type error formatting

[MigMit]

At the very least, "bound at" should help IDEs (Emacs in particular) show exactly the right places.

> On 24 Oct 2015, at 22:14, Evan Laforge <qdunkan at gmail.com> wrote:
> 
> Ok, ticket created.  I'll go see how much I can figure out on my own.
> 
> https://ghc.haskell.org/trac/ghc/ticket/11014
> 
> WRT the "bound at" bits in "relevant bindings", I have no strong opinion.  What about omitting them if they are in the same file as the main error?  Or maybe they always are?  I'm not totally clear how it chooses which bindings are relevant.
> 
> On Sat, Oct 24, 2015 at 12:50 PM, Simon Peyton Jones <simonpj at microsoft.com> wrote:
> I’m all for it.   Can advise.  (Make a ticket.)
> 
>  
> 
> Thanks!
> 
>  
> 
> Simon
> 
>  
> 
> From: Glasgow-haskell-users [mailto:glasgow-haskell-users-bounces at haskell.org] On Behalf Of Evan Laforge
> Sent: 24 October 2015 03:48
> To: GHC users
> Subject: type error formatting
> 
>  
> 
> 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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