Editing Pattern matching (section)

===Primitive patterns===
The simplest pattern in pattern matching is an explicit value or a variable. For an example, consider a simple function definition in Haskell syntax (function parameters are not in parentheses but are separated by spaces, = is not assignment but definition):

<syntaxhighlight lang="haskell">
f 0 = 1
</syntaxhighlight>

Here, 0 is a single value pattern. Now, whenever f is given 0 as argument the pattern matches and the function returns 1. With any other argument, the matching and thus the function fail. As the syntax supports alternative patterns in function definitions, we can continue the definition extending it to take more generic arguments:

<syntaxhighlight lang="haskell">
f n = n * f (n-1)
</syntaxhighlight>

Here, the first <code>n</code> is a single variable pattern, which will match absolutely any argument and bind it to name n to be used in the rest of the definition. In Haskell (unlike at least [[Hope (programming language)|Hope]]), patterns are tried in order so the first definition still applies in the very specific case of the input being 0, while for any other argument the function returns <code>n * f (n-1)</code> with n being the argument.

The wildcard pattern (often written as <code>_</code>) is also simple: like a variable name, it matches any value, but does not bind the value to any name. Algorithms for [[matching wildcards]] in simple string-matching situations have been developed in a number of [[recursion|recursive]] and non-recursive varieties.<ref>{{cite web| last=Cantatore| first=Alessandro| title=Wildcard matching algorithms| year=2003| url=http://xoomer.virgilio.it/acantato/dev/wildcard/wildmatch.html}}</ref>