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

[GHC]

#15147: Type checker plugin receives Wanteds that are not completely unflattened
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        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:                    |
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Comment (by nfrisby):

 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.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15147#comment:15>
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