Pattern guard

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Revision as of 14:57, 23 June 2007

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...-}

See also

• Simon Peyton Jones' note on pattern guards
• Pattern Guards and Transformational Patterns.

Categories: Glossary | Language extensions | Syntax