[Haskell-cafe] Data types and Haskell classes

[Dylan Thurston]

On Tue, May 17, 2005 at 01:13:17PM +0200, Jens Blanck wrote:
> > > How would I introduce number classes that are extended with plus and
> > > minus infinity? I'd like to have polymorphism over these new classes,
> > > something like a signature
> > >
> > > f :: (Real a, Extended a b) => b -> b
> > >
> > > which clearly is not part of the current syntax, but I hope you get
> > > the picture. What are the elegant ways of doing this?
> > 
> > How about
> >     f :: Real a => Extended a -> Extended a
>  
> Not quite what I had in mind. I'd like to have extended integers and
> extended rationals, and possibly extended dyadic numbers. So I can't
> have just a single type ExtendedRational (unless I'm prepared to do
> some ugly coersing).

You're missing the point.  Try:

> data Extended a = PlusInf | NegInf | Finite a

Peace,
	Dylan
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