[electrum]

These gradual type checking frameworks allow migrating massive code bases incrementally. Doing a complete big-bang rewrite into a different language isn't feasible (for the obvious software engineering and business reasons).

[growtofill]

So you can introduce a type system in a module, but you can’t write new version of that module in a different language? To me these seem comparable in scope.

[netheril96]

They are in no way comparable in scope. Introducing a type system gradually doesn't break any compatibility, within the module or outside the module. It doesn't require you to understand what the legacy code is doing. Rewriting in a different language also means different libraries, different glue code, different interfaces, different everything.

[alangpierce]

I've been adding TypeScript types to a bunch of JavaScript code recently, and it really is much safer and faster than trying to move to a different language. As long as you are only adding types, you don't have any chance of introducing a bug, since the types just compile away, and plenty of real-world code (especially untyped code) tends to have subtle details that you might overlook if you're reading it for the first time.

Another thing to keep in mind is that type annotations can be done incrementally. You can't take 1000 Python files and port them to Go one by one, but you can add types a little bit at a time with a completely usable codebase at every intermediate step.