RBS, Ruby’s new type signature language

I'm really puzzled by the decision to use a separate file for this. The stated justification ("it doesn't require changing Ruby code") doesn't make sense, and my personal experience with languages with external type specifications is strongly negative. It's an unbelievable pain to keep multiple interface files in sync over time.

`.h` files are not something to emulate! External interfaces should be generated by tools where needed.

FWIW, you can use inline syntax with Sorbet[0], one of the two typecheckers that will work with the RBS format (the other being Steep, which does not have inline syntax).

Here's a full example, complete with a typo, based on the example in the blog post: https://bit.ly/3hMEMSp

Here's a truncated excerpt to get the basic idea across:

    # typed: true

    class Merchant
      extend T::Sig

      sig {returns(String)}
      attr_reader :name

      sig {returns(T::Array[Employee])}
      attr_reader :employees

      sig {params(token: String, name: String).void}
      def initialize(token, name)
        @token = token
        @name = name
      end

    end

Disclaimer, I used Sorbet while I was an employee at Stripe. I found it to be a terrific typechecker. It's also just absurdly fast (most of the time).

[0] https://sorbet.org