Pattern guard
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...-}will check to see if both lookups succeed, and bind the results to val1 and val2 before proceeding to use the equation.