But what is the meaning of "pure" here? Stretching back over the last forty years of Lisp, then Prolog, etc., it's a moving target. "Pure" once meant using rout

But what is the meaning of "pure" here? Stretching back over the last forty years of Lisp, then Prolog, etc., it's a moving target. "Pure" once meant using routines instead of GOTO. Then it meant structured instead of GOTO. Then recursion rather than iteration, followed by structured recursion with fold rather than explicit recursion. Most recently, it meant first-class functions. Read a lot of papers from the 90's and you'll see this being used as the definition of a real functional language. And lambdas and currying. Now that we've all seen the power these things give, us, they're now mainstream and we have a new definition of "functional programming" in terms of type-controlled effects.