Play didn't invent the iteratee concept, Oleg Kiselyov did (http://okmij.org/ftp/Streams.html http://okmij.org/ftp/Streams.html). I asked myself the same questi

Play didn't invent the iteratee concept, Oleg Kiselyov did (http://okmij.org/ftp/Streams.html http://okmij.org/ftp/Streams.html). I asked myself the same question, as I'm doing the reactive programming course on Coursera, where Rx was the topic of the last couple weeks. I came to the conclusion that Rx focuses on side-effects while iteratees concentrate on iterative calculations. Iteratees calculate a result while Rx-observers just perform side-effects. Maybe someone else can further elaborate on this topic.