This Web

Other Webs

Schematics

RSS

Scheme Links

Pattern Matching

Introduction

Pattern matching is a particular convenient way to specify a case analysis on compound data and at the same time to deconstruct the data.

The essence of pattern matching is to compare the input data with set of patterns and when the first matching pattern is found, to perform an associated action. A typical match expression has the following form:

(match <expression>
  (<pattern_1> <expression_1>)
  ...)

The result of <expression> is compared to the patterns <pattern1> ... in turn, until the first matching pattern is found, then the associated <expression_n> is evaluated. If the pattern <pattern_n> contains pattern variables used to match arbitrary input, then these variables in <expression_n> are bound to the pieces of matched. Example: If the datum (1 2) is matched by the pattern (a b), then an associated expression (+ a b) will evaluate to 3.

There are several pattern matching libraries for Scheme. PLT Scheme comes with two, namely (lib "match.ss" "mzlib") and (lib "plt-match.ss" "mzlib"). The most important difference between them is pattern syntax, where plt-match is open to future enchancements.

You can view pattern matching as being similar to regular expressions, but acting on S-expressions instead of text strings. As in Regular Expressions, patterns allow you to bind parts of the data to variables that can be used in the expression part. Unlike regular expression systems, patterns can be used to do things like

1. Match against the fields of a struct.
2. Match based on the result of evaluating an expression.
3. Use quasiquoting to make pattern templates.

Pattern matching is useful in handling markup (HTML or XML translated to S-Expressions), dispatching functions, and analyzing Scheme code, among other tasks.

Recipes

• How to Match Struct Fields by Name (Recipe)

Other matchers

IU-Match

The pattern matcher original written by Dan Friedman, then rewritten by Erik Hilsdale and later modified by Kent Dybvig.

This pattern matcher is used in (among others) Chez Scheme and SISC. match.ss

A PLT version can by found here

Mayer Goldbers pattern matcher

Goldbergs matcher

A naive pattern matcher implemented with syntax-rules

-- JensAxelSoegaard - 15 Aug 2004

• Set ALLOWTOPICCHANGE = SchematicsEditorsGroup