Pattern patching library for Common Lisp.
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README.md
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README.md

Match

A lightweight, extensible pattern matching library for Common Lisp

Created by Michael Dickens.

See the .lisp files for full documentation of individual macros and functions.

Installation

To install Match, simply clone the repository and begin using the library. Load Match by putting (load "path/to/match.lisp") in your Lisp file.

All of the macros and functions in match.lisp are defined in the package :match. If you want to use them without adding the match: prefix, you must import them.

Features

Match Predicate matchp

Use matchp to determine whether an expression matches a form. If you write new patterns using defpattern, matchp will automatically know how to use these patterns.

matchp implements the core functionality of Match, but it isn't very useful by itself. The real benefits come from match.

Match Form match

match provides a structure similar to cond or case: given an expression and a series of forms, it attempts to match the expression against each form. If it finds a match, it executes a given expression. It looks very similar to case on the surface, but the functionality of match goes beyond case in that it allows binding new variables: a variable used in a match form is bound to the corresponding value in the expression. This functionality may be used to greatly simplify code.

Consider a simple example -- the recursive implementation of the factorial function that we all know and love:

(defun factorial (n)
  (if (= n 0)
      1
      (* n (factorial (1- n)))))

In Haskell, we might implement factorial using pattern matching:

factorial :: Int -> Int
factorial 0 = 1
factorial n = n * (factorial (n - 1))

The Match library allows us to do something similar using Lisp.

(defun factorial (x)
  (match x
    (0 1)
    (n (* n (factorial (1- n))))))

Using the special defmatch macro, we can simplify this further:

(defmatch factorial
  (0 1)
  (n (* n (factorial (1- n)))))

Note: match sometimes automatically initializes variables that may be used in each form. If you do not use these variables, you may get a style warning to that effect.

Feedback

I would love to hear any comments you may have about Match. I'm also interested to hear if you add any functionality or write interesting new patterns. Leave a comment on the project's GitHub page: https://github.com/michaeldickens/Match