[Haskell-cafe] type/class question: toString

Graham Fawcett graham.fawcett at gmail.com
Tue Nov 6 16:23:22 EST 2007

On Nov 6, 2007 3:29 PM, Graham Fawcett <graham.fawcett at gmail.com> wrote:
> On Nov 6, 2007 2:21 PM, Jeff Polakow <jeff.polakow at db.com> wrote:
> >   Have you tried using -fglasgow-exts? That should enable all ghc
> > extensions.

If anyone's interested, I had best results when I added the flag
-fallow-incoherent-instances. Without it, I could not handle numbers
without declaring their types, e.g. 'toString (33 :: Int)' would work,
but 'toString 33' would lead to:

    Ambiguous type variable `t' in the constraints:
      `ToString t'
        arising from use of `toString'
        at /home/graham/tmp/ToString.hs:13:15-25
      `Num t'
        arising from the literal `33'
        at /home/graham/tmp/ToString.hs:13:24-25
    Probable fix: add a type signature that fixes these type variable(s)

Here's the code I ended up with.

{-# OPTIONS -fglasgow-exts -fallow-overlapping-instances #-}
{-# OPTIONS -fallow-incoherent-instances -fallow-undecidable-instances #-}

module ToString (ToString(..)) where

class Show a => ToString a      where toString :: a -> String
instance ToString String        where toString s = s
instance (Show a) => ToString a where toString s = show s

Thanks to all who responded; I learned a lot from this.


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