[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 Iceland_jack):

 Replying to [comment:3 nomeata]:
 > Interestingly, had you had
 >
 > {{{
 > newtype NotOp cat a b = NotOp (cat b a)
 > }}}
 > then `:t Coercion :: Coercion (NotOp (NotOp cat)) cat` would work.

 Did you mean

 {{{#!hs
 newtype NotOp cat a b = NotOp (cat a b)
 }}}

 > The thing with `Either` is a completely different story. Note that you
 are coercing _an argument of `Either`_ (still `Either` on both sides)
 whereas originally you are coercing between `Op` and it’s representation.

 With quantified constraints, could we write?


 {{{#!hs
 type OpOp cat = forall xx yy. cat xx yy `Coercible` Op (Op cat) xx yy

 wit :: OpOp cat => Op (Op cat) `Coercion` cat
 wit = Coercion
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14386#comment:4>
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