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egison / egison


Egison is a purely functional programming language with non-linear pattern-matching against non-free data types. We can directly pattern-match against a wide range of data types such as lists, multisets, sets, trees and graphs with Egison.


The Egison Programming Language

Egison is the pattern-matching-oriented, purely functional programming language. We can directly represent pattern-matching against lists, multisets, sets, trees, graphs and any kind of data types. This is the repository of the interpreter of Egison.

For more information, visit our website.

Non-Linear Pattern-Matching against Non-Free Data Types

We can do non-linear pattern-matching against non-free data types in Egison. An non-free data type is a data type whose data have no canonical form, a standard way to represent that object. It enables us to write elegant programs.

Twin Primes

We can use pattern-matching for enumeration. The following code enumerates all twin primes from the infinite list of prime numbers with pattern-matching!

Poker Hands

The following code is the program that determines poker-hands written in Egison. All hands are expressed in a single pattern.


We can write a pattern even against mahjong tiles. We modularize patterns to represent complex mahjong hands.


We can pattern-match against graphs. We can write program to solve the travelling salesman problem in a single pattern-matching expression.

Aren't these exciting? The pattern-matching of Egison is very powerful. We can use it for pattern-matching also against graphs and tree-structures such as XML.

Comparison with Related Work

There are a lot of existing work for pattern-matching.

The advantage of Egison is that it achieves all of the following features at the same time.

  • Modularization of the way of pattern-matching for each data type
  • Pattern-matching with multiple results (backtracking)
  • Non-linear pattern-matching with lexical scoping
  • Parametric polymorphism of pattern-constructors

The Pattern-Matching Mechanism section in Egison developer's manual explains how we achieve that.

Please read our paper on for details.


At first, you should install Haskell Platform.

After you installed Haskell Platform, run the following commands on the terminal.

% cabal update
% cabal install egison

Now, you can try Egison.

% egison
Egison Version X.X.X(C) 2011-2014 Satoshi Egi
Welcome to Egison Interpreter!
> ^D
Leaving Egison Interpreter.

If you are a beginner of Egison, it would be better to install egison-tutorial.

% cabal update
% cabal install egison-tutorial
% egison-tutorial
Egison Tutorial Version 3.3.6 (C) 2013-2014 Satoshi Egi
Welcome to Egison Tutorial!
** Information **
We can use a 'Tab' key to complete keywords on the interpreter.
If we type a 'Tab' key after a closed parenthesis, the next closed parenthesis will be completed.
List of sections in the tutorial.
1: Calculate numbers                             (10 minutes)
2: Basics of functional programming              (10 minutes)
3: Basics of pattern-matching                    (10 minutes)
4: Pattern-matching against infinite collections (5 minutes)
Choose a section to learn.
(1-4): 1
We can do arithmetic operations with '+', '-', '*', '/', 'modulo' and 'power'.

  (+ 1 2)
  (- 30 15)
  (* 10 20)
  (/ 20 5)
  (modulo 17 4)
  (power 2 10)

We can try it also online. Enjoy!

Note for Developers

How to Run Test

% cabal test

How to Profile the Interpreter

% sudo apt-get install haskell-platform-doc haskell-platform-prof
% cabal install --enable-library-profiling --enable-executable-profiling
% egison +RTS -p -RTS -l sample/sequence.egi
% cat


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I thank Ryo Tanaka, Takahisa Watanabe, Takuya Kuwahara and Kentaro Honda for their help to implement the interpreter.


Copyright (c) 2011-2015, Satoshi Egi

Egison is released under the MIT license.

I used husk-scheme by Justin Ethier as reference to implement the base part of the previous version of the interpreter.


Egison is sponsored by Rakuten, Inc. and Rakuten Institute of Technology.