[Haskell-beginners] How do I return Nothing from a function?

How do I return Nothing from a function? When there is no edge in the graph this code fails. *** Exception: Graph.hs:65:9-26: Irrefutable pattern failed for pattern (Data.Maybe.Just w) *Main> :t weight weight :: (Ix t1, Ix t2) => t1 -> t2 -> Array (t1, t2) (Maybe t) -> t weight x y g = w where (Just w) = g!(x,y) Thank you for your time. -- -- Sent from an expensive device which will be obsolete in a few months! :D Casey