[GHC] #15147: Type checker plugin receives Wanteds that are not completely unflattened

[Nicolas Frisby]

Whoops, I replied via email instead of commenting on the ticket. I've done
so now. Sorry for the mailing list noise.

On Mon, Jun 11, 2018, 08:21 Nicolas Frisby <nicolas.frisby at gmail.com> wrote:

> That way of saying it clarifies the expectations for me. And doesn't seem
> too burdensome for the plugin author.
>
> Thus I think this ticket could be resolved by updating the documentation.
> (Though I still would like for a plugin to be able to request the flattened
> Wanteds. Separate ticket?)
>
> In particular this sentence in the User Guide
>
> "[The plugin] will be invoked at two points in the constraint solving
> process: after simplification of given constraints, and after unflattening
> of wanted constraints."
>
> would benefit from some elaboration. Specifically, "unflattening of wanted
> constraints" is somewhat ambiguous: until you spelled it out, I was
> thinking that if a constraint is flattened, it doesn't have any flattening
> variables in it. However, I'm inferring here that the jargon is used to
> mean that "unflattening a wanted constraint" only eliminates fmvs, possibly
> leaving fsks behind? That's what I've been confused about (until now, I
> think). Thanks.
>
>
> On Tue, Jun 5, 2018, 01:48 GHC <ghc-devs at haskell.org> wrote:
>
>> #15147: Type checker plugin receives Wanteds that are not completely
>> unflattened
>>
>> -------------------------------------+-------------------------------------
>>         Reporter:  nfrisby           |                Owner:  (none)
>>             Type:  bug               |               Status:  new
>>         Priority:  normal            |            Milestone:  8.6.1
>>        Component:  Compiler (Type    |              Version:  8.4.1
>>   checker)                           |             Keywords:
>>       Resolution:                    |  TypeCheckerPlugins
>> Operating System:  Unknown/Multiple  |         Architecture:
>>                                      |  Unknown/Multiple
>>  Type of failure:  None/Unknown      |            Test Case:
>>       Blocked By:                    |             Blocking:
>>  Related Tickets:                    |  Differential Rev(s):
>>        Wiki Page:                    |
>>
>> -------------------------------------+-------------------------------------
>>
>> Comment (by simonpj):
>>
>>  > Perhaps I'm misunderstanding something
>>
>>  I didn't express it very clearly.  As it stands, the Given CFunEqCan's
>>  remain, and hence so do the fsks.  The Wanted CFunEqCans are removed
>>  (currently) along with the fmvs.
>>
>>  So yes, currently Wanteds can contain fsks, whose definition is given by
>> a
>>  CFunEqCan.  I would have thought that most plugins would not find it hard
>>  to deal with that.
>>
>> --
>> Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15147#comment:14>
>> GHC <http://www.haskell.org/ghc/>
>> The Glasgow Haskell Compiler
>>
>
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