Overlapping and incoherent instances

[Simon Peyton Jones]

Friends, in sending my message below, I should also have sent a link to
            https://ghc.haskell.org/trac/ghc/ticket/9242#comment:25
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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