[GHC] #9118: Can't eta-reduce representational coercions

[GHC]

#9118: Can't eta-reduce representational coercions
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        Reporter:  goldfire          |            Owner:
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  #9117
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Comment (by rwbarton):

 (Type families are like type synonyms in this regard; you can't define
 `type Pair a = (a, a)` and then talk about `Functor Pair`.)

 It looks like #8555 is for `Coercion (f a) (f b) -> Coercion a b`, while
 this ticket is for `Coercion (f a) (g a) -> Coercion f g`. I don't know
 whether those are literally the same construction in Core but I expect
 they are equally easy if not.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9118#comment:5>
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