Dear all I am sure this is a very beginner problem, but I am not sure how to solve it: I have a myFunct function: myFunct :: Int -> [a] -> a defined as: myFunct _ [] = error "empty list provided as arg" myFunct a [b] | length [x|x<-[b],fid x == a] == 0 = error "no match" | otherwise = head [x|x<-[b],fid x == a] Which works fine (may not be elegant, but works). Now, I am chaining it in a larger function, where at some stage I use a graph object from Data.Graph and check for its edges, resulting in [(Int,Int)] trying to run this fails: myFunct (fst (head (edges graph))) myList *** Exception: FunctStruct.hs:(136,1)-(138,48): Non-exhaustive patterns in function myFunct contrived example, but myList is a list of [a]). The problem arrises from the definition of the type of Edge as (Vertex,Vertex), but type Vertex = Int (http://hackage.haskell.org/packages/archive/containers/0.2.0.1/doc/html/Data...), so that should be ok, or am I making a mistake somewhere and I should go from vertex to Integer explicitly? Thanks a lot! Martin