Consider this:
module Test3 where
class C a b | a -> b where
m :: a -> b
data D a = forall b. (C a b) => MkD a
f :: (C a b) => D a -> b
f (MkD a) = m a
This compiles fine under GHC 5.02.2. But under 5.03, it gives an error:
Model/Test3.hs:9:
Inferred type is less polymorphic than expected
Quantified type variable `b' escapes
When checking an existential match that binds
and whose type is D a -> b1
In the definition of `f': f (MkD a) = m a
I consider that the 5.02.2 behaviour is preferable, and that this is a
perfectly good program. 'b' does not escape because it is fundep on 'a',
which is specified in the type-signature. There can be only one.
What was changed in 5.03 and why?
--
Ashley Yakeley, Seattle WA
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