[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      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by goldfire):

 I'm afraid I don't quite see what you're getting at. The original post
 says, "[This modified version] is much easier to visually process than
 GHC's current messages." My question is: Why, precisely? I don't mean to
 be defensive or dismissive, but trying to generate grounds for a
 meaningful conversation. For example, here are a few things that you might
 be getting at:

 * Having different colors/font weights (i.e. boldness) makes the error
 messages more visually interesting and therefore easier to pay attention
 to and read.
 * Having blank lines in the middle of single error messages makes them
 less imposing.
 * Using position marker in a line below some code is easier to follow than
 an ever-growing context.
 * In the example, the type of `f` is given explicitly, so the context in
 which the error was made is more apparent.

 Short of re-engineering the entire way that GHC handles error messages, it
 would certainly be hard to produce exactly the output that you are
 requesting. But, it may be possible to address bulletpoints like my
 suggested ones above piecemeal and nudge ourselves in the direction of
 better errors.

 It's also worth pointing out that each of the bulletpoints above has
 reasons "against", such as:

 * Not every terminal supports these extra modes. In particular, GHC has
 already had some trouble getting "smart" quotes working in all possible
 environments (or, indeed, figuring out when to fall back onto dumb
 quotes).
 * An automated processor of error messages (that is, an IDE built around
 GHC) could easily get confused around the blank lines. In fact, I believe
 I've run into this exact problem when running clang from emacs -- the
 extra "context-setting" output gets interpreted as fresh warnings.
 * It's unclear to me, personally, if having the position marker on a
 separate line is necessarily better than the current output.
 * The user of a DSL in Haskell is generally unaware of the full, general
 type of a function they are using. Perhaps including the full type in the
 error message would make it scarier, not friendlier.

 In any case, I'm curious to hear more about the specific things GHC can do
 to improve. I think we all want "better" error messages, but we need to
 agree on a definition of "better" first. And, the changes should probably
 be incremental, unless we have an eager volunteer to examine the whole
 error-message generation mechanism holistically. There is quite a bit of
 code dedicated to error messages, so this is not a task to be taken on
 lightly!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8809#comment:2>
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