Convert a list of constraints to one

conj_constraints.hs

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

import GHC.Exts

type A a = Num a ': Eq a ': Ord a ': '[]

type family ListOfConstraints (a :: [Constraint]) :: Constraint where
  ListOfConstraints '[] = ()
  ListOfConstraints (c ': cs) = (c, ListOfConstraints cs)

f :: ListOfConstraints (A a) => a -> a -> a
f x y = max x y * 3