I'm just catching up with some old mail here.
Iavor writes:
| class C a b | a -> b
| class C a b => D a
|
| vs.
|
| class C a b | a -> b
| class C a b => D a b
|
| Hugs accepts both of those, while GHC insists on the second.
| The first example is a little shorter and one might argue that if we
| know "a" we also know "b", because of the functional depency,
| so it is not really a parameter.
|
| On the other hand, i would expect writing "b" in the type
| signatures of any of the methods to refer to the particular "b"
| determined by the "a", rather than it becomeing universally
| quantified, and perhaps this is more explicit in the second form.
Dead right! Imagine there was a method in class D:
class C a b => D a where
op :: a -> b
The type of 'op' is
op :: D a => a -> b
You can't really expect that the 'b' here is determined by 'a'!
Still, I suppose that if the methods in class D mention
only 'a', then it is strange to require D to be parameterised over
'b'. Thus, this seems more reasonable:
class C a b => D a where
op :: a -> a
Even this seems odd, because we would get the deduction
D a |- C a b
(i.e. from a (D a) dictionary you can get a (C a b) dictionary)
and that's only true for a particular 'b'. My brain is too small
to be sure what I'm talking about here, but I'm strongly inclined
to be conservative here, which GHC is.
Simon
_______________________________________________
Haskell mailing list
[EMAIL PROTECTED]
http://www.haskell.org/mailman/listinfo/haskell
