[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:  merge
        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):

 >  To be specific: what goes wrong if you simply remove Coercible from
 ghcPrimExports?

 It cannot be imported any more.

 And if I export it in `GHC.Types` and import it from there, then haddock
 and ghci (IIRC) display it as regular data type, not as a constraint,
 which I found very confusing. This might be a considered a bug or a
 misfeature; should I open a bug for it?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8888#comment:13>
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