superconstraints

A way of inferring instance constraints given an instance.

Suppose you have a class like this:

class C a

instance C a => C [a]

Normally, given C [a], you cannot obtain C a; however, superconstraints allows you to add that capability:

class HasSuper (C a) => C a

instance C a => C [a]
makeSuper "C [a]"

Then, you can retrieve the superconstraint by doing:

case super (Proxy :: Proxy (C [a])) of
  Dict -> ...

The superconstraint dictionary will include all of the constraints required by the instance.

Future Improvements

• Replace the crazy string argument to makeSuper with something more sensible