The `(ice-9 match)`

module provides a *pattern matcher*,
written by Alex Shinn, and compatible with Andrew K. Wright’s pattern
matcher found in many Scheme implementations.

A pattern matcher can match an object against several patterns and
extract the elements that make it up. Patterns can represent any Scheme
object: lists, strings, symbols, records, etc. They can optionally contain
*pattern variables*. When a matching pattern is found, an
expression associated with the pattern is evaluated, optionally with all
pattern variables bound to the corresponding elements of the object:

(let ((l '(hello (world)))) (match l ;; <- the input object (('hello (who)) ;; <- the pattern who))) ;; <- the expression evaluated upon matching ⇒ world

In this example, list `l` matches the pattern `('hello (who))`

,
because it is a two-element list whose first element is the symbol
`hello`

and whose second element is a one-element list. Here
`who` is a pattern variable. `match`

, the pattern matcher,
locally binds `who` to the value contained in this one-element
list—i.e., the symbol `world`

. An error would be raised if
`l` did not match the pattern.

The same object can be matched against a simpler pattern:

(let ((l '(hello (world)))) (match l ((x y) (values x y)))) ⇒ hello ⇒ (world)

Here pattern `(x y)`

matches any two-element list, regardless of
the types of these elements. Pattern variables `x` and `y` are
bound to, respectively, the first and second element of `l`.

Patterns can be composed, and nested. For instance, `...`

(ellipsis) means that the previous pattern may be matched zero or more
times in a list:

(match lst (((heads tails ...) ...) heads))

