Unnecessary prohibition of unquantified higher-order typeclass constraints
|Reported by:||Owned by:|
|Component:||Compiler (Type checker)||Version:||6.6|
|Type of failure:||None/Unknown||Test Case:|
|Related Tickets:||Differential Rev(s):|
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') => ...) -> ...