Once, on Facebook I wrote:
I believe in Rich Hickey now: and not just because he quoted Heraclitus. His metaphor for imperative programming as a baseball stadium is a powerful one. Loosely: imperative models require that at the moment ball meets bat, we stop the world and update every spectator to update their reaction; move forward one instant of time; re-update; etc. Finding a model for declaring how they ought react without a stoppage of time is something we need in order to to scale to big problems.