[GHC] #8423: contraint solver doesn't reduce reducible closed type family expressions (even with undecidable instances!)

[GHC]

#8423: contraint solver doesn't  reduce reducible closed type family expressions
(even with undecidable instances!)
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        Reporter:  carter                   |            Owner:
            Type:  feature request          |           Status:  new
        Priority:  normal                   |        Milestone:  7.10.1
       Component:  Compiler (Type checker)  |          Version:  7.7
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  None/Unknown             |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:  #4259
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Comment (by carter):

 thanks! this is great! Would I want to compare the core? (or just use
 criterion to benchmark?). I'll add that to my infinite todo list.

 This example seems to work only in ghc 7.8, how would i adapt those ideas
 to work on 7.6? (can I even?)

 that said, having more examples like this about how to prove things in
 haskell is probably a good idea! Also, figuring out how to erase such
 proofs as much as possible also would likely be worth while. I'm happy to
 help contribute a few braincells to chewing on that post 7.8

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8423#comment:8>
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