>It's definitely not harder to prototype in - and I say this as a functional programmer

Have you used haskell to make the comparison?

>if you're writing Go idiomatically, you really shouldn't be thinking very much about the types as you write them.

I don't understand where you are coming from here. I am not thinking about types, that is why I need the compiler to point out when I mess them up. The problem is go has such a limited type system, that you have to change much more code when you refactor, and the type system is inadequate for catching many errors, in particular dealing with error handling. The combination makes go worse for refactoring than haskell. It is certainly much better than python for example, but you seem to be convinced that go is the top of the spectrum and nothing can exist above it.