7.2 The pattern language

The syntax for <pattern> is:

<pattern> →                Matches:

  <atom>                    the <atom>.
| (kwote <atom>)            any expression eq? to <atom>.
| (and <pat1> ... <patn>)   if all of <pati> match.
| (or <pat1> ... ...<patn>) if any of <pat1> through <patn> matches.
| (not <pat>)               if <pat> doesn't match.
| (? <predicate>)           if <predicate> is true.
| (<pat1> ... <patn>)       a list of n elements. Here, ... is a
                            meta-character denoting a finite repetition
                            of patterns.
| <pat> ...                 a (possibly empty) repetition
                            of <pat> in a list.
| #(<pat> ... <patn>)       a vector of n elements.
| #{<struct> <pat> ... }    a structure.
| ?<id>                     anything, and binds id as a variable.
| ?-                        anything.
| ??-                       any (possibly empty) repetition of anything
                            in a list.
| ???-                      any end of list.

Remark: and, or, not, check and kwote must be quoted in order to be treated as literals. This is the only justification for having the kwote pattern since, by convention, any atom which is not a keyword is quoted.

• ?- matches any s-expr
• a matches the atom 'a.
• ?a matches any expression, and binds the variable a to this expression.
• (? integer?) matches any integer
• (a (a b)) matches the only list '(a (a b)).
• ???- can only appear at the end of a list, and always succeeds. For instance, (a ???-) is equivalent to (a . ?-).
• when occurring in a list, ??- matches any sequence of anything: (a ??- b) matches any list whose car is a and last car is b.
• (a ...) matches any list of a’s, possibly empty.
• (?x ?x) matches any list of length 2 whose car is eq to its cadr
• ((and (not a) ?x) ?x) matches any list of length 2 whose car is not eq to 'a but is eq to its cadr
• #(?- ?- ???-) matches any vector whose length is at least 2.
• #{foo (?- . ?-) (? integer?)} matches any structure or record foo whose first and second fields are respectively a pair and an integer. You can provide only the fields you want to test. The order is not relevant.

Remark: ??- and ... patterns can not appear inside a vector, where you should use ???-: For example, #(a ??- b) or #(a...) are invalid patterns, whereas #(a ???-) is valid and matches any vector whose first element is the atom a.

This document was generated on March 31, 2014 using texi2html 5.0.