type error formatting

[Ryan Trinkle]

This looks like an improvement to me.  I love the idea of a visual
demarcation between sections, too; the bullets seem like a good choice
there (the horizontal lines seem like they'd take up more space).


Ryan

On Sat, Oct 24, 2015 at 6:07 AM, Roman Cheplyaka <roma at ro-che.info> wrote:

> I have the same issue with the current error messages. I think these are
> all good ideas.
>
> On 10/24/2015 05:48 AM, Evan Laforge wrote:
> > Here's a typical simple type error from GHC:
> >
> > Derive/Call/India/Pakhawaj.hs:142:62:
> >     Couldn't match type ‘Text’ with ‘(a1, Syllable)’
> >     Expected type: [([(a1, Syllable)], [Sequence Bol])]
> >       Actual type: [([Syllable], [Sequence Bol])]
> >     Relevant bindings include
> >       syllables :: [(a1, Syllable)]
> >         (bound at Derive/Call/India/Pakhawaj.hs:141:16)
> >       best_match :: [(a1, Syllable)]
> >                     -> Maybe (Int, ([(a1, Syllable)], [(a1, Sequence
> Bol)]))
> >         (bound at Derive/Call/India/Pakhawaj.hs:141:5)
> >     In the second argument of ‘mapMaybe’, namely ‘all_bols’
> >     In the second argument of ‘($)’, namely
> >       ‘mapMaybe (match_bols syllables) all_bols’
> >
> > I've been having more trouble than usual reading GHC's errors, and I
> > finally spent some time to think about it.  The problem is that this new
> > "relevant bindings include" section gets in between the expected and
> > actual types (I still don't like that wording but I've gotten used to
> > it), which is the most critical part, and the location context, which is
> > second most critical.  Notice the same effect in the previous sentence
> > :)  After I see a type error the next thing I want to see is the where
> > it happened, so I have to skip over the bindings, which can be long and
> > complicated.  Then I usually know what to do, and only look into the
> > bindings if something more complicated is going on, like wonky
> > inference.  So how about reordering the message:
> >
> > Derive/Call/India/Pakhawaj.hs:142:62:
> >     Couldn't match type ‘Text’ with ‘(a1, Syllable)’
> >     Expected type: [([(a1, Syllable)], [Sequence Bol])]
> >       Actual type: [([Syllable], [Sequence Bol])]
> >     In the second argument of ‘mapMaybe’, namely ‘all_bols’
> >     In the second argument of ‘($)’, namely
> >       ‘mapMaybe (match_bols syllables) all_bols’
> >     Relevant bindings include
> >       syllables :: [(a1, Syllable)]
> >         (bound at Derive/Call/India/Pakhawaj.hs:141:16)
> >       best_match :: [(a1, Syllable)]
> >                     -> Maybe (Int, ([(a1, Syllable)], [(a1, Sequence
> Bol)]))
> >         (bound at Derive/Call/India/Pakhawaj.hs:141:5)
> >
> > After this, why not go one step further and set off the various sections
> > visibly to make it easier to scan.  The context section can also be
> > really long if it gets an entire do block or record:
> >
> > Derive/Call/India/Pakhawaj.hs:142:62:
> >   * Couldn't match type ‘Text’ with ‘(a1, Syllable)’
> >     Expected type: [([(a1, Syllable)], [Sequence Bol])]
> >       Actual type: [([Syllable], [Sequence Bol])]
> >   * In the second argument of ‘mapMaybe’, namely ‘all_bols’
> >     In the second argument of ‘($)’, namely
> >       ‘mapMaybe (match_bols syllables) all_bols’
> >   * Relevant bindings include
> >       syllables :: [(a1, Syllable)]
> >         (bound at Derive/Call/India/Pakhawaj.hs:141:16)
> >       best_match :: [(a1, Syllable)]
> >                     -> Maybe (Int, ([(a1, Syllable)], [(a1, Sequence
> Bol)]))
> >         (bound at Derive/Call/India/Pakhawaj.hs:141:5)
> >
> > Or alternately, taking up a bit more vertical space:
> >
> > Derive/Call/India/Pakhawaj.hs:142:62:
> >     Couldn't match type ‘Text’ with ‘(a1, Syllable)’
> >     Expected type: [([(a1, Syllable)], [Sequence Bol])]
> >       Actual type: [([Syllable], [Sequence Bol])]
> >         -----------------------------
> >     In the second argument of ‘mapMaybe’, namely ‘all_bols’
> >     In the second argument of ‘($)’, namely
> >       ‘mapMaybe (match_bols syllables) all_bols’
> >         -----------------------------
> >     Relevant bindings include
> >       syllables :: [(a1, Syllable)]
> >         (bound at Derive/Call/India/Pakhawaj.hs:141:16)
> >       best_match :: [(a1, Syllable)]
> >                     -> Maybe (Int, ([(a1, Syllable)], [(a1, Sequence
> Bol)]))
> >         (bound at Derive/Call/India/Pakhawaj.hs:141:5)
> >
> > Thoughts?  It seems simple enough that I could do myself, but of course
> > not without buy-in.
>
>
>
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