simple type level predicates in haskell (with functional dependencies)

Prolog.hs

{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, UndecidableInstances #-}

module Prolog (Petja, Vasja, Serg, Father, Son, GrandFather) where 

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

class Father f s | s -> f where
	isFather :: f -> s -> ()
	isFather x y = ()

instance Father Vasja Petja
instance Father Petja Serg

class Son s f | s -> f where
	isSon :: s -> f -> ()
	isSon x y = ()

instance (Father f s) => Son s f

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

instance (Father g s, Father s c) => GrandFather g c
-- instance (Son c s, Son s g) => GrandFather g c