Add support for formatting .yrl and .xrl files

[michalmuskala]

cc @ilya-klyuchnikov

[slaykachu]

Jumping on this task, I have some questions:

erlfmt can process arbitrary files. So am I correct by summarising the task as:

• Manually identify the rules that shouldn't be applied to yrl/xrl.
• Filter them in the code when file have extension yrk/xrl (maybe the ignore-list is different for each, not big deal).
• Mute the warnings (syntax error) for non erlang-like sections (or should we plug a dedicated grammar?)
• Process yrl/xrl files when formatting a whole directory.

[awalterschulze]

Each erlang form ends with a .
In a .yrl file, these forms can be mixed and either be a normal erlang form or a yrl specific form.
Currently we warn on any forms (including .yrl specific forms) that we cannot parse.
We should keep on warning on forms we cannot parse, but we want to add the ability to parse .yrl specific forms.

Here are some things we will need to add:

1. I think we will need to add the ability to parse .yrl specific forms into our erlfmt_parse.yrl grammar OR
   we can create a separate grammar that can only parse the specific .yrl forms and use it as a fallback when parsing normal erlang forms fails.
2. erlfmt_recomment.erl will also need to be able to add comments back into the AST
3. Finally we need to add the ability to erlfmt_format.erl to be able to format these new expressions.

[awalterschulze]

@michalmuskala I think I saw a specification for .yrl grammars previously, but I can't find it again for some reason. Do you know if I was imagining things or if this exists somewhere?

[michalmuskala]

It's in here a bit lower: https://erlang.org/doc/man/yecc.html#more-examples

I agree adding first-class support for yecc forms would be probably the "nicest" solution

[ilya-klyuchnikov]

It can be done gradually. - There is the explicit "Erlang code." part of .yrl files - adding support for understanding and formatting it may be the first step.

[awalterschulze]

It's in here a bit lower: https://erlang.org/doc/man/yecc.html#more-examples

I agree adding first-class support for yecc forms would be probably the "nicest" solution

Amazing thank you, I somehow missed that.

[michalmuskala]

It can be done gradually. - There is the explicit "Erlang code." part of .yrl files - adding support for understanding and formatting it may be the first step.

Looks like this actually mostly works today - just issues a lot of warnings. #289