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Diffstat (limited to 'txr.1')
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@@ -41356,6 +41356,131 @@ is a standard \*(TL notation with the same meaning as - > (t (1 2 3)) .brev +.coNP Pattern macro @ sme +.synb +.mets @(sme < spat < mpat < epat >> [ mvar <> [ evar ]]) +.syne +.desc +The pattern macro +.code sme +(start, middle, end) is a notation defined using the +.code defmatch +macro. + +The +.code sme +macro generates a complex pattern which matches three non-overlapping +parts of a list object using three patterns. The +.meta spat +pattern is required to match a prefix of the input list. If that match is +successful, then the remainder of the list is searched for a match for +.metn mpat , +using the +.code scan +operator. If that match, in turn, is successful, then the suffix of +the remainder of the list is required to match +.codn epat . + +The optional +.meta mvar +and +.meta evar +arguments must be bindable symbols, if they are specified. +These symbols specify lexical variables which are bound to, respectively, +the object matched by +.meta mpat +and +.metn epat , +using the fresh binding semantics of the +.code as +pattern operator. + +The first two patterns, +.meta spat +and +.metn mpat , +must be possibly dotted list patterns. +The last pattern, +.metn epat , +must be either an atom or a possibly dotted list pattern. + +Important to the semantics of +.code sme +is the concept of the length of a list pattern. + +The length of a pattern with a pattern variable or operator +in the dotted position is the number of items before that variable +or operator. The length of +.code "(1 2 . @(and a b))" +is 2; likewise the length of +.code "(1 2 . @nil)" +is also 2. +The length of a pattern which does not have a variable or +operator in the dotted position is simply its list length. +For instance, the pattern +.code "(1 2 3)" +has length 3, and so does the pattern +.codn "(1 2 3 . 4)" . +The length is determined by the list object structure of the +pattern, and not the printed syntax used to express it. Thus, +.code "(1 . (2 3))" +is still a length 3 pattern, because it denotes the same +.code "(1 2 3)" +object, using the dot notation unnecessarily. + +The non-overlapping semantics of +.code sme +develops as follows. When the +.meta spat +pattern matches a prefix of the input object, then a middle suffix is +calculated of the input object by dropping leading elements from it. The number +of elements dropped is equal to the length +.metn spat . +The +.meta mpat +is then similarly matched against a prefix of this middle suffix. If that match +is successful, a number of leading elements equal to the length of +.meta mpat +is dropped from the middle suffix to determine the final suffix. +Then +.meta epat +is matched against the tail portion of the final suffix which is equal +to its length. If the final suffix is shorter than +.metn epat , +then the match isn't possible. + +.TP* Examples: + +.verb + (when-match @(sme (1 2) (3 4) (5 . 6) m e) + '(1 2 3 4 5 . 6) + (list m e)) + -> ((3 4 5 . 6) (5 . 6)) + + (when-match @(sme (1 2) (3 4) (5 . 6) m e) + '(1 2 abc 3 4 def 5 . 6) + (list m e)) + ((3 4 def 5 . 6) (5 . 6)) + + ;; backreferencing + (when-match @(sme (1 @y) (@z @x @y @z) (@x @y)) '(1 2 3 1 2 3 1 2) + (list x y z)) + -> (1 2 3)) + + ;; collect odd items starting at 3, before 7 + (when-match @(and @(sme (1 @x) (3) (7) m e) + @(with @(coll @(oddp @y)) (ldiff m e))) + '(1 2 3 4 5 6 7) + (list x y)) + -> (2 (3 5))) + + ;; no overlap + (when-match @(sme (1 2) (2 3) (3 4)) '(1 2 3 4) t) -> nil + + ;; The atom 5 is like a "zero-length improper list". + (when-match @(sme () () 5) 5 t) -> t +.brev + .SS* Pattern Matching Macros .coNP Macros @ when-match and @ if-match |