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

9 Oct 2017

      #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):

 Replying to [comment:2 goldfire]:
...
Do you have a concrete use case?
My use case is [https://github.com/Icelandjack/deriving-
 via/blob/master/examples/Generics.hs this] where `swap` is required to
 express symmetry. I use this to witness a coercion between `Rep a x` and
 `Rep b x`

 {{{#!hs
 sameRep :: SameRep a b :- (Rep a x `Coercible` Rep b x)
 }}}

 but I also need

 {{{#!hs
 SameRep a b :- (Rep b x `Coercible` Rep a x)
 }}}

 for which I use `swap`

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14333#comment:4
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