Sorbet appears to be a language extension rather than a library, similar to TypeScript or Flow in the JavaScript ecosystem. I’m guessing that one of their goals

Sorbet appears to be a language extension rather than a library, similar to TypeScript or Flow in the JavaScript ecosystem. I’m guessing that one of their goals was ease of use for type annotations. I’ve had a quick look at RDL and while I quite like its syntax it is somewhat more cumbersome than having type annotations “inline”, as it were. Furthermore, the annotations in RDL are themselves stringly typed, which comes with its own issues.