Good morning, Am Samstag, den 27.07.2013, 20:16 +0000 schrieb Simon Peyton-Jones:
So the change I propose to make IncoherentInstances to pick arbitrarily among instances that match. More precisely, when trying to find an instance matching a target constraint (C tys),
a) Find all instances matching (C tys); these are the candidates
b) Eliminate any candidate X for which another candidate Y is strictly more specific (ie Y is a substitution instance of X), if either X or Y was complied with -XOverlappingInstances
c) Check that any non-candidate instances that *unify* with (C tys) were compiled with -XIncoherentInstances
d) If only one candidate remains, pick it. Otherwise if all remaining candidates were compiled with -XInccoherentInstances, pick an arbitrary candidate
All of this is precisely as now, except for the "Otherwise" part of (d). One could imagine different flags for the test in (c) and (d) but I really don't think it's worth it.
I believe it would be more consistent to change the otherwise part of (d) to “Otherwise, if all but at most one remaining candidates were compiled with -XInccoherentInstances, pick the one that does not have the flag, or any other”. The rationale is that it goes much better with (c): Consider a typical example for (c): class C a b where foo :: (a,b) instance C [a] b instance [incoherent] [Int] b instance [incoherent] C a Int now foo :: ([a],b]) works (only one instance matches, the others unify, but are incoherent. So I can write (foo :: ([a],b])) :: ([Int], Int]). But I cannot write foo :: ([Int], Int]) as now all three instances from above match. The second is ruled out in step (b), but the third is not, so we are in case (d) and by the original logic, things fail here. If we allow one non-incoherent instance (and, for consistency with (b), pick that), it would work. I’ll prepare the patch in that variant, but of course I’ll change it if it turns out I am wrong here. Greetings, Joachim -- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata@debian.org