[GHC] #14362: Allow: Coercing (a:~:b) to (b:~:a)

[GHC]

#14362: Allow: Coercing (a:~:b) to (b:~:a)
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      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 simonpj):

 But I don't want the most general type of `convert`!  The general
 principle of `Coercible` is that it simply automates what you could do by
 hand.  E.g
 {{{
 newtype Age = MkAge Int

 convert :: [Int] -> [Age]
 convert xs = map MkAge xs
 }}}
 And back.  So we can reasonably claim `Coercible [Age] [Int]`.

 It's the same with `Coercible (a :~: b) (b :~: a)`, which is what you
 asked for.  I can do it by hand (with `convert`); using `coerce` just
 automates it.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14362#comment:7>
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