[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 simonpj):

 I'm delighted to see activity on the "better error messages" front.  Thank
 you!

 There are two separate lines of thought here:

 1. Using colour etc in `SDoc`, in a way that is portable across platforms.
 This seems to be non-trivial (comment:33 and comment:34), but it's non-
 controversial and it'd be great to have.  I think Phab:D2716 and
 Phab:D2717 are about this.

 2. Some infrastructure to allow more info to be displayed in error
 messages.  That's what Phab:D2718 is about.

 I suggest we split (2) into a separate ticket.  On (1) let's just do it.

 On the (2) front, I confess that I'm not keen on the approach shown in
 Phab:D2718, which allows an `SDoc` to contain an I/O action.  It smells
 wrong in principle to me; it would fork GHC away from our goal of using
 the Hackage `pretty` library (#10735); and I think we can do better.

 I would love to make progress on adopting David Christiansen's idea for
 pretty printing, which he calls "A pretty printer that says what it
 means":

 * [https://www.youtube.com/watch?v=m7BBCcIDXSg Video],
 * [https://hackerfall.com/story/a-pretty-printer-that-says-what-it-means
 Blog post]
 * [https://wiki.haskell.org/wikiupload/4/4c/Hiw-2015-david-
 christiansen.pdf Slides]
 * [https://github.com/david-christiansen/hindent Git repo for hindent, an
 extensible Haskell pretty printer]

 That's what Ben is alluding to in comment:31.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8809#comment:35>
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