22 Feb
2006
22 Feb
'06
3:51 a.m.
On Tue, Feb 21, 2006 at 07:13:17PM -0800, oleg@pobox.com wrote:
I'm afraid that may still be insufficient, as the following counter-example shows. It causes GHC 6.4.1 to loop in the typechecking phase. I haven't checked the latest GHC.
The HEAD is more cautious: F.hs:12:0: Variable occurs more often in a constraint than in the instance head in the constraint: E m a b (Use -fallow-undecidable-instances to permit this) In the instance declaration for `Foo m (a -> ())' This is required for all instances. GHC 6.4 relaxed it in the presence of FDs on the classes in the context, but as you've shown that is dangerous.