Hello, I am writing a function which returns an exception/error string should an unexpected parameter is passed in. Borrowing from the Maybe module I have the following: ------------------------------------------------------------ data Result a = Ex String | Value a deriving Show -- Testing the Result a type first_h :: [a] -> Result a first_h [] = Ex "No list" first_h (x:xs) = Value x -- Trying out the first_h main :: IO() main = print (first_h []) ------------------------------------------------------------ Which the compiler complains: Ambiguous type variable(s) `a' in the constraint `Show a' arising from use of `print' at atype.hs:8 In the definition of `main': print (first_h []) This is understandable since it is unable to unify the empty list with a concrete list of type 'a' i.e. there are an infinite types which would match. My question is how can I indicate to the compiler that it does not matter which type the empty list is since the return result is of a concreate type. Thanks Tee