Difference between revisions of "Pattern guard"
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(Changed introduction to mention Haskell 2010; corrected links, added link to the Haskell Prime wiki page of Pattern Guards) |
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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. |
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. |
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| + | From the GHC user's guide, |
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| − | Stealing a function from the note, |
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<haskell> |
<haskell> |
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| + | lookup :: FiniteMap -> Int -> Maybe Int |
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| + | |||
addLookup env var1 var2 |
addLookup env var1 var2 |
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| Just val1 <- lookup env var1 |
| Just val1 <- lookup env var1 |
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Revision as of 20:48, 9 May 2013
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.