[GHC] #9725: Constraint deduction failure

[GHC]

#9725: Constraint deduction failure
-------------------------------------+-------------------------------------
              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:1 rwbarton]:
 > ...
 >
 > Maybe adding kind annotations for some of the type variables involved
 would help?

 I tried that. These are the most generic kinds:
 {{{
 ent :: Entity -> k -> Entity
 Monitor :: Entity -> Bool -> Entity
 par :: Entity
 mp :: Bool
 }}}
 It looks like the equation `(*)` does not seem to establish the kind-level
 equality `Entity -> k -> Entity === Entity -> Bool -> Entity` and thus `k
 === Bool`. So yes, your analysis is correct, that kind variables mess up
 the decomposition.

 When changing `ent` to `Monitor`, everything works, probably because the
 kind variable `k` is eliminated.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9725#comment:5>
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