No, you don't. But the question at hand isn't using a dynamically vs. a statically typed language, it is, given that you are in a statically typed language, is

No, you don't. But the question at hand isn't using a dynamically vs. a statically typed language, it is, given that you are in a statically typed language, is a Java-style one sufficient? Indeed, this is one of the benefits of dynamically typed languages: you're not restricted to any particular type discipline; you don't have to worry that the type system of the language you're in is too restrictive. OTOH, if you want it to be easy to write reusable code, having static typeclasses with type inference can be quite useful. You could write a monad transformer stack in a dynamically typed language, but you can't always write code that's generic across any monad without explicitly passing around an object that represents the monad in question. (Racket's "units" might be a reasonable solution to this problem, though.)