Comment by belmont_sup

To be clear, I myself prefer sound type systems.

But the reality is that teams have started with untyped Python, Ruby, and Javascript, have been productive, and now need to gradually add static types to remain productive.

> Every caller would need to check that the function...

The nice part here is where the gradual part comes in. As you are able to type more of your code, you're able to move where you add your runtime validation, and eventually you'll be able to move all validation to the edges of your system.