Re: [Haskell-cafe] type/class question: toString

6 Nov 2007

      On Nov 6, 2007 10:30 AM, Bayley, Alistair
 wrote:
...
...
From: haskell-cafe-bounces@haskell.org
[mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Graham Fawcett
Is there a way to declare a 'toString' function, such that
toString x | <x is a String> = x
toString x |  = show x
I'm assuming you're not fond of the way the print function handles
Strings?
More a curiosity about the flexibility of the type system -- but yes,
I could see cases where such a thing could be useful.
...
With GHC you can do this:
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{-# OPTIONS -fallow-overlapping-instances #-}
{-# OPTIONS -fallow-undecidable-instances #-}
class Show a => MyShow a where show_ :: a -> String
instance MyShow String where show_ s = s
instance (Show a) => MyShow a where show_ s = show s
This doesn't want to compile for me:

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.6.1

$ ghc ToString.hs   # your code, verbatim

ToString.hs:5:0:
    Illegal instance declaration for `MyShow String'
        (The instance type must be of form (T a b c)
         where T is not a synonym, and a,b,c are distinct type variables)
    In the instance declaration for `MyShow String'

ToString.hs:6:0:
    Illegal instance declaration for `MyShow a'
        (The instance type must be of form (T a b c)
         where T is not a synonym, and a,b,c are distinct type variables)
    In the instance declaration for `MyShow a'

I'll read up on those two GHC options, and try to figure it out myself
(but hints are welcome).

Thanks,
Graham