Now if only we could somehow find a way to do the same thing for AllowAmbiguousTypes. :)
I have a 2500 line file that I'm forced to turn on AllowAmbiguousTypes in for 3 definitions, and checking that I didn't accidentally make something else ambiguous to GHC's eyes is a rather brutal affair. (I can't break up the file without inducing orphans) This is just a passing comment, while I'm thinking about it, not a serious attempt to derail the topic! -Edward On Thu, Jul 31, 2014 at 4:13 AM, Simon Peyton Jones <simo...@microsoft.com> 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-boun...@haskell.org] On Behalf Of > | Andreas Abel > | Sent: 31 July 2014 08:59 > | To: Simon Peyton Jones; ghc-devs; GHC users; Haskell Libraries > | (librar...@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-boun...@haskell.org] *On Behalf Of > | > *Simon Peyton Jones > | > *Sent:* 29 July 2014 10:11 > | > *To:* ghc-devs; GHC users; Haskell Libraries (librar...@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-class- > | 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 > | > librar...@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.a...@gu.se > | http://www2.tcs.ifi.lmu.de/~abel/ > | _______________________________________________ > | Libraries mailing list > | librar...@haskell.org > | http://www.haskell.org/mailman/listinfo/libraries > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users >
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