Patterns

Patterns are written to look like the printed representation of the objects they match. The basic usage is

(match expr (pat body ...) ...)

where the result of expr is matched against each pattern in turn, and the corresponding body is evaluated for the first to succeed. Thus, a list of three elements matches a list of three elements.

(let ((ls (list 1 2 3))) (match ls ((1 2 3) #t)))

If no patterns match an error is signalled.

Identifiers will match anything, and make the corresponding binding available in the body.

(match (list 1 2 3) ((a b c) b))

If the same identifier occurs multiple times, the first instance will match anything, but subsequent instances must match a value which is equal? to the first.

(match (list 1 2 1) ((a a b) 1) ((a b a) 2))

The special identifier _ matches anything, no matter how many times it is used, and does not bind the result in the body.

(match (list 1 2 1) ((_ _ b) 1) ((a b a) 2))

To match a literal identifier (or list or any other literal), use quote.

(match 'a ('b 1) ('a 2))

Analogous to its normal usage in scheme, quasiquote can be used to quote a mostly literally matching object with selected parts unquoted.

(match (list 1 2 3) (`(1 ,b ,c) (list b c)))

Often you want to match any number of a repeated pattern. Inside a list pattern you can append ... after an element to match zero or more of that pattern (like a regexp Kleene star).

(match (list 1 2) ((1 2 3 ...) #t))

(match (list 1 2 3) ((1 2 3 ...) #t))

(match (list 1 2 3 3 3) ((1 2 3 ...) #t))

Pattern variables matched inside the repeated pattern are bound to a list of each matching instance in the body.

(match (list 1 2) ((a b c ...) c))

(match (list 1 2 3) ((a b c ...) c))

(match (list 1 2 3 4 5) ((a b c ...) c))

More than one ... may not be used in the same list, since this would require exponential backtracking in the general case. However, ... need not be the final element in the list, and may be succeeded by a fixed number of patterns.

(match (list 1 2 3 4) ((a b c ... d e) c))

(match (list 1 2 3 4 5) ((a b c ... d e) c))

(match (list 1 2 3 4 5 6 7) ((a b c ... d e) c))

___ is provided as an alias for ... when it is inconvenient to use the ellipsis (as in a syntax-rules template).

The ..1 syntax is exactly like the ... except that it matches one or more repetitions (like a regexp "+").

(match (list 1 2) ((a b c ..1) c))

(match (list 1 2 3) ((a b c ..1) c))

The boolean operators and, or and notcan be used to group and negate patterns analogously to their Scheme counterparts.

The and operator ensures that all subpatterns match. This operator is often used with the idiom (and x pat) to bind x to the entire value that matches pat(c.f. "as-patterns" in ML or Haskell). Another common use is in conjunction with not patterns to match a general case with certain exceptions.

(match 1 ((and) #t))

(match 1 ((and x) x))

(match 1 ((and x 1) x))

The or operator ensures that at least one subpattern matches. If the same identifier occurs in different subpatterns, it is matched independently. All identifiers from all subpatterns are bound if the or operator matches, but the binding is only defined for identifiers from the subpattern which matched.

(match 1 ((or) #t) (else #f))

(match 1 ((or x) x))

(match 1 ((or x 2) x))

The not operator succeeds if the given pattern doesn't match. None of the identifiers used are available in the body.

(match 1 ((not 2) #t))

The more general operator ? can be used to provide a predicate. The usage is (? predicate pat ...) where predicate is a Scheme expression evaluating to a predicate called on the value to match, and any optional patterns after the predicate are then matched as in an and pattern.

(match 1 ((? odd? x) x))

The field operator = is used to extract an arbitrary field and match against it. It is useful for more complex or conditional destructuring that can't be more directly expressed in the pattern syntax. The usage is (= field pat), where field can be any expression, and should result in a procedure of one argument, which is applied to the value to match to generate a new value to match against pat.

Thus the pattern (and (= car x) (= cdr y)) is equivalent to (x . y), except it will result in an immediate error if the value isn't a pair.

(match '(1 . 2) ((= car x) x))

(match 4 ((= sqrt x) x))

The record operator $ is used as a concise way to match records defined by SRFI-9 (or SRFI-99). The usage is ($ rtd field ...), where rtd should be the record type descriptor specified as the first argument to define-record-type, and each field is a subpattern matched against the fields of the record in order. Not all fields must be present.

(let ()
  (define-record-type employee
    (make-employee name title)
    employee?
    (name get-name)
    (title get-title))
  (match (make-employee "Bob" "Doctor")
    (($ employee n t) (list t n))))

The set! and get! operators are used to bind an identifier to the setter and getter of a field, respectively. The setter is a procedure of one argument, which mutates the field to that argument. The getter is a procedure of no arguments which returns the current value of the field.

(let ((x (cons 1 2))) (match x ((1 . (set! s)) (s 3) x)))

(match '(1 . 2) ((1 . (get! g)) (g)))

The new operator *** can be used to search a tree for subpatterns. A pattern of the form (x *** y) represents the subpattern y located somewhere in a tree where the path from the current object to y can be seen as a list of the form (x ...). y can immediately match the current object in which case the path is the empty list. In a sense it's a 2-dimensional version of the ... pattern.

As a common case the pattern (_ *** y) can be used to search for y anywhere in a tree, regardless of the path used.

(match '(a (a (a b))) ((x *** 'b) x))

(match '(a (b) (c (d e) (f g))) ((x *** 'g) x))

Syntax

turn, according to the pattern rules described in the previous section, until the the first pattern matches. When a match is found, the corresponding _body_s are evaluated in order, and the result of the last expression is returned as the result of the entire match. If a failure is provided, then it is bound to a procedure of no arguments which continues, processing at the next pattern. If no pattern matches, an error is signalled.

Shortcut for lambda + match. Creates a procedure of one argument, and matches that argument against each clause.

Similar to match-lambda. Creates a procedure of any number of arguments, and matches the argument list against each clause.

Matches each var to the corresponding expression, and evaluates the body with all match variables in scope. Raises an error if any of the expressions fail to match. Syntax analogous to named let can also be used for recursive functions which match on their arguments as in match-lambda*.

Similar to match-let, but analogously to letrecmatches and binds the variables with all match variables in scope.

Similar to match-let, but analogously to let*matches and binds the variables in sequence, with preceding match variables in scope.