Editing Guard (computer science) (section)

==Pattern guard==
In addition to a guard attached to a pattern, '''pattern guard''' can refer to the use of [[pattern matching]] in the context of a guard. In effect, a match of the pattern is taken to mean pass. This meaning was introduced in a proposal for Haskell by [[Simon Peyton Jones]] titled [https://web.archive.org/web/20160203203811/http://research.microsoft.com/en-us/um/people/simonpj/Haskell/guards.html A new view of guards] in April 1997 and was used in the implementation of the proposal. The feature provides the ability to use patterns in the guards of a pattern.

An example in extended Haskell:

<syntaxhighlight lang="haskell">
 clunky env var1 var2
 | Just val1 <- lookup env var1
 , Just val2 <- lookup env var2
 = val1 + val2
 -- ...other equations for clunky...
</syntaxhighlight>

This would read: "Clunky for an environment and two variables, ''in case the lookups of the variables from the environment produce values'', is the sum of the values. ..." As in [[list comprehension]]s, the guards are in series, and if any of them fails the branch is not taken.