It's not exactly what you want, but if you want a statically typed ruby-like you should check out crystal (http://crystal-lang.org/)

Crystal looks interesting but I can't really migrate to a new language. Something that compiles down to plain Ruby would be great though.

Not sure if this will do, but there's Contracts for Ruby: https://github.com/egonSchiele/contracts.ruby

You might want to check out rubocop[1]. It does static checking, and we're pretty happy with it.

1: https://github.com/bbatsov/rubocop

Does it actually do static type checking? Because I don't see anything about types in the documentation. It looks like just a more sophisticated linter, which is very different from what TypeScript and Flow bring to the table.

Ah, you're right. It's "just" a linter... but a pretty good one.

I started using rubocop earlier this year and I've become a huge fan. It's done a lot to improve the quality of my code and the consistency of code on projects with multiple developers.