| My proposal is to have just one pragma, e.g. OVERLAP, that allows | overlap in either direction. But if you have examples whether the | extra sophistication introduced by a separation into OVERLAPPABLE and | OVERLAPPING is needed, I am happy to go along... Great! As you'll see the proposal, "OVERLAPS" is precisely what you want. I don't care whether it is called "OVERLAP" or "OVERLAPS". So it sounds as if you are content. (I assume you don't want to *prevent* careful programmers from saying something more precise.) Simon | | On 31.07.2014 10:13, Simon Peyton Jones wrote: | > Andreas, remember that GHC 7.8 already implements (essentially) the | same algorithm. The difference is that 7.8 offers only the brutal - | XOverlappingInstances to control it. In your example of the decision | you make when writing | > instance Bla a => Bla [a] | > vs | > instance {-# OVERLAPPABLE #-} Bla a => Bla [a] you are, with GHC | > 7.8, making precisely the same decision when you decide whether or | not to add {-# LANGUAGE OverlappingInstances #-} to that module. | Perhaps that wasn't clear in what I wrote; apologies. | > | > So your proposal seems to be this | > | > don't remove -XOverlappingInstances, because that will prevent | > programmers from "flipping on/off pragmas until their program | > goes through". | > | > It's hard to argue AGAINST providing the opportunity for more careful | programmers to express their intentions more precisely, which is what | the OVERLAP/OVERLAPPABLE pragmas do. | > | > Concerning deprecating OverlappingInstances, my gut feel is that it | is positively a good thing to guide programmers towards a more robust | programming style. But my reason for starting this thread was to see | whether or not others' gut feel is similar. | > | > Simon | > | > | -----Original Message----- | > | From: Libraries [mailto:libraries-bounces@haskell.org] On Behalf Of | > | Andreas Abel | > | Sent: 31 July 2014 08:59 | > | To: Simon Peyton Jones; ghc-devs; GHC users; Haskell Libraries | > | (libraries@haskell.org) | > | Subject: Re: Overlapping and incoherent instances | > | | > | On 31.07.2014 09:20, Simon Peyton Jones wrote: | > | > Friends, in sending my message below, I should also have sent a | > | > link to | > | > | > | > https://ghc.haskell.org/trac/ghc/ticket/9242#comment:25 | > | | > | Indeed. | > | | > | Quoting from the spec: | > | | > | * Eliminate any candidate IX for which both of the following | hold: | > | * There is another candidate IY that is strictly more specific; | > | that is, IY is a substitution instance of IX but not vice | versa. | > | | > | * Either IX is overlappable or IY is overlapping. | > | | > | Mathematically, this makes a lot of sense. But put on the hat of | > | library writers, and users, and users that don't rtfm. Looking out | > | from under this hat, the one may always wonder whether one should | > | make one's generic instances OVERLAPPABLE or not. | > | | > | If I create a library with type class Bla and | > | | > | instance Bla a => Bla [a] | > | | > | I could be a nice library writer and spare my users from declaring | > | their Bla String instances as OVERLAPPING, so I'd write | > | | > | instance {-# OVERLAPPABLE #-} Bla a => Bla [a] | > | | > | Or maybe that would be malicious? | > | | > | I think the current proposal is too sophisticated. There are no | > | convincing examples given in the discussion so far that demonstrate | > | where this sophistication pays off in practice. | > | | > | Keep in mind that 99% of the Haskell users will never study the | > | instance resolution algorithm or its specification, but just flip | > | on/off pragmas until their code goes through. [At least that was | my | > | approach: whenever GHC asks for one more LANGUAGE pragma, just | throw | > | it in.] | > | | > | Cheers, | > | Andreas | > | | > | | > | > Comment 25 describes the semantics of OVERLAPPING/OVERLAPPABLE | > | > etc, which I signally failed to do in my message below, leading | to | > | > confusion in the follow up messages. My apologies for that. | > | > | > | > Some key points: | > | > | > | > *There is a useful distinction between /overlapping/ and | > | > /overlappable/, but if you don't want to be bothered with it you | > | > can just say OVERLAPS (which means both). | > | > | > | > *Overlap between two candidate instances is allowed if /either/ | > | > has the relevant property. This is a bit sloppy, but reduces the | > | > annotation burden. Actually, with this per-instance stuff I | think | > | > it'd be perfectly defensible to require both to be annotated, but | > | > that's a different discussion. | > | > | > | > I hope that helps clarify. | > | > | > | > I'm really pretty certain that the basic proposal here is good: | it | > | > implements the current semantics in a more fine-grained manner. | > | > My main motivation was to signal the proposed deprecation of the | > | > global per-module flag -XoverlappingInstances. Happily people | > | > generally | > | seem | > | > fine with this. It is, after all, precisely what deprecations | are | > | for | > | > ("the old thing still works for now, but it won't do so for ever, | > | > and you should change as soon as is convenient"). | > | > | > | > Thanks | > | > | > | > Simon | > | > | > | > *From:*Libraries [mailto:libraries-bounces@haskell.org] *On | Behalf | > | > Of *Simon Peyton Jones | > | > *Sent:* 29 July 2014 10:11 | > | > *To:* ghc-devs; GHC users; Haskell Libraries | > | > (libraries@haskell.org) | > | > *Subject:* Overlapping and incoherent instances | > | > | > | > Friends | > | > | > | > One of GHC's more widely-used features is overlapping (and | > | > sometimes | > | > incoherent) instances. The user-manual documentation is here | > | > <http://www.haskell.org/ghc/docs/latest/html/users_guide/type- | clas | > | > s- | > | extensions.html#instance-overlap>. | > | > | > | > The use of overlapping/incoherent instances is controlled by | > | > LANGUAGE | > | > pragmas: OverlappingInstances and IncoherentInstances | respectively. | > | > | > | > However the overlap/incoherent-ness is a property of the | > | > **instance | > | > declaration** itself, and has been for a long time. Using | > | > LANGUAGE OverlappingInstances simply sets the "I am an | overlapping instance" | > | > flag for every instance declaration in that module. | > | > | > | > This is a Big Hammer. It give no clue about **which** particular | > | > instances the programmer is expecting to be overlapped, nor which | are | > | > doing the overlapping. It brutally applies to every instance | in | > | the | > | > module. Moreover, when looking at an instance declaration, there | > | > is no nearby clue that it might be overlapped. The clue might be | > | > in the command line that compiles that module! | > | > | > | > Iavor has recently implemented per-instance-declaration pragmas, | > | > so you can say | > | > | > | > instance {-# OVERLAPPABLE #-} Show a => Show [a] where ... | > | > | > | > instance {-# OVERLAPPING #-} Show [Char] where ... | > | > | > | > This is much more precise (it affects only those specific | > | > instances) and it is much clearer (you see it when you see the | > | > instance | > | declaration). | > | > | > | > This new feature will be in GHC 7.10 and I'm sure you will be | > | > happy about that. *But I propose also to deprecate the LANGUAGE | > | > pragmas OverlappingInstances and IncoherentInstances*, as way to | > | > encourage everyone to use the new feature instead of the old big | > | > hammer. The old LANGUAGE pragmas will continue to work, of | > | > course, for at least another complete release cycle. We could | > | > make that two cycles if it | > | was helpful. | > | > | > | > However, if you want deprecation-free libraries, it will entail a | > | wave | > | > of library updates. | > | > | > | > This email is just to warn you, and to let you yell if you think | > | > this | > | is | > | > a bad idea. It would actually not be difficult to retain the | old | > | > LANGUAGE pragmas indefinitely - it just seems wrong not to | > | > actively push authors in the right direction. | > | > | > | > These deprecations of course popped up in the test suite, so I've | > | been | > | > replacing them with per-instance pragmas there too. | Interestingly | > | > in some cases, when looking for which instances needed the | > | > pragmas, I found...none. So OverlappingInstances was entirely | > | > unnecessary. Maybe library authors will find that too! | > | > | > | > Simon | > | > | > | > | > | > | > | > _______________________________________________ | > | > Libraries mailing list | > | > Libraries@haskell.org | > | > http://www.haskell.org/mailman/listinfo/libraries | > | > | > | | > | | > | -- | > | Andreas Abel <>< Du bist der geliebte Mensch. | > | | > | Department of Computer Science and Engineering Chalmers and | > | Gothenburg University, Sweden | > | | > | andreas.abel@gu.se | > | http://www2.tcs.ifi.lmu.de/~abel/ | > | _______________________________________________ | > | Libraries mailing list | > | Libraries@haskell.org | > | http://www.haskell.org/mailman/listinfo/libraries | > _______________________________________________ | > Libraries mailing list | > Libraries@haskell.org | > http://www.haskell.org/mailman/listinfo/libraries | > | | | -- | Andreas Abel <>< Du bist der geliebte Mensch. | | Department of Computer Science and Engineering Chalmers and Gothenburg | University, Sweden | | andreas.abel@gu.se | http://www2.tcs.ifi.lmu.de/~abel/