To be honest I'm not even sure what reactive programming means. I took the Reactive Programming course from Coursera, by Odersky, Kuhn & Meijer, and in the forums Kuhn explicitly said it's not functional reactive programming.

I'll admit I'm confused by the relationship between FRP, RP and Akka's brand of RP. The only thing I'm certain is that Akka's RP != FRP. The assignments from the course were full of side-effects, mutability and global variables.

Rx is not FRP, for sure, but even most impure FRP systems (like flapjax) are not really FRP given a pure definition.

"Reactive" is quite vague. The earliest programming systems labelled "reactive" were meant for expressing low level real time control behavior, but these days the word could just as well be related to UI or enterprise/web data processing (note, that the above projects don't really help with UI, they are more for the latter!).

Because reactive programming is a pretty specific (though not overly specific) programming paradigm, while the manifesto talks about ideals in software design. You can achieve these ideals