diff options
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 108 |
1 files changed, 104 insertions, 4 deletions
@@ -4273,10 +4273,10 @@ Define several Lisp functions using @(do): 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 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. +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. @@ -4363,6 +4363,21 @@ with the innermost quote. The quote between the commas protects the (+ 1 2) from repeated evaluations: the two unquotes call for two evaluations, but we only want (+ 1 2) to be evaluated once. +.SS The DWIM Brackets + +.IP [...] + +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. + +More details are given in the documentation for the dwim operator. + .SS Lisp Operators When the first element of a compound expression is an operator symbol, @@ -4753,6 +4768,11 @@ Currently, these forms are recognized as places: (vecref <vector> <index>) + (dwim <obj> ...) + + [<obj> ...] ;; equivalent to (dwim <obj> ...) + + A <symbol> place denotes a variable. If the variable does not exist, it is an error. @@ -4769,6 +4789,86 @@ 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, or hash table, +depending on the type of obj. + +.SS Operator dwim + +.TP +Syntax: + + (dwim <argument>*) + + [<argument>*] + +.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. How many are required depends on the type of +object to which the first argument expression evaluates: of the first argument. +The possibilities are: + +.IP [<function> <argument>*] +Call the given the function object to the given arguments. + +.IP [<symbol> <argument>*] +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 [<list> <index>] +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 [<vector> <index>] +Retrieve the specified element of a vector. This is equivalent to +(vecref <vector> <index>). + +.IP [<string> <index>] +Retrieve the specified element of a string. This is equivalent to +(chr-str <string> <index>). + +.IP [<hash-table> <key> <default-value>] +Retrieve a value from the hash table corresponding to <key>, +or <default-value> if there is no such entry. + +The list, vector and hash table forms of dwim denote places +that can be assigned. + +.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. 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. When round +parentheses are used, the normal Lisp-2 rules apply. A "best of both worlds" +situation is achieved. + +Lisp-1 dialects can 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. + .SS Operators for and for* .TP |