ambiguous type variables in existential classes

[Simon Peyton-Jones]

Hal

I don't quite understand the intuitions behind your program, but the
bug is easy enough:

| instance (Eq e, Foo p) =3D> Foo (Wrap p) where
|   foo (Bar p) e q =3D foo p e q

This instance declaration is guaranteed to give problems if it is
ever used, and GHC should probably bleat about it.  Suppose=20
you have the constraint=20

	Foo (Wrap T)

where T is some type.  Using the instance declaration we can simplify
this to the constraints

	Eq e
	Foo T

where e is a fresh type variable.  But we are never going to learn any
thing more about 'e', and so it is correctly reported as ambiguous.
It's
like writing

	reverse :: Eq x =3D> [a] -> [a]

Any call to this 'reverse' will give a constraint (Eq x) that is
ambiguous.


Really, GHC should reject the instance declaration.

Simon