[GHC] #14333: GHC doesn't use the fact that Coercible is symmetric

[GHC]

#14333: GHC doesn't use the fact that Coercible is symmetric
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeFamilies,
                                     |  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 Iceland_jack):

 I attached an example, I found another need for this: encoding that the
 opposite category is coercible to the current:

 {{{#!hs
 type Cat obj = obj -> obj -> Type

 class (forall xx yy. Coercible (cat xx yy) (Op cat yy xx)) => Category
 (cat :: Cat obj) where
   type Op cat :: Cat obj
 }}}

 The use case is similar as the other one but I can elaborate if needed

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14333#comment:6>
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