Overlapping and incoherent instances

[Andreas Abel]

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
>
>
>
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-- 
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/