Pattern guard
Categories: Glossary | Language extensions | Syntax
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.
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See also
- Simon Peyton Jones' note on pattern guards
- Pattern Guards and Transformational Patterns.