[GHC] #9123: Need for higher kinded roles

[GHC]

#9123: Need for higher kinded roles
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        Reporter:  simonpj           |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by nomeata):

 > If we do allow users to write their own instances, we seem to lose the
 guarantee that coerce is free at runtime, no?

 Did anyone say guarantee? It’s a promise, not more :-)

 What I’m saying: By passing around boxed `Coercible` witnesses (e.g. in
 the `Coercion` data type) you can probably create quite complex terms that
 the compiler will not be able to simplify completely. Then `coerce` still
 incurs the cost of evaluating that box. So I would not worry that a ill-
 meaning user can make `coerce` expensive by writing strange
 `Representational` instances, as long as he can write good instances where
 there are good instances.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9123#comment:8>
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