Hello all,

Please consider the following code:

{-# OPTIONS -Wall                 #-}
{-# LANGUAGE FlexibleContexts     #-}
{-# LANGUAGE FlexibleInstances    #-}
{-# LANGUAGE TypeOperators        #-}
{-# LANGUAGE TypeFamilies         #-}
{-# LANGUAGE OverlappingInstances #-}
{-# LANGUAGE UndecidableInstances #-}

module Test where

-- Some view
class Viewable a where
type View a
to   :: a -> View a

-- Structural representations
data Unit    = Unit
data a :+: b = L a | R b

instance Viewable Unit where
type View Unit = Unit
to = id

instance (Viewable a, Viewable b) => Viewable (a :+: b) where
type View (a :+: b) = a :+: b
to = id

-- Worker class
class F' a where
f' :: a -> ()

instance F' Unit where
f' Unit = ()

instance (F a, F b) => F' (a :+: b) where
f' (L x) = f x
f' (R x) = f x

-- Dispatcher class
class (Viewable a, F' (View a)) => F a where
f :: a -> ()
f = f' . to

instance F Unit where
f = f'

instance (F a, F b) => F (a :+: b) where
f = f'

-- All generic instances
instance (Viewable a, F' (View a)) => F a

-- A recursive datatype
data Nat = Zero | Succ Nat

-- Instance of Viewable
instance Viewable Nat where
type View Nat = Unit :+: Nat
to = undefined

-- Uncommenting the line below causes the typechecker to loop (GHC 6.10.1, Windows)
--test = f Zero

Is this expected behavior or a bug?

Thanks,
Pedro