[Haskell-cafe] (Un)termination of overloading resolution
ross at soi.city.ac.uk
Wed Feb 22 03:51:32 EST 2006
On Tue, Feb 21, 2006 at 07:13:17PM -0800, oleg at 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:
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.
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