pattern-match-lambda for R7RS-Scheme

Introduction

This package is easy pattern match library for R7RS. It provides only one macro named pattern-match-lambda.

Install

This library is R7RS portable. However, only the install script for gauche is available.

./configure
make install

For another Scheme implementation, copy pattern-match-lambda.sld to appropriate path.

How to use

The following examples shows how to use pattern-match-lambda.

(define fact
  (pattern-match-lambda ()
   ((0) 1)
   ((n) (* n (fact (- n 1))))))

(fact 5) ;; -> 120

(define string-input?
  (pattern-match-lambda ()
    ((x) (string? x) x)
    ((x) 'not-string)))

(string-input? "ok") ;; -> "ok"
(string-input? 'ng)  ;; ->  not-string

test.scm contains more examples.

Syntax

The pattern-match-lambda's syntax is the following.

(pattern-match-lambda (<pattern literal> ...) <clause> ...)

(_<pattern literal>_ ...) must be either null or a list of unique identifiers. If this is not null, then specified identifiers are treated as if they are keyword of the pattern-match-lambda macro.

(define literal
  (pattern-match-lambda (foo)
    ((foo x) x)
    ((_   x) 'ng)))

(literal 'foo 'x) ;; -> x
(literal 'bar 'x) ;; -> ng

Literal matching are done against mere symbols not syntax identifiers. This is because pattern-match-lambda macro creates an procedure not a macro.

<clause> must have one of the following forms:

• (<pattern> <expr>)
• (<pattern> <fender> <expr>)

A <pattern> is either an identifier, a constant, or one of the followings.

(<pattern> ...)
(<pattern> <pattern> ... . <pattern>)
#(<pattern> ...)

A <fender> is an expression which is evaluated when <pattern> is matched. If the result of evaluation is true value, then the following <expr> is evaluated, otherwise pattern-match-lambda continues matching. The following example shows how it works:

(define foo
  (pattern-match-lambda ()
    ((a) (eq? a 'fender) 'fender)
    ((a) 'fallback)))

(foo 'fender) ;; -> fender
(foo 'a)      ;; -> fallback

Semantics

A pattern-match-lambda expression evaluates to a procedure that accepts a variable number of arguments and is lexically scoped in the same manner as a procedure resulting from a lambda expression. When the procedure is called, the first <clause> for which the arguments match with <pattern> is selected, where argument is specified as for the <pattern> of a syntax-rules like expression.

Difference between <pattern> of syntax-rules and pattern-match-lambda is ellipsis. Ellipsis is not able to use in pattern-match-lambda's <pattern>.

The variables of <pattern> are bound to fresh locations, the values of the arguments are stored in those locations, the <expr> is evaluated in the extended environment, and the results of <expr> are returned as the results of the procedure call. It is an error for the arguments not to match with the <pattern> of any <clause>.

An identifier appearing within a <pattern> can be an underscore (_), a literal identifier listed in the list of <pattern-literal>. All other identifiers appearing within a <pattern> are variables.

Variables in <pattern> match arbitrary input elements and are used to refer to elements of the input in the body. It is an error for the same variable to appear more than once in a <pattern>. Underscores also match arbitrary input elements but are not variables and so cannot be used to refer to those elements. If an underscore appears in the <pattern literal> list, then that takes precedence and underscores in the <pattern> match as literals. Multiple underscores can appear in a <pattern>.

Identifiers that appear in (_<pattern literal>_ ...) are interpreted as literal identifiers to be matched against corresponding elements of the input. An element in the input matches a literal identifiers if and only if it is an symbol and equal to literal identifier in the sense of the eqv? procedure.

License

Copyright (c) 2014 SAITO Atsushi

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
3. Neither the name of the authors nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.