Design a way to emit warnings without spamming the user

[rosun82]

Description of the problem / feature request:

Currently skylark provides a "print" function, which produce a line prefixed with "DEBUG". It is often useful to print a line with prefix "WARNING", however, this is no such function right now.

[laszlocsomor]

/cc @vladmos

[laurentlb]

Hi,

Thanks for the feedback. You're not the first person to ask for it, but we have to be careful.

• The person who sees the warning is not necessarily the person who can fix it.
• When warnings accumulate on the output, users will start ignoring them. They will perceive the output stream as spammy. They will not notice when a new warning was introduced.

I think it's hard to do warnings correctly based on dynamic information. Imagine you want to warn about a specific value your function received. How to know who is responsible for it? How can we know where the value comes from exactly? The immediate caller of your function is not necessarily responsible for the value.

I think warnings based on static information are more reliable in general. As a proof of concept, we have a linter that can detect calls to deprecated functions: https://github.com/bazelbuild/bazel/blob/master/site/docs/skylark/skylint.md#deprecating-functions-docstring-format-deprecated-symbol

My preference would be to develop more static checks like this, instead of adding a new function.

[pauldraper]

@laurentlb I love those ideas in theory.

I would also love those ideas in practice....if you could get the rules in Bazel core to follow them. (e.g. strict java deps).

[benjaminp]

I don't understand the relevance of strict Java deps. By default, those are errors not warnings.

[vmax]

A fine-ish approach would be to use colored print:

def warn(msg):
    print('{red}{msg}{nc}'.format(red='\033[0;31m', msg=msg, nc='\033[0m'))

[pauldraper]

I don't understand the relevance of strict Java deps.

Strict Java deps uses warnings.

It shows a good example of when rule authors (in this case, the Java rule authors) needed warning capabilities.

[brandjon]

I agree that warnings are generally spammy to the majority of people who will see them, and that we shouldn't encourage rule, macro, and repository rule authors to add more of them. I also agree that we could be doing more to provide better alternatives, whether those be static linters or opt-in filters of some sort.

I don't think we can prioritize this anytime soon though. Leaving the issue open for further design discussion.

[celestialorb]

Wouldn't it make sense to have the ability to produce warnings if you're creating custom rules? What if I want to warn the user about potentially nonsensical inputs to a rule? I don't see how it would be different from sanity checking inputs to a function in another language.

What about deprecating inputs to a rule? I'd prefer to warn the user about the deprecation for some time before forcing an error. Without a way to produce a warning I'd have to create a new rule and have users migrate to that if I wanted to clean up a rule. Granted this shouldn't happen often, but it does happen.