I do understand that aspect of their proposal. My point is that causes unnecessary amounts of work. In typical Clojure, there's a very high ratio of pure functi

I do understand that aspect of their proposal. My point is that causes unnecessary amounts of work. In typical Clojure, there's a very high ratio of pure functional code to STM code. i.e. you'll probably make 10 or 100 pure functional calls between each STM interaction. Under the current pypy proposal, they're going to STM-ize every function call, regardless of whether it needs to do STM work. If the language had been built with STM in mind, with an explicit STM call, more user and library level code would adapt to that, and use STM sparingly.