Unnecessary prohibition of unquantified higher-order typeclass constraints

In GHC 6.6, attempting to declare a function (say f) with a type of the form

forall t. ... -> (Num t => ...) -> ...

produces the error:

All of the type variables in the constraint `Num t' are already in scope
    (at least one must be universally quantified here)

This used to make sense -- after all, there's no way f can deduce a Num constraint on t that wasn't provided by its caller, so a higher-order argument with this sort of type is uncallable. But with the addition of GADTs, this is no longer true, since another of f's arguments might, when deconstructed, reveal that t is in fact an Int or a Ratio u. In fact, GADT deconstructors can have types like this. E.g.

data Foo a where
  Bar :: forall a. Num a => a -> Foo a

has a deconstructor of type

forall a r. Foo a -> (Num a => a -> r) -> r

So I think that these types should be allowed, although I haven't found a use for them yet except for writing GADT deconstructors.

On the other hand, one can't write deconstructors for most GADTs even with this change, so perhaps it's the wrong change. If explicit type equality constraints were allowed in Haskell, then I could write

forall t. ... -> (forall t'. (t ~ t', Num t') => ...) -> ...

instead.

Trac metadata

Trac field	Value
Version	6.6
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Multiple
Architecture	Multiple