simple type level predicates in haskell

Prolog.hs

{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, UndecidableInstances #-}
module Prolog (Petja, Vasja, Serg, Father) where 

data Petja = Petja
data Vasja = Vasja
data Serg = Serg

class Father a b where
	isFather :: a -> b -> ()
	isFather x y = ()
	  

instance Father Vasja Petja
instance Father Petja Serg

class Son a b where
	isSon :: a -> b -> ()
	isSon x y = ()


instance (Father b a) => Son a b

class GrandFather a b c where
	isGrandFather :: a -> c -> ()
	isGrandFather x y = ()

instance (Father a b, Father b c) => GrandFather a b c