Rule formatting tool.

[juliusv]

Like "gofmt" for Go, we ought to have a "promfmt" for Prometheus since we have a syntax tree. The idea being that the system produces uniform style that minimizes deviation and learning curve.

[grobie]

👍 * 💯

[fabxc]

Yep, that'd be great. Preserving comments is the biggest (and basically only) problem. And agreement on how to split expressions over multiple lines.

[brian-brazil]

Aren't we in the middle of adding this to promtool?

[mattbostock]

Related: #1779

[beorn7]

Some observations:

• With the 2.x rule format (YAML embedding PromQL), we want this to act on YAML files, too.
• It should preserve both comments in YAML and in PromQL.
• Indentation is crucial (not just line breaks).
• It has to be seen if certain line breaks in the input should be preserved in the output (cf. how gofmt does it).
• Wherever a rule is rendered, the same formatting should apply, in particular on the Alerts and Rules tabs of the Prometheus web UI. This implies that the AST needs to keep comments (and possibly certain line breaks, see previous point).

[brian-brazil]

It should preserve both comments in YAML

Our current YAML library doesn't do this, but is hoping to "soon".

[geekodour]

I'd like to include this in my gsoc proposal. @brian-brazil, can you link to the YAML library you're referring to?

[brian-brazil]

https://github.com/go-yaml/yaml, but I'd scope this to just the PromQL as this stands.

[juliusv]

@brian-brazil But since the PromQL from rules is always embedded in YAML files, it at least has to preserve YAML comments, right? Otherwise the tool wouldn't be very useful as nobody wants to lose their comments the benefit of formatting.

[brian-brazil]

That'd be nice, but we can get benefits without that. I'd rather also not depend on something with an unclear timeline for a gsoc project.

[beorn7]

Let's start with the PromQL part. The YAML part should be almost trivial once the library preserves comments and things like the |2 indentation hint for multiline strings.

[juliusv]

Ok!

[brian-brazil]

v3 of the YAML library is out, which should allow for all of this.