Hello,
GHC 6.12.3 allows to omit the explicit quantification of
higher-rank type variables using 'forall' in data types if they
appear in a type class context
{-# LANGUAGE RankNTypes #-}
data Foo = Foo (Eq a => a)
Is this implicit introduction of 'forall' intended? If it is, why
does it not work in function types? The following is not accepted
by my GHC:
bar :: Eq b => (Eq a => a) -> b
bar x = x
The error message is
All of the type variables in the constraint `Eq a'
are already in scope (at least one must be universally quantified
here)
(Use -XFlexibleContexts to lift this restriction)
Using `FlexibleContexts` the signature of `bar` seems to be
interpreted as
bar :: (Eq b, Eq a) => a -> b
because then the error becomes
Couldn't match expected type `b' against inferred type `a'
So unlike in data-type declarations, a 'forall' in a function type
must be written explicitly even if the quantified variable appears in
a local type class constraint.
bar :: Eq b => (forall a . Eq a => a) -> b
bar x = x
I have not yet installed GHC 7. Is this inconsistency between data and
function declarations intended or has it been changed in the new type
checker?
Sebastian
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