Difference between revisions of "Pattern guard"

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The idea is to change the syntax for [[guard]]s 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.
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Haskell 2010 changes the syntax for [[guard]]s 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.
   
Stealing a function from the note,
 
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From the GHC user's guide,
 
<haskell>
 
<haskell>
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lookup :: FiniteMap -> Int -> Maybe Int
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addLookup env var1 var2
 
addLookup env var1 var2
 
| Just val1 <- lookup env var1
 
| Just val1 <- lookup env var1
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==See also==
 
==See also==
*[http://research.microsoft.com/Users/simonpj/Haskell/guards.html Simon Peyton Jones' note] on pattern guards
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*[https://web.archive.org/web/20170109011924/http://research.microsoft.com/en-us/um/people/simonpj/Haskell/guards.html Simon Peyton Jones' note] on pattern guards
*[http://research.microsoft.com/%7Esimonpj/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].
*[http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#PATTERN-GUARDS GHC Manual on pattern guards]
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*{{GHCUsersGuide|exts/pattern_guards||a very short section on PatternGuards}}
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* [https://gitlab.haskell.org/haskell/prime/-/wikis/PatternGuards The Haskell Prime wiki page of Pattern Guards]
   
 
[[Category:Glossary]]
 
[[Category:Glossary]]

Latest revision as of 23:22, 24 July 2021

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