[Haskell-cafe] Class

Slavomir Kaslev slavomir.kaslev at gmail.com
Wed Nov 1 18:06:05 EST 2006


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
      `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

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