Hello. I am new to Haskell and I am going through "Haskell: The craft of functional programming". I am trying to grasp haskell's classes and instances, so here is slightly modified code from the book: class Show a => Visible a where toString :: a -> String toString = show size :: a -> Int size = length . show instance Visible a => Visible [a] where toString = concat . map toString size = foldl (+) 0 . map size vSort :: (Visible a, Ord a) => [a] -> String vSort = toString . List.sort s = vSort [1..3] Unfortunetly in ghc it gives the following type error: Ambiguous type variable `a' in the constraints: `Visible a' arising from use of `vSort' at d:/tmp.hs:83:4-8 `Enum a' arising from the arithmetic sequence `1 .. 3' at d:/tmp.hs:83:10-15 `Num a' arising from the literal `3' at d:/tmp.hs:83:14 `Ord a' arising from use of `vSort' at d:/tmp.hs:83:4-8 Probable fix: add a type signature that fixes these type variable(s) Failed, modules loaded: none. As you can see, Visible is nothing more than an adapter to the Show class. How I got thing so far, [1..3] :: (Num a, Enum a) => [a], has a Show instance so does class Num (which 'subclasses' Show). Therefore, I can't see any reason why toString function can't call show from those instances. Can someone please enlighten my (still) C++ thinking head? -- Slavomir Kaslev