Is it possible to place inequality constraints on haskell type variables?

Question

Is it possible to place an inequality constraint on the typevariables of a function, à la foo :: (a ~ b) => a -> b as in GHC type family docs, except ineq

Answer 1

From GHC 7.8.1. closed type families are available. The solution is much simpler with them:

data True
data False

type family TypeEqF a b where
  TypeEqF a a = True
  TypeEqF a b = False

type TypeNeq a b = TypeEqF a b ~ False