Overlapping and incoherent instances

Thu Jul 31 08:18:55 UTC 2014

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 <simonpj at 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-bounces at haskell.org] On Behalf Of
> | Andreas Abel
> | Sent: 31 July 2014 08:59
> | To: Simon Peyton Jones; ghc-devs; GHC users; Haskell Libraries
> | (libraries at 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 at haskell.org] *On Behalf Of
> | > *Simon Peyton Jones
> | > *Sent:* 29 July 2014 10:11
> | > *To:* ghc-devs; GHC users; Haskell Libraries (libraries at 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
> | > Libraries at 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 at gu.se
> | http://www2.tcs.ifi.lmu.de/~abel/
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