On Mon, Feb 06, 2006 at 01:53:17PM +0100, Doaitse Swierstra wrote:
I have not followed this completely, but do these new rules now allow:
class F a b c where f: a -> b -> c
and then
instance F a a a where
Yes. Indeed they allow any unconstrained instance. They would also allow instance C a a => F a a a
On the other hand:
instance F [a] [a] [a] where ... posed no problem, although I still see no non-type-variable in the head, or is [] a non-type-variable? I assumed the complaint was a left-over from the single parameter classes, where indeed having a single variable in an instance declaration does not make much sense. The second example does not follow the old rule (2) below!
The argument [a] is not a type variable, so this is OK under the old rule (2).