Pattern guard
Revision as of 21:27, 24 November 2009 by Henk-Jan van Tuyl (talk | contribs) (Changed introduction to mention Haskell 2010; corrected links, added link to the Haskell Prime wiki page of Pattern Guards)
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.
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.