{-# OPTIONS -fglasgow-exts #-}
{-# OPTIONS -fallow-overlapping-instances #-}
{-# OPTIONS -fallow-undecidable-instances #-}

module FooModule(Dependent(..), Abstract(..), Concrete(..)) where

class Dependent a

class (Dependent b) => Abstract a b | a -> b

class Abstract a b => Concrete a b where
	foo :: a -> String
	foo x = bar x
	bar :: a -> String
	bar _ = "default"

instance (Show a, Abstract a b) => Concrete a b where
	foo x = show x

{--
-- Uncommenting this solves the problem:
data Dummy = Dummy
instance Dependent Dummy
instance Abstract Dummy Dummy
instance Concrete Dummy Dummy
--}

{--
-- Uncommenting this results in error:
instance (Abstract a b) => Concrete a b 
--}