Add support for Tag Expressions

[aslakhellesoy]

Summary

Tag Expressions is a more intuitive way to specify tags in tagged hooks and on the command line's --tags argument.

Expected Behavior

Tagged Hooks should accept a single string:

Before("@foo and not @bar") do
end

Likewise, the --tags command line argument should allow the same. (Specifying it more than once should cause an error)

--tags "@foo and not @bar"

Context & Motivation

The current mechanism to specify or with comma and and with multiple --tags is incredibly awkward. Tag Expressions are intuitive!

[nodo]

@aslakhellesoy I am quite new to the project, but I would be happy to give this feature a try. What do you think?

[aslakhellesoy]

That would be fantastic!

The first step would be to port the TagExpressions library to Ruby. See cucumber/common#60. It's a tiny codebase! Just use the JavaScript or Java code as inspiration.

When that's done, send a PR to cucumber/cucumber and I'll mirror it at https://github.com/cucumber/tag-expressions-ruby so it can be consumed as a git gem dependency from the cucumber-ruby project while you're working on it. Here is how I recommend you do that:

• First make sure you can run all cucumber-ruby tests successfully
• Locate all the old tag expression code and modify/delete it to use the new tag-expressions gem.
• Update existing tests (maybe add some and delete some)
• Send a PR to cucumber-ruby when it's working
• If all is good, I can release tag-expressions to rubygems, and we can depend on that instead of git.

How does that sound?

[nodo]

Sounds good, thanks a lot @aslakhellesoy :) I am planning to dedicate some times next week and keep you updated here.

[aslakhellesoy]

That's great @nodo - let us know if you have any questions. You can also try the contributors chat room.

[mattwynne]

If someone wants to take a crack at integrating @nodo's work in cucumber/common#67, I'd like to ask that they help us to stay backwards-compatible for one major release by making the choice of old / new tag expression syntax a configuration option. That will enable us to transition users over to the new syntax gradually. We can make it opt-in first, then make it the default for the next major release.

[aslakhellesoy]

Great work so far @nodo! I have released cucumber-tag_expressions 1.0.0!

[aslakhellesoy]

I second @mattwynne's comment about backwards compatibility.

In the next version, use the new tag-expression library only if:

• Only one --tags option is used on the CLI or only one string is passed to a tagged hook.
• Its value doesn't contain any commas (that's old school)

Also, print a deprecation warning in the other case, telling people that:

• They should start using the new tag expressions (link to http://docs.cucumber.io/tag-expressions/)
  • We might need to add a migration guide here with some examples
• Old school tag expressions will go away in the next major version
  • No more multiple --tags on the CLI or multiple strings passed to tagged hooks - that's an error
  • No more commas in tag strings (that's a tag expression parse error)

WDYT?

[nodo]

Sounds good @aslakhellesoy , I will give it a try and keep you updated.

Thanks for the hints.

[mattwynne]

Automatic identification of legacy tag expressions (with a deprecation warning) would be awesome UX if we can achieve that.

Remember tags are used in hooks as well as on the CLI.

[aslakhellesoy]

Hi @nodo did you make any progress on this?

[nodo]

@aslakhellesoy not yet, after seeing added to milestone 4.0 I thought to concentrate on #954. I would like to pick it up asap though.