[GHC] #14386: GHC doesn't allow Coercion between partly-saturated type constructors

[GHC]

#14386: GHC doesn't allow Coercion between partly-saturated type constructors
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:  roles
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by nomeata):

 >  With quantified constraints, could we write?

 I don’t think so. It would require a rule “eta-expansion rule”

 {{{
 ∀a. T1 a ~R T2 b
 −−−−−−−−−−−−−−
     T1 ~R T2
 }}}
 which does not exist. But others will have to tell you if that can exist
 or if it would break things.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14386#comment:5>
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