Difference between revisions of "Pattern guard"

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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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*[http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#PATTERN-GUARDS GHC Manual on pattern guards]
   
 
[[Category:Glossary]]
 
[[Category:Glossary]]

Revision as of 11:24, 5 November 2008

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