[GHC] #12390: List rules for `Coercible` instances

[GHC]

#12390: List rules for `Coercible` instances
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        Reporter:  Iceland_jack      |                Owner:  tellah
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:  ghci059
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3634
       Wiki Page:                    |
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Comment (by goldfire):

 Here are some design notes off the top of my head:

 I don't want to clutter `TyThing` with this, as it's unclear where within
 `TyThing` the documentation should go. In `TyCon`? Perhaps, but I don't
 love it.

 Instead, `InteractiveEval.getInfo` could return 5 things instead of 4; the
 fifth is an `SDoc` with extra text that should be printed. (It would
 normally be `empty`.) Naturally, you will have to add the fifth return
 value to `HscMain.hscTcRnGetInfo` and `TcRnDriver.tcRnGetInfo`. That last
 function can just call a new `lookupCustomDoc` function. That function can
 be defined in `PrelInfo`; it would refer to a `UniqFM SDoc` that
 associates `Unique`s with `SDoc`s. The `SDoc`s could (for now) just be
 hard-coded in `PrelInfo`. Certainly baked-in goodies other than
 `Coercible` can benefit from this new behavior. Perhaps it would be better
 to store the printed strings outside of a Haskell source file, but we
 don't have to over-design to begin with.

 Before implementing this, @tellah, perhaps someone else who is not a
 newcomer can opine on my design choices here. And, thanks!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12390#comment:10>
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