How to put constraints on type variable of kind `Constraint`?

Question

I\'m playing around with the ConstraintKinds extension of GHC. I have the following data type, which is just a box for things fulfilling some one parameter constrai

Answer 1

The new QuantifiedConstraints extension allows this.

class (a => b) => Implies a b where
instance (a => b) => Implies a b where
instance (forall a. c a `Implies` Show a) => Show (Some c) where
  show (Some x) = show x

Within the body of the Show instance, it is as if there is a

instance forall a. Implies (c a) (Show a)

in scope. If you then have T :: Type and know c T, then the superclass of c T => Show T of the specialized Implies (c T) (Show T) instance allows you to derive Show T. It is necessary to use Implies instead of a straight forall a. c a => Show a constraint. This incorrect constraint acts like

instance forall a. c a => Show a

which overlaps with every Show instance, causing weird breakage. Forcing an indirection through the superclass of an otherwise useless constraint fixes everything.