Recently I wrote that it was already possible to run RuboCop with Ruby’s new Prism parser, but that required a bit of manual work. I also outlined some plans to add built-in Prism support in RuboCop. 1 Today I’m happy to report that last week we’ve released RuboCop 1.62 which features (experimental) support for Prism!

Now using RuboCop with Prism is just a matter of adding Prism to your Gemfile (assuming you’re using Bundler, that is):

gem 'prism'

and adding the following to your RuboCop configuration:

AllCops:
  ParserEngine: parser_prism
  TargetRubyVersion: 3.3

Magic! Initial benchmarks we’ve done suggest that using Prism results in about 70% speedup of the parsing speed, but I’d take those with a grain of salt as we’re very early in the process of optimizing RuboCop and rubocop-ast for Prism, plus the results you’d get would be strongly correlated with what you’re parsing.2 At any rate - it’s pretty certain that Prism is faster than whitequark/parser.

Why is the Prism support considered “experimental”? There a few reasons for this:

  • Prism currently supports parsing only Ruby 3.3+, compared to whitequark/parser supporting Ruby 2.0+. This means Prism is still not a complete replacement for whitequark/parser in all use-cases.
  • There are a bunch of small issues with Prism’s parser translation that have to be fixed. In other words - for the time being this means that some cops won’t be working properly with Prism. You can monitor the list of outstanding Prism issues related to RuboCop here.
  • We might decide to make some changes to the configuration options. (although this seems unlikely)

I expect that as more people start to use RuboCop with Prism we’ll quickly identify and fix any outstanding problems. I’d encourage the more adventurous people to play a bit with RuboCop and Prism and report their findings. (e.g. issues discovered, benchmarks, etc)

RuboCop will keep supporting whitequark/parser for the foreseeable future, so there’s no rush for anyone to switch to using Prism today. Also, it’s not like we can stop supporting whitequark/parser until we switch to using Prism’s AST format natively - currently we’re using Prism as a backend for whitequark/parser.3

That’s all I have for you today. Keep hacking!

  1. See my previous article. ↩

  2. See https://github.com/rubocop/rubocop-ast/pull/277#issuecomment-1962383316 ↩

  3. See https://github.com/ruby/prism/blob/main/docs/parser_translation.md for details. ↩