[GHC] #8809: Prettier error messages?

[GHC]

#8809: Prettier error messages?
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        Reporter:  joelteon          |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.9
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
  #8809,#10073,#10179                |
       Wiki Page:                    |
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Comment (by bgamari):

 > I've run into this while trying to hack together something like
 ​http://gcc.godbolt.org/, but for Haskell,

 Ahh, interesting.



 > `​pretty` appears to be the same library GHC uses and as pointed out by
 bgamari already supports annotations. Does anyone know if they diverged in
 functionality/semantics? If not, would it be a useful step to migrate
 GHC's `Doc` to annotated version?
 There were a few deviations, although I think most of them have been
 patched up by thomie (see #10735).

 > I think an argument against it is that it doesn't have a `Monad`
 instance, but I don't immediately see the benefit of having it.

 I think you ultimately **need** a `Monad` instance since this allows you
 to project your annotations back to the output currently producted by GHC.
 AFAICT the Hughes pretty-printer doesn't admit an efficient, correct
 monadic bind.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8809#comment:21>
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