defpat

7.7

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defpat

(require defpat)

package: defpat

1 defpat

2 match-case-lambda

3 opt-case-lambda

4 opt-match-case-lambda

1 defpat

source code: https://github.com/AlexKnauth/defpat

(require defpat/defpat)

package: defpat

This module provides the forms defpat and pat-lambda.

defpat is a version of define for functions where the arguments can be match patterns.

see also define/match from racket/match and ~define from generic-bind

pat-lambda is a version of lambda where (again) the arguments can be match patterns.

see also match-lambda, match-lambda*, and match-lambda** from racket/match, and ~lambda from generic-bind

syntax
(defpat id expr)
(defpat (head args) body ...+)

head = id
| (head args)

args = arg ...
| arg ... . rest-id

arg = arg-pat
| [arg-pat default-expr]
| keyword arg-pat
| keyword [arg-pat default-expr]

like define, except that each arg-pat can be an arbitrary match pattern.

>

(require defpat/defpat)
>

(defpat (distance (list x1 y1) (list x2 y2))
(sqrt (+ (* (- x2 x1) (- x2 x1))
(* (- y2 y1) (- y2 y1)))))
>

(distance (list 0 0) (list 3 4))
5
>

(distance (list 0 3) (list 4 0))
5

The arg-pat can’t start with a [ though, because square brackets are used to specify optional arguments:

>

(require defpat/defpat)
>

;; If the second point is not specified, it computes the
;; distance to the origin.
(defpat (distance (list x1 y1) [(list x2 y2) (list 0 0)])
(sqrt (+ (* (- x2 x1) (- x2 x1))
(* (- y2 y1) (- y2 y1)))))
>

(distance (list 0 3) (list 4 0))
5
>

(distance (list 3 4))
5

(defpat (head . args) body ...)

expands to

(defpat head (pat-lambda args body ...))

syntax
(pat-lambda kw-formals body ...+)

kw-formals = (arg ...)
| (arg ...+ . rest-id)
| rest-id

arg = arg-pat
| [arg-pat default-expr]
| keyword arg-pat
| keyword [arg-pat default-expr]

like lambda, except that each arg-pat can be an arbitrary match pattern. Just as with defpat, the arg-pat can’t start with a [, and you have to use square brackets to specify an optional argument

It is very similar to match-lambda**, except that it doesn’t support multiple clauses, and it allows optional arguments, keyword arguments, and a rest argument.

As an example,

(pat-lambda ((list x y) (vector z))
body)

expands to

(lambda (%1 %2)
  (match-define (list x y) %1)
  (match-define (vector z) %2)
  body)

and for keyword-arguments,

(pat-lambda (#:kw (list x y))
body)

expands to

(lambda (#:kw %#:kw)
(match-define (list x y) %#:kw)
body)

2 match-case-lambda

(require defpat/match-case-lambda)

package: defpat

syntax
(match*-case-lambda clause ...)

clause = [args body ...+]
| [args (=> id) body ...+]
| [args #:when cond-expr body ...+]

args = [arg-pat ...]
| rest-id
| [arg-pat ... . rest-id]

like case-lambda, except that each arg-pat can be an arbitrary match pattern.

As an example,

(match*-case-lambda
[[(list x y) (vector z)]
body])

is equivalent to

(case-lambda
  [(tmp1 tmp2)
   (match* [tmp1 tmp2]
     [[(list x y) (vector z)]
      body])])

Clauses with the same arity are grouped together into a single case-lambda clause with multiple match* clauses within it.

3 opt-case-lambda

(require defpat/opt-case-lambda)

package: defpat

syntax
(case-lambda/opt clause ...)

clause = [args body ...+]

args = (arg-id ... [arg-id default-expr] ...)
| rest-id
| (arg-id ... [arg-id default-expr] ... . rest-id)

like case-lambda, except that it supports optional arguments.

4 opt-match-case-lambda

(require defpat/opt-match-case-lambda)

package: defpat

syntax
(match*-case-lambda/opt clause ...)

clause = [args body ...+]

args = (arg-pat ... [arg-pat default-expr] ...)
| rest-id
| (arg-pat ... [arg-pat default-expr] ... . rest-id)

like match*-case-lambda, except that it supports optional arguments.

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1	defpat
2	match-case-lambda
3	opt-case-lambda
4	opt-match-case-lambda