{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, FlexibleContexts, OverlappingInstances, UndecidableInstances #-} {- The HList library (C) 2004, Oleg Kiselyov, Ralf Laemmel, Keean Schupke A generic implementation of a type equality predicate. The given implementation only works for GHC. It relies on two properties of GHC instance selection: (i) selection is lazy, and the negation of the constraints of the more specific instance is assumed for the more general instance. The specific encoding given here makes use of TypeCast, and by transitive closure therefore relies on separate compilation of TypeCast clients and the TypeCast instance. There is another encoding in TypeEqGeneric2.hs. -} module Data.HList.TypeEqGeneric1 where import Data.HList.FakePrelude instance TypeEq x x HTrue instance (HBool b, TypeCast HFalse b) => TypeEq x y b -- instance TypeEq x y HFalse -- would violate functional dependency class HBool b => TupleType t b | t -> b instance TupleType () HTrue instance TupleType (x,y) HTrue instance TupleType (x,y,z) HTrue -- Continue for a while instance (HBool b, TypeCast HFalse b) => TupleType x b -- instance TupleType x HFalse -- would violate functional dependency