This expression returns the first element of each list within `lst`.
For proper lists of proper lists, it is equivalent to ```
(map car
lst)
```

. However, it performs additional checks to make sure that
`lst` and the lists therein are proper lists, as prescribed by the
pattern, raising an error if they are not.

Compared to hand-written code, pattern matching noticeably improves
clarity and conciseness—no need to resort to series of `car`

and
`cdr`

calls when matching lists, for instance. It also improves
robustness, by making sure the input *completely* matches the
pattern—conversely, hand-written code often trades robustness for
conciseness. And of course, `match`

is a macro, and the code it
expands to is just as efficient as equivalent hand-written code.

The pattern matcher is defined as follows:

- Scheme Syntax:
**match**`exp clause1 clause2 …`¶ Match object

`exp`against the patterns in`clause1``clause2`… in the order in which they appear. Return the value produced by the first matching clause. If no clause matches, throw an exception with key`match-error`

.Each clause has the form

`(pattern body1 body2 …)`

. Each`pattern`must follow the syntax described below. Each body is an arbitrary Scheme expression, possibly referring to pattern variables of`pattern`.

The syntax and interpretation of patterns is as follows:

patterns: matches: pat ::= identifier anything, and binds identifier | _ anything | () the empty list | #t #t | #f #f | string a string | number a number | character a character | 'sexp an s-expression | 'symbol a symbol (special case of s-expr) | (pat_1 ... pat_n) list of n elements | (pat_1 ... pat_n . pat_{n+1}) list of n or more | (pat_1 ... pat_n pat_n+1 ooo) list of n or more, each element of remainder must match pat_n+1 | #(pat_1 ... pat_n) vector of n elements | #(pat_1 ... pat_n pat_n+1 ooo) vector of n or more, each element of remainder must match pat_n+1 | #&pat box | ($ record-name pat_1 ... pat_n) a record | (= field pat) a ``field'' of an object | (and pat_1 ... pat_n) if all of pat_1 thru pat_n match | (or pat_1 ... pat_n) if any of pat_1 thru pat_n match | (not pat_1 ... pat_n) if all pat_1 thru pat_n don't match | (? predicate pat_1 ... pat_n) if predicate true and all of pat_1 thru pat_n match | (set! identifier) anything, and binds setter | (get! identifier) anything, and binds getter | `qp a quasi-pattern | (identifier *** pat) matches pat in a tree and binds identifier to the path leading to the object that matches pat ooo ::= ... zero or more | ___ zero or more | ..1 1 or more quasi-patterns: matches: qp ::= () the empty list | #t #t | #f #f | string a string | number a number | character a character | identifier a symbol | (qp_1 ... qp_n) list of n elements | (qp_1 ... qp_n . qp_{n+1}) list of n or more | (qp_1 ... qp_n qp_n+1 ooo) list of n or more, each element of remainder must match qp_n+1 | #(qp_1 ... qp_n) vector of n elements | #(qp_1 ... qp_n qp_n+1 ooo) vector of n or more, each element of remainder must match qp_n+1 | #&qp box | ,pat a pattern | ,@pat a pattern

The names `quote`

, `quasiquote`

, `unquote`

,
`unquote-splicing`

, `?`

, `_`

, `$`

, `and`

,
`or`

, `not`

, `set!`

, `get!`

, `...`

, and
`___`

cannot be used as pattern variables.

Here is a more complex example:

(use-modules (srfi srfi-9)) (let () (define-record-type person (make-person name friends) person? (name person-name) (friends person-friends)) (letrec ((alice (make-person "Alice" (delay (list bob)))) (bob (make-person "Bob" (delay (list alice))))) (match alice (($ person name (= force (($ person "Bob")))) (list 'friend-of-bob name)) (_ #f)))) ⇒ (friend-of-bob "Alice")

Here the `$`

pattern is used to match a SRFI-9 record of type
`person` containing two or more slots. The value of the first slot
is bound to `name`. The `=`

pattern is used to apply
`force`

on the second slot, and then checking that the result
matches the given pattern. In other words, the complete pattern matches
any `person` whose second slot is a promise that evaluates to a
one-element list containing a `person` whose first slot is
`"Bob"`

.

The `(ice-9 match)`

module also provides the following convenient
syntactic sugar macros wrapping around `match`

.

- Scheme Syntax:
**match-lambda**`clause1 clause2 …`¶ Create a procedure of one argument that matches its argument against each clause, and returns the result of evaluating the corresponding expressions.

(match-lambda clause1 clause2 …) ≡ (lambda (arg) (match arg clause1 clause2 …))

((match-lambda (('hello (who)) who)) '(hello (world))) ⇒ world

- Scheme Syntax:
**match-lambda***`clause1 clause2 …`¶ Create a procedure of any number of arguments that matches its argument list against each clause, and returns the result of evaluating the corresponding expressions.

(match-lambda* clause1 clause2 …) ≡ (lambda args (match args clause1 clause2 …))

((match-lambda* (('hello (who)) who)) 'hello '(world)) ⇒ world

- Scheme Syntax:
**match-let**`((pattern expression) …) body`¶ Match each pattern to the corresponding expression, and evaluate the body with all matched variables in scope. Raise an error if any of the expressions fail to match.

`match-let`

is analogous to named let and can also be used for recursive functions which match on their arguments as in`match-lambda*`

.(match-let (((x y) (list 1 2)) ((a b) (list 3 4))) (list a b x y)) ⇒ (3 4 1 2)

- Scheme Syntax:
**match-let**`variable ((pattern init) …) body`¶ Similar to

`match-let`

, but analogously to*named let*, locally bind VARIABLE to a new procedure which accepts as many arguments as there are INIT expressions. The procedure is initially applied to the results of evaluating the INIT expressions. When called, the procedure matches each argument against the corresponding PATTERN, and returns the result(s) of evaluating the BODY expressions. See Iteration, for more on*named let*.

- Scheme Syntax:
**match-let***`((variable expression) …) body`¶ Similar to

`match-let`

, but analogously to`let*`

, match and bind the variables in sequence, with preceding match variables in scope.(match-let* (((x y) (list 1 2)) ((a b) (list x 4))) (list a b x y)) ≡ (match-let (((x y) (list 1 2))) (match-let (((a b) (list x 4))) (list a b x y))) ⇒ (1 4 1 2)

- Scheme Syntax:
**match-letrec**`((variable expression) …) body`¶ Similar to

`match-let`

, but analogously to`letrec`

, match and bind the variables with all match variables in scope.

Guile also comes with a pattern matcher specifically tailored to SXML
trees, See `sxml-match`

: Pattern Matching of SXML.