Haskell' - class aliases

[Wolfgang Jeltsch]

Am Donnerstag, 24. April 2008 21:27 schrieb John Meacham:
> On Thu, Apr 24, 2008 at 08:48:15PM +0200, Wolfgang Jeltsch wrote:
> […]

> > I also have some remark: Why not write
> >
> > > class Eq a => Num a = (Additive a, Multiplicative a)
> >
> > instead of
> >
> > > class Num a = Eq a => (Additive a, Multiplicative a)
>
> Well, because you can think of 'Num a' as an alias for 'Eq a =>
> (Additive a, Multiplicative a)', not that Eq is a superclass of Num
> which the class declaration syntax implies.

Hmm, in what way is Num a an alias for Eq a => (Additive a, Multiplicative a)?  
You cannot write this:

> square :: (Eq a => (Additive a, Multiplicative a)) => a -> a

I would say: “Under the condition that Eq a holds, Num a is an alias for 
(Additive a, Multiplicative a).  And this seems to be perfectly expressed by 
my above proposal.

Best wishes,
Wolfgang