[GHC] #8888: Document Coercible in user's guide

[GHC]

#8888: Document Coercible in user's guide
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        Reporter:  goldfire          |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:  7.8.1
       Component:  Documentation     |          Version:  7.8.1-rc2
      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):

 >  And, despite its unique weirdness, there is no comment with
 ghcPrimExports.

 Added.

 >  None of this happens for (~). Why it needed for Coercible?

 Hmm. Good question. I believe that `(~)` needs to not to be importable at
 all (it is handled by the Parser). Also commented in the Note.

 >  There may be some Deep Reason for all this strangeness, but I can't see
 it.

 It is just what happened to work and seems reasonable, starting with the
 example of `(~)`. Quite likely that a less strange implementation is
 possible (with little change to make sure haddock and ghci use the updated
 kind); I do not know my way around all related parts of the compiler yet.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8888#comment:8>
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