I agree. I've migrated multiple large codebases from a weakly-typed language to a strongly-typed language, and in my experience this is the only way to do it. A

I agree. I've migrated multiple large codebases from a weakly-typed language to a strongly-typed language, and in my experience this is the only way to do it. And it's also not as bad as the OP makes it sound: you get your stronger guarantees gradually, and that's fine. One gotcha, though: a stronger type system encourages the removal of runtime checks. When doing a partial conversion, I've found it's best to leave any existing runtime checks in place, even if the type system makes them "redundant", because it doesn't really until the calling code has also been converted. Once it has, you can go back and remove them.