diff options
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 97 |
1 files changed, 94 insertions, 3 deletions
@@ -40271,6 +40271,25 @@ one for each list position. This is because macro-style parameter lists are oriented toward writing macros, and macros usually make use of every parameter position. +.NP* Application-defined Patterns + +Application-defined pattern operators are possible. When the +.meta operator +symbol in the +.mono +.meti >> @( operator << argument *) +.onom +syntax doesn't match any built-in operator, a search takes +place to determine whether +.meta operator +is a pattern macro. If so, the pattern macro is expanded, and +its result of the expansion treated as a pattern to process recursively, +unless it is the original macro form, in which case it is treated +as a predicate pattern. Application-defined pattern macros are defined +using the +.code defmatch +macro. + .SS* Pattern Matching Notation The pattern-matching notation is documented in the following @@ -41136,9 +41155,14 @@ operator. .mets >> @(@ rvar >> ( function << arg * . <> @ avar )) .syne .desc -Whenever the operator position of a pattern consists of a symbol which is not -the name of a pattern operator, the expression denotes a predicate pattern, -expected to conform to one of the first three syntax variations above. +Whenever the operator position of a pattern consists of a symbol which is +neither the name of a pattern operator, nor the name of a macro, the expression +denotes a predicate pattern. An expression is also a predicate pattern if +it is handled by a pattern macro which declines to expand it by yielding +the original expression. + +An operator pattern is expected to conform to one of the first three +syntactic variations above. Together, these three variations constitute the .I "first form" of the pattern predicate operator. @@ -41810,6 +41834,73 @@ in which case it inserts its own generated symbol. width 0 height 0) .brev +.coNP Macro @ defmatch +.synb +.mets (defmatch < name < macro-style-params +.mets \ \ << body-form *) +.syne +.desc +The +.code defmatch +macro allows for the definition of pattern macros: application-defined pattern +operators which are implemented via expansion into existing operator syntax. + +The +.code defmatch +macro has the same syntax as +.codn defmacro . +It specifies a macro transformation for a compound form which has the +.meta name +symbol in its leftmost position. + +This macro transformation is performed when +.meta name +is used as a pattern operator: an expression of the form +.mono +.meti >> @( name << argument *) +.onom +occurring in pattern matching syntax. + +The behavior is unspecified if +.meta name +is the name a built-in pattern operator, or a predefined pattern macro. + +The pattern macro bindings are stored in a hash table held by the variable +.code *match-macro* +whose keys are symbols, and whose values are expander functions. +There are no lexically scoped pattern macros. + +.TP* Example: + +.verb + ;; Create an alias called let for the @(as var pattern) operator: + ;; Note that the macro produces @(as ...) and not just (as ...) + + (defmatch let (var pattern) + ^@(as ,var ,pattern)) + + ;; use the macro in matching: + (when-match @(let x @(or foo bar)) 'foo x) +.brev + +.coNP Special variable @ *match-macro* +.desc +The +.code *match-macro* +special variable holds the hash table of associations between +symbols and pattern macro expanders. + +If the expression +.code "[*place-macro* 'sym]" +yields a function, then symbol +.code sym +has a binding as a pattern macro. If that +expression yields +.codn nil , +then there is no such binding: pattern operator forms based on +.code sym +do not undergo place macro expansion. + .SS* Quasiquote Operator Syntax .coNP Macro @ qquote .synb |