We're collaborating with @yukihiro_matz, @mametter, @soutaro and Jeff Forster to make sure that types are not disruptive to Ruby. Thus, types are optional. The intention is to deliver value for unmodified Ruby programs. Hear more from Matz at
https://youtu.be/cmOt9HhszCI?t=2148
So, something along the lines of https://github.com/soutaro/steep but without the annotations in the original source code, because Matz said "no annotations". That's nice because it doesn't pollute the code. I use Ruby because of Rails and because I don't have to write types. I can use many other languages if I want to write them.
As long as types can be required to be explicit where ambiguous (e.g., TypeScript) in the file itself (via a magic comment or similar), I'm all for it. I am happy to declare types for external calls if I need to.
I have said for awhile that "Ruby with types" would be my favorite language to work in. I recently returned to Ruby briefly and had to integrate with a poorly-documented API. I spent more time digging through third-party code trying to figure out what certain parameters were supposed to be than writing the program itself.
