I think it's a genuine weakness of static types that they inhibit prototyping when you first begin. HM type systems tend to be liberal and powerful enough that

I think it's a genuine weakness of static types that they inhibit prototyping when you first begin. HM type systems tend to be liberal and powerful enough that once you learn them well they're easier to prototype in—but that comes with time. In particular, type-driven-development is wonderful. You can write light, compiler-checked specifications of code you haven't bothered to write yet and see if your logic all typechecks even without implementation. I can run through lots of ideas that way very quickly since the only thing I have to produce is the high-level type skeleton of my program. Then I just go through and implement it all. It's a lot like test driven development except that your executable tests are much more lightweight and carry a strong sense of the logic of your program. If your tests don't hang together, you'll find out eventually. If your types don't hang together, your compiler tells you immediately.