I didn't enumerate the reasons that I don't use dynamic languages, because that would have been flame-bait territory.

I see this suggestion as a way in which static languages could be better at prototyping in the same way that dynamic languages sometimes are, so I think you noticed my (implied) assumption that there was something desirable about this idiom that is not as readily achievable using existing methods in languages like ML or Haskell.

That said, I can't see that there is any reason why what I suggest wouldn't be useful in a dynamic language, too. The implementation details just change a bit.

Even bringing dynamic languages into the conversation was an just to counter to people saying "Use ___" where ___ is one of Python, Ruby etc, which is basically a non-solution for my purposes.