[Haskell-beginners] help with error

I'm getting an error and I do not know why... This is what I have: class Finite a where elements' :: [a] instance (Finite a, Show a, Show b) => Show (a -> b) where show f = concat (map show [ a | a <- elements'::[a] ]) and this is what ghci gives me: Could not deduce (Finite a1) from the context () arising from a use of `elements'' at Hw06.lhs:119:42-50 Possible fix: add (Finite a1) to the context of an expression type signature In the expression: elements' :: [a] In a stmt of a list comprehension: a <- elements' :: [a] In the second argument of `map', namely `[a | a <- elements' :: [a]]' What's going on here? Thanks for the help! - Tyler