Difference between revisions of "Pattern guard"
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==See also== |
==See also== |
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| − | *[http://research.microsoft.com/en-us/um/people/simonpj/Haskell/guards.html Simon Peyton Jones' note] on pattern guards |
+ | *[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 |
*[https://www.microsoft.com/en-us/research/publication/pattern-guards-and-transformational-patterns/ Pattern Guards and Transformational Patterns]. |
*[https://www.microsoft.com/en-us/research/publication/pattern-guards-and-transformational-patterns/ Pattern Guards and Transformational Patterns]. |
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*[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] |
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Revision as of 16:02, 17 November 2020
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.