Print 10 violations in normal mode

[mpickering]

Using --verbose on a large project prints out too much output (see haskell/cabal#10744)

Printing out all the violations if there are a lot of them is quite slow, printing out a lot to stdout is not that fast.

Therefore we reach a compromise where we print up to 10 reasons for failure and omit the rest. If you want all the failures you can use --verbose.

Fixes #48

---

What do you think @andreasabel ? The other solution I can think of was to add a flag which controlled whether violations were printed out.

[ulysses4ever]

Therefore we reach a compromise where we print up to 10 reasons for failure and omit the rest. If you want all the failures you can use --verbose.

Sounds good to me.

[ulysses4ever]

On the second thought... I think the issue in the past was that collecting violations slowed down the tool quite a bit. That's why it was hidden behind the --verbose flag:

• Print line violations only under --verbose flag #49

So, this patch probably gets back to square one?

Re-using the verbose flag was also a bad idea because on big projects just printing out file names (what verbose have been always doing) quickly becomes overwhelming. So, I think, more flags are necessary anyway. E.g. verbose should be unrelated to printing violations, and a new flag with a numeric value and a default would say whether to print violations and if yes, how many. For instance --violations with 0 as the default -- and --violations 10 to do what this patch does.

[mpickering]

@ulysses4ever I checked the profile and the slow part is printing the violations to stdout. Always collecting warnings is slightly slower, but not noticeable.

[ulysses4ever]

Oh, that’s good then. I still think decoupling violations printing from --verbose would be a good improvement but may be it can be done separately.

Just FYI, I can’t approve things here, just sticking around as a past contributor.

[mpickering]

Thanks Artem.

I don't mind splitting up the flag if that's what @andreasabel prefers. My goal is a solution which can run on CI and you can see what the violations are.

[andreasabel]

Sorry for the delay in looking into this.
How about the following refinement of --verbose?

• no flag: no error printed
• --verbose=all : all errors printed
• --verbose=N : at most N errors printed, defaults to 10, so
• --verbose : at most 10 errors printed

[andreasabel]

I pushed a commit to this extend. What do you think, @mpickering and @ulysses4ever ?

[ulysses4ever]

@andreasabel it's a fine interface but I don't think it solves the original problem. If you follow the link to the cabal ticket, you will see the crux of it well-put by Matt:

Either, the failures should be reported at the end, or run in non-verbose mode.

haskell/cabal#10744

Right now it's still hard to find one violation in a project with hundreds of modules, like Cabal, because of all the filenames that --verbose prints.

[mpickering]

Right, @ulysses4ever is correct, if --verbose still prints out each file it checks then your proposal doesn't fix the problem.

To be clear, the issue is this:

1. Normal mode, doesn't print out why the check fails.
2. --verbose prints out the reason the check failed (useful for CI)
3. But the output of --verbose also includes thousands of lines listing each files which are checked.

In Cabal CI we want to print the reason for the failure, but not all the files which are checked. Therefore we need a way to enable (2) but not (3). The original proposal was to make mode (1) more verbose to print the violations by default (up to a limit).

[ulysses4ever]

happy 2026! it'd be good to do something about this this year: we keep eating these endless [checked] lines and it's not tasty...