RBS, Ruby’s new type signature language
Didn't realize Square was interested in Ruby type checking, just like their competitors over at Stripe. Lots of money riding on Ruby, I guess :)It does seem useful to have a _standard_ for type definitions - RBS as the equivalent to a .d.ts file - as that allows for different type checking implementations to use the same system under the hood. This was a big problem for Flow, and why it lost the fight as soon as TypeScript's definitely-typed repository started gaining momentum - users wanted to use the type-checker that they knew had definitions for the libraries they used.On the other hand, RBS as hand-written seems rather dangerous, to me. Nothing wrong with using them to define previously-untyped external code, as long as you know the caveats, but I think you really want to have definitions generated from your code. Sorbet cleverly (and unsurprisingly, given it's Ruby) used a DSL for definitions in code, which had the (excellent) additional boost of runtime checking, so you actually could know whether your types were accurate - by far the biggest pain-point of erased-type systems like TypeScript.Given that Ruby 3 was supposed to "support type checking," I'm surprised that it does not seem to have syntax for type definitions in code, and instead will focus on external type checking. I might be missing a piece of the full puzzle not covered in the blog post, however.