[GHC] #9376: More informative error messages when closed type families fail to simplify

[GHC]

#9376: More informative error messages when closed type families fail to simplify
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              Reporter:              |            Owner:
  MikeIzbicki                        |           Status:  new
                  Type:  feature     |        Milestone:
  request                            |          Version:  7.8.2
              Priority:  normal      |         Keywords:
             Component:  Compiler    |     Architecture:  Unknown/Multiple
  (Type checker)                     |       Difficulty:  Unknown
            Resolution:              |       Blocked By:
      Operating System:              |  Related Tickets:
  Unknown/Multiple                   |
       Type of failure:              |
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by rwbarton):

 To get the desired behavior in the original program you should help GHC
 out a little by factoring out the common part of the two constraints.
 {{{
 type OrdRec (f :: * -> *) a b = ( Ord a, Ord b, OrdRec' f a b )
 type family OrdRec' (f :: * -> *) a b :: Constraint where
     OrdRec' f a (f b) = OrdRec f a b
     OrdRec' f a b = ()
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9376#comment:6>
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