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>