Patterns

Figure 0.1 gives the full syntax for patterns. Explanations of these patterns follow.

• identifier ( excluding the reserved names ?, $, _, and, or, not, set!, get!, ..., and ..k for non-negative integers k): matches anything, and binds a variable of this name to the matching value in the body.
• _: matches anything, without binding any variables.
• (), #t, #f, string, number, character, 's-expression: These constant patterns match themselves, ie., the corresponding value must be equal? to the pattern.
• : matches a proper list of n elements that match through .
• : matches a (possibly improper) list of at least n elements that ends in something matching .
• : matches a proper list of n or more elements, where each element of the tail matches . Each pattern variable in is bound to a list of the matching values. For example, the expression:

  (match '(let ([x 1][y 2]) z)
    [('let ((binding values) ...) exp)  body])

  binds binding to the list '(x y), values to the list '(1 2), and exp to 'z in the body of the match-expression. For the special case where is a pattern variable, the list bound to that variable may share with the matched value.
• : This pattern is similar to the previous pattern, but the tail must be at least k elements long. The pattern keywords ..0 and ... are equivalent.
• : matches a vector of length n, whose elements match through .
• : matches a box containing something matching pat.
• : matches a structure declared with define-structure or define-const-structure.
• : matches if all of the subpatterns match. This pattern is often used as to bind x to to the entire value that matches pat (cf. ``as-patterns'' in ML or Haskell).
• : matches if any of the subpatterns match. At least one subpattern must be present. All subpatterns must bind the same set of pattern variables.
• : matches if none of the subpatterns match. The subpatterns may not bind any pattern variables.
• : In this pattern, predicate must be an expression evaluating to a single argument function. This pattern matches if predicate applied to the corresponding value is true, and the subpatterns all match. The predicate should not have side effects, as the code generated by the pattern matcher may invoke predicates repeatedly in any order. The predicate expression is bound in the same scope as the match expression, ie., free variables in predicate are not bound by pattern variables.
• : matches anything, and binds identifier to a procedure of one argument that mutates the corresponding field of the matching value. This pattern must be nested within a pair, vector, box, or structure pattern. For example, the expression:

  (define x (list 1 (list 2 3)))
  (match x [(_ (_ (set! setit)))  (setit 4)])

  mutates the cadadr of x to 4, so that x is '(1 (2 4)).
• : matches anything, and binds identifier to a procedure of zero arguments that accesses the corresponding field of the matching value. This pattern is the complement to set!. As with set!, this pattern must be nested within a pair, vector, box, or structure pattern.
• Quasipatterns: Quasiquote introduces a quasipattern, in which identifiers are considered to be symbolic constants. Like Scheme's quasiquote for data, unquote (,) and unquote-splicing (,@) escape back to normal patterns.