Interested to help with error messages

[Siddharth Bhat]

Elm has a interesting take on this: a collection of all error messages (
https://github.com/elm-lang/error-message-catalog).

I created an empty repo (
https://github.com/bollu/hask-error-messages-catalog) and shot an email to
haskell-cafe asking for examples where GHC generates unintuitive error
messages. Hopefully, I can start collecting these so there's context the
next time this comes up.

(PRs welcome :)

Thanks,

~Siddharth

On Mon 5 Jun, 2017, 22:56 Evan Laforge, <qdunkan at gmail.com> wrote:

> This is a bit off topic, but is there a collection of not-so-great
> error messages along with opinions about what they should be?  Like a
> wiki page or something?
>
> I just stumbled across one and was going to complain, but didn't know
> the most productive way to do that, aside from try to fix it myself.
>
> Specifically, I'm working in a small EDSL that uses lots of small
> expressions composed together with a repurposed (.) operator.  The
> pretty printer layout causes the context section of each type error to
> become huge, for instance:
>
> Derive/Solkattu/MridangamScore.hs:110:60:
>     Couldn't match type ‘Derive.Solkattu.Sequence.Note
>                            (Derive.Solkattu.Solkattu.Note
>                               Derive.Solkattu.MridangamDsl.Stroke)’
>                    with ‘[Derive.Solkattu.Sequence.Note
>                             (Derive.Solkattu.Solkattu.Note
>                                Derive.Solkattu.MridangamDsl.Stroke)]’
>     Expected type: [Sequence]
>       Actual type: [Derive.Solkattu.Sequence.Note
>                       (Derive.Solkattu.Solkattu.Note
>                          Derive.Solkattu.MridangamDsl.Stroke)]
>     In the second argument of ‘($)’, namely
>       ‘nadai 7
>        $ repeat 2 kook
>          . od
>            . __
>              . k
>                . d
>                  . __
>                    . k
>          [ keeps walking to the right for many many lines ]
>     In the second argument of ‘($)’, namely
>       ‘korvai adi
>        $ nadai 7
>          $ repeat 2 kook
>            . od
>              [ again ]
>     In the second argument of ‘($)’, namely
>        [ and again ]
>
>
> You can probably easily reproduce this by making a lot of little
> expressions and putting an error in someplace.  What I think it should
> be is put more things on a horizontal line, or maybe cap the
> expression at a certain number of lines.  Or maybe the "zooming out"
> contexts could replace the previous expression with {- previous
> expression -} to emphasize the difference.
>
>
> Also, the other thing to think about before changing type errors is
> that tons of ghc tests rely on (almost) exact error output.  Probably
> the first task is to make an abstract output, then change the tests to
> rely on that (or at least a legacy formatting mode), so that every
> change doesn't break a million tests.
>
> On Sun, Jun 4, 2017 at 8:20 PM, Richard Eisenberg <rae at cs.brynmawr.edu>
> wrote:
> >
> >> On Jun 3, 2017, at 11:50 AM, Ben Gamari <ben at smart-cactus.org> wrote:
> >>
> >> In particular, Richard Eisenberg
> >> advocated that error message documents look like,
> >>
> >>    -- A dynamically typed value embedded in an error message
> >>    data ErrItem = forall a. (Outputable a, Typeable a). ErrItem a
> >>
> >>    type ErrDoc = Doc ErrItem
> >
> > I retract this argument.
> >
> > Otherwise, I contribute only a hearty thanks to whoever is working on
> this, which likely has a larger impact on the adoption of Haskell than
> anything I've done. :)
> >
> > Richard
> > _______________________________________________
> > ghc-devs mailing list
> > ghc-devs at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
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-- 
Sending this from my phone, please excuse any typos!
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