[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:15 goldfire]:
 > After some struggle, I came up with a working construction, attached.
 Bonus points if you can make it prettier and/or bind the scoped type
 variables without adding proxies.

 Thanks, Richard, this is very cool. It will take a while for me to analyse
 what exactly you are doing. I attached a version based on your workaround
 that makes the kind `En` a bit more interesting. I believe I can now
 transfer this into my real code :-)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9725#comment:16>
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