[GHC] #9725: Constraint deduction failure

[GHC]

#9725: Constraint deduction failure
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              Reporter:  heisenbug   |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
  (Type checker)                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  GHC         |  Related Tickets:
  rejects valid program              |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by heisenbug):

 Replying to [comment:8 simonpj]:
 > No it's not just kind inference.  Do you want that poly-kinded type for
 `test`?  Sounds as if the answer is "yes".  Well, then `k` is a skolem
 variable, and if it is is locally known to be equal to `Bool`, that's a
 local kind equality (a la GADTs).
 >

 While I suspected that this could be the answer, I am still happy I asked!
 Thanks for explaining.

 > From what you say, in some branches of `h`, you have a local equality `k
 ~ Bool` and in others you have `k ~ Nat`.

 Exactly. `k` will be instantiated to different kinds in the pattern
 matching process.

 >
 > This is just what Richard is working on!

 Good to hear. Is there an ETA for this work? Is there a chance to have a
 stripped-down version of that feature as a preview, that handles just such
 plain equations, without the full bells&whistles goodness? Would a plain
 mortal like me be in a position to make the former happen?

 >
 > Simon

 Thanks again,

     Gabor

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9725#comment:9>
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