Wording of non-exhaustivity warning is confusing for beginners

[vicuna]

Original bug ID: 7169
Reporter: @gasche
Assigned to: @gasche
Status: resolved (set by @gasche on 2016-07-01T15:17:26Z)
Resolution: fixed
Priority: normal
Severity: text
Version: 4.02.3
Target version: 4.03.1+dev
Fixed in version: 4.04.0 +dev / +beta1 / +beta2
Category: ~DO NOT USE (was: OCaml general)
Related to: #7504
Monitored by: @hcarty

Bug description

In discussion https://www.reddit.com/r/ocaml/comments/49g5yi/doubt_about_pattern_matching/ , a beginner has a two-field records {player1; player2} that he pattern-matches on, and he gets the following non-exhaustivity warning (one possible player value is not handled);

Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
{player1=Spock}

The beginner asks:

AFAIK, I can not create a record with only the first item:

utop[17]> let x = {player1=Spock};;
Error: Some record fields are undefined: player2

I think the beginner is right: this error message is confusing.

I would propose to rephrase it as follows:

Here is an example of a case that is not matched:
{player1=Spock}

(It would also possibly be a good idea to print partial record patterns with a trailing "; _" in this case, to highlight that this is not expected to be a constructible value, but I'm not sure whether that part is worth the trouble.)

[vicuna]

Comment author: @Armael

I think it would be quite beneficial to print partial record patterns with a "; _". The fact that the pattern looks like a valid OCaml value was in my eyes the major cause of confusion.

[vicuna]

Comment author: @Octachron

I also think that printing a trailing " _ " when the record pattern is not complete is a good idea. Moreover, it seems quite easy to implement this by reusing the warning on non-closed records.

I have proposed a PR to this effect in #501.

[vicuna]

Comment author: @gasche

Solved by merging octachron's PR. Thanks a lot!