[GHC] #10735: Smooth out the differences between `compiler/utils/Pretty.hs` and `libraries/pretty`

[GHC]

#10735: Smooth out the differences between `compiler/utils/Pretty.hs` and
`libraries/pretty`
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        Reporter:  thomie            |                   Owner:  thomie
            Type:  task              |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.0.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #1062, #1176,     |  Differential Revisions:
  #7666                              |
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Comment (by bgamari):

 Replying to [comment:1 thomie]:
 > I have patches.
 Lovely. Will we be seeing them on Phab soon?

 On a related note, I've recently been thinking about error reporting with
 an eye for making the messages produced by GHC more useful to external
 tools. I've described one possible approach in ticket:8809#comment:3 which
 would involve making `Doc` a sort of free functor, allowing the insertion
 of rich AST objects into the document. Monadic semantics would certain
 patterns of projecting out these annotations into vanilla documents quite
 convenient.

 Unfortunately, the Hughes/Peyton-Jones pretty-printer isn't terribly well-
 suited for this as you need to know the width of a node's parent during
 construction. I've been meaning to explore adopting the Wadler/Leijen
 pretty-printer as Edward Kmett has already
 [http://hackage.haskell.org/package/wl-pprint-extras-3.5.0.5/docs/Text-
 PrettyPrint-Free.html demonstrated] that an implementation with
 annotations along the lines of my proposal is possible.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10735#comment:2>
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