Type checker loops with type families, overlapping and undecidable instances

[Lennart Augustsson]

Turning on UndecidableInstances is the same as saying: OK typechcker,
you can loop if I make a mistake.
I've not looked closely at your code, but if you turn on that flag,
looping is probably not a bug.

  -- Lennart

2008/12/4 José Pedro Magalhães <jpm at cs.uu.nl>:
> 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
>
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