*.