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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.
   
Stealing a function from the note,
+
From the GHC user's guide,
 
<haskell>
 
<haskell>
  +
lookup :: FiniteMap -> Int -> Maybe Int
  +
 
addLookup env var1 var2
 
addLookup env var1 var2
 
| Just val1 <- lookup env var1
 
| Just val1 <- lookup env var1

Latest 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.

[edit] See also