| So I would claim that these two types are the same:
|
| forall x. Class x => (forall y. Class y => y -> y) -> x -> x
| (forall y. Class y => y -> y) -> (forall x. Class x => x -> x)
|
| ...so you should be able to do this:
|
| combinator :: (forall y. Class y => y -> y) -> (forall x.
| Class x => x -> x)
| combinator f x = combinator' f x
|
| but for some reason GHC 5.02.2 complains. I think this is a bug.
Indeed the two types are the same. In fact GHC does "forall-lifting"
on type signatures to bring the foralls to the front. But there's a bug
in 5.02's forall-lifting... it doesn't bring the constraints to the
front too.
I fixed this in 5.03 a while ago, but didn't back-propagate the fix to
5.02. And indeed, 5.03 is happy with the pure rank-2 program.
class Class x where
combinator' :: (forall y. Class y => y -> y) -> x -> x
combinator :: (forall y. Class y => y -> y)
-> (forall x. Class x => x -> x)
combinator f = combinator' f
It's quite a bit of extra work propagating fixes into the 5.02 branch,
so I probably won't do so for this one, since only a small minority
of people will trip over it. Perhaps you can try the 5.03 snapshot
release?
Simon
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