Difference between revisions of "Pattern guard"

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==See also==
 
==See also==
 
*[http://research.microsoft.com/en-us/um/people/simonpj/Haskell/guards.html Simon Peyton Jones' note] on pattern guards
 
*[http://research.microsoft.com/en-us/um/people/simonpj/Haskell/guards.html Simon Peyton Jones' note] on pattern guards
*[http://research.microsoft.com/en-us/um/people/simonpj/Papers/pat.htm Pattern Guards and Transformational Patterns].
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*[https://www.microsoft.com/en-us/research/publication/pattern-guards-and-transformational-patterns/ Pattern Guards and Transformational Patterns].
 
*[https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#pattern-guards GHC Manual on pattern guards]
 
*[https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html#pattern-guards GHC Manual on pattern guards]
 
* [http://hackage.haskell.org/trac/haskell-prime/wiki/PatternGuards#BriefExplanation The Haskell Prime wiki page of Pattern Guards]
 
* [http://hackage.haskell.org/trac/haskell-prime/wiki/PatternGuards#BriefExplanation The Haskell Prime wiki page of Pattern Guards]

Revision as of 14:53, 7 September 2016

Haskell 2010 changes the syntax for guards by replacing the use of a single condition with a list of qualifiers. These qualifiers, which include both conditions and pattern guards of the form pat <- exp, serve to bind/match patterns against expressions. The syntax is comparable that of a list comprehension, where instead the types of pat and exp match. This makes it easy, for instance, to pattern match against (possibly failing) table lookups while deciding which definition of a function to use.

From the GHC user's guide,

lookup :: FiniteMap -> Int -> Maybe Int

addLookup env var1 var2
   | Just val1 <- lookup env var1
   , Just val2 <- lookup env var2
   = val1 + val2
{-...other equations...-}

will check to see if both lookups succeed, and bind the results to val1 and val2 before proceeding to use the equation.

See also