Type family that disallows certain types for type variables.

gistfile1.hs

{-# LANGUAGE ConstraintKinds, DataKinds, PolyKinds, TypeFamilies, TypeOperators #-}

import GHC.Exts (Constraint)

type family Restrict (a :: k) (as :: [k]) :: Constraint where
    Restrict a (a ': as) = ("Error!" ~ "Tried to apply a restricted type!")
    Restrict x (a ': as) = Restrict x as
    Restrict x '[] = ()

foo :: Restrict a [(), Int] => a -> a
foo = id