Pattern guard
From HaskellWiki
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.
From the GHC user's guide,
lookup :: FiniteMap -> Int -> Maybe Int addLookup env var1 var2 | Just val1 <- lookup env var1 , Just val2 <- lookup env var2 = val1 + val2 {-...other equations...-}
val1
val2
