Just to add one more example to Ralf's reply: We get exactly the same kind of problem if we try *Main> null [fromEnum] <interactive>:1:6: Ambiguous type variable `a' in the constraint: `Enum a' arising from use of `fromEnum' at <interactive>:1:6-13 Probable fix: add a type signature that fixes these type variable(s) Indeed, fromEnum is a polymorphic function |fromEnum :: (Enum a) => a -> Int| The expression [fromEnum] has the type |(Enum a) => [a -> Int]| that carries the constraint Enum a. That constraint has to be eventually resolved somehow. For a top-level expression (not a function), the constraint must be resolved. Granted, the function 'null' could care less what the list is made of, and whether all constraints are resolved. But Haskell does. One may say that instead of inferring the type [fromEnum] :: forall a. (Enum a) => [a -> Int] Haskell should have inferred [fromEnum] :: [forall a. (Enum a) => a -> Int] and the problem would have been solved. There is no longer an (outermost, top-level) constraint to resolve. When GHC does implement MLF and start explicitly supporting impredicative types, the problem would be fixed indeed... Currently, the only way to achieve the same effect is to do this manually: newtype W = W (forall a. (Enum a) => a -> Int) *Main> null [W fromEnum] False Perhaps indeed we should move to Cafe...