In <http://www.dcs.gla.ac.uk/~simonpj/multi-param.html>,
Simon L Peyton Jones <[EMAIL PROTECTED]> wrote:
| 5. In the signature of a class operation, every constraint must
| mention at least one type variable that is not a class type
| variable. Thus:
...
| > class C a where
| > op :: Eq a => (a,b) -> (a,b)
|
| is not OK because the constraint (Eq a) mentions on the class type
| variable a, and no others.
What's the rationale for this rule?
| (GHC 3.02 enforces a more restrictive rule, but that's definitely
| wrong.)
| A yet more relaxed rule would allow the context of a class-op
| signature to mention only class type variables. I don't like that
| from an implementation point of view;
Could you be more specific? What's the implementation difficulty?
--
Fergus Henderson <[EMAIL PROTECTED]> | "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh> | of excellence is a lethal habit"
PGP: finger [EMAIL PROTECTED] | -- the last words of T. S. Garp.
