Mitar, and others
Yes, I've spent most of the last month working on the new type checker, and
unless there's a major hiccup it'll be in GHC 7.0. We'll produce a release
candidate just before ICFP.
However, as it happens both your tests compile with GHC 6.12, if you add
ScopedTypeVariables. The trouble is that your type signatures like (IO n) are
read as (forall n. IO n).
It's a bit unfortunate that there isn't a more coherent warning about this.
Suppose you write
class C a where
op :: a -> a
op = .....(e :: a -> a)....
Then the "a->a" signature means (in Haskell98) forall a. a->a.
I suppose that I could add a warning that says more or less
Warning: type signature implicitly quantifies over 'a',
but if ScopedTypeVariables was on, type variable 'a'
would be in scope here, so the type signature would not
be quantified
It's not trivial to add, but not really hard either. Has anyone else been
bitten by this?
Simon
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