type error formatting

[Simon Peyton Jones]

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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