[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:  bug                      |            Status:  new
       Priority:  normal                   |         Milestone:  7.8.1
      Component:  Compiler (Type checker)  |           Version:  7.7
       Keywords:                           |  Operating System:
   Architecture:  Unknown/Multiple         |  Unknown/Multiple
     Difficulty:  Unknown                  |   Type of failure:
     Blocked By:                           |  None/Unknown
Related Tickets:                           |         Test Case:
                                           |          Blocking:
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 attached is an example where the type checker isn't "computing" in the
 closed type families, or at least doing a one step reduction.

 this makes sense, given that type families only compute when
 instantiated...
 But if we could partially evaluate closed type families (or at least do a
 one step reduction), the attached example code would type check!

 interestingly, depending on what order the cases for the PSum are written,
 the type error changes!

 I guess I want an "eager matching" closed type family, that when partially
 instantiated will patch on the first pattern it satisfies, to complement
 ordered type families.

 attached is a toy example where I otherwise need an unsafeCoerce to make
 things type check

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