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Pattern guard

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The idea is to change 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.
 
The idea is to change 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.
   
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*[http://research.microsoft.com/Users/simonpj/Haskell/guards.html Simon Peyton Jones' note] on pattern guards
 
*[http://research.microsoft.com/Users/simonpj/Haskell/guards.html Simon Peyton Jones' note] on pattern guards
 
*[http://research.microsoft.com/%7Esimonpj/Papers/pat.htm Pattern Guards and Transformational Patterns].
 
*[http://research.microsoft.com/%7Esimonpj/Papers/pat.htm Pattern Guards and Transformational Patterns].
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[[Category:Glossary]]
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[[Category:Language]]
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[[Category:Syntax]]

Revision as of 11:42, 8 November 2006

The idea is to change 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.

Stealing a function from the note,

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