Open-sourcing Sorbet: a fast, powerful type checker for Ruby
I have mixed feelings about adding type annotations to an existing project. IDEs become easier to use, you can avoid certain bugs, refactoring becomes a bit less error-prone. But this comes at a cost: you need a very high type coverage, which means you need to rewrite a lot of code to deal with the different style of polymorphism. It's very likely that you end up with code that looks as if it were written in a statically typed language but without any of the performance benefits of such a language.