I'm sure there will be more of a point to this later, but all you've demonstrated is recursive style vs non-recursive style which doesn't have much of anything to do with imperative vs declarative style. You're SUM() example could be just as easily coded as a 'single assignment' recursive function in any procedural language that support s recursion (in as few lines with as much programmer efficiency). But as you say "how the rules are executed is up the declarative language". How the rules are executed is also up to the imperative language. Each coding method can be equally tuned (or broken) for parallel execution. »