@noelrap For me it's about tightening up the feedback loop.

I want the type checker to tell me that assumptions I've made are wrong and that, for example, the method I've been testing with a symbol parameter is going to be passed a string from the API layer. It sucks to only find that one out in production when the alerts start going off.

Stupid mistakes like that should be surfaced as soon as I've finished typing the method call/signature.