I want more than this. This article talks about adding type annotations, and if the variable has an annotation, then the type is checked. That is, you have to change the source code to use it.

I want a knob I can turn at compile time. "Compile this dynamically, so I can run it and play with the algorithm. OK, now I think I've iterated the algorithm enough times that I'm fairly confident in it, now turn on type errors as warnings. Now that I've cleaned up all of that, let's go back to the default, which is to compile with type issues being errors."