[Haskell-cafe] type/class question: toString

[Graham Fawcett]

On Nov 6, 2007 10:30 AM, Bayley, Alistair
<Alistair_Bayley at invescoperpetual.co.uk> wrote:
> > From: haskell-cafe-bounces at haskell.org
> > [mailto:haskell-cafe-bounces at 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 | <x's type is an instance of Show> = 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:
> > {-# 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