Stripe has built a type checker for Ruby that they're using internally. I don't think it has been released yet, but you can get a preview of it at https://sorbet.run/.
So it's something along the lines of Erlang / Elixir dialyzer. I don't like it much (dialyzer) because of the verbosity and unDRYness. Every function definition becomes two lines and the name of the function is repeated in both. Type names are longer than they should and are separated from argument names.
