I don't know much about type theory but I did eventually manage to learn to interpret OCaml's type errors well enough to get things to compile. It still feels like driving a car by scraping it along a highway guardrail but it's often better than testing. I have to write twice as much code as in Python, it takes me longer to get it to run, and I still can't call it from C or Lua, but it runs twenty times as fast as Python and the process of thinking through the types helps me a lot to write code that works.

I keep hoping I'll internalize the type system enough that my programs run the first time they way they do in Python, but it hasn't happened yet.

I still often write a prototype in Python, though.