Thank you Ross. I've committed your patch, and added documentation. Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Ross Paterson | Sent: 06 February 2006 11:36 | To: Simon Peyton-Jones; glasgow-haskell-users@haskell.org | Subject: Re: instance inference | | A patch implementing a relaxed termination constraint is at | | http://www.soi.city.ac.uk/~ross/instance-termination.patch | | Here is the description: | | With -fglasgow-exts but not -fallow-undecidable-instances, GHC 6.4 | requires that instances be of the following form: | | (1) each assertion in the context must constrain distinct variables | mentioned in the head, and | | (2) at least one argument of the head must be a non-variable type. | | This patch replaces these rules with the requirement that each | assertion in the context satisfy | | (1) no variable has more occurrences in the assertion than in the | head, and | | (2) the assertion has fewer constructors and variables (taken together | and counting repetitions) than the head. | | This allows all instances permitted by the old rule, plus such | instances as | | instance C a | instance Show (s a) => Show (Sized s a) | instance (Eq a, Show b) => C2 a b | instance C2 Int a => C3 Bool [a] | instance C2 Int a => C3 [a] b | instance C4 a a => C4 [a] [a] | | but still ensures that under any substitution assertions in the context | will be smaller than the head, so context reduction must terminate. | | This is probably the best we can do if we consider each instance in | isolation. | | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users