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

11 Jun 2018


      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  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  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
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