Circular Instance declarations

[oleg@pobox.com]

Ashley Yakeley wrote:

> Would it be reasonable for the compiler to check back through the
> stack and allow the circularity? It will just create an ordinary
> recursive function.

The following works.

Flags: -fglasgow-exts -fallow-undecidable-instances

data D r = ZeroD | SuccD (r (D r))
    
instance (Eq (r (D r))) => Eq (D r) where
   ZeroD == ZeroD = True
   (SuccD a) == (SuccD b) = a == b
   _ == _ = False

    
newtype C a = MkC a

instance Eq (C (D C)) where
   (MkC ZeroD) == (MkC ZeroD) = True
   (MkC (SuccD a)) == (MkC (SuccD b)) = a == b
   _ == _ = False

equalDC :: D C -> D C -> Bool
equalDC = (==)

c2 = SuccD (MkC (SuccD (MkC ZeroD)))
c1 = SuccD (MkC (ZeroD))

*Main> equalDC c2 c2
True
*Main> equalDC c2 c1
False
*Main> equalDC ZeroD c1
False
*Main> equalDC ZeroD ZeroD
True
*Main>