Sorbet: Stripe’s type checker for Ruby

By 2017, Stripe had grown to the point where hundreds of engineers had written millions of lines of code. Most of that code was—and still is—written in Ruby, which is famous for helping engineers iterate quickly (if somewhat notorious for encouraging inscrutable code). Unfortunately we were starting to see Ruby come apart at the seams: new engineers found it hard to learn the codebase, and existing engineers were scared to make sweeping changes. Everyone faced a constant tradeoff: run the fast, local tests which might not catch many breakages, or run all the tests, even the slow ones. Ruby was becoming a source of friction more than a source of productivity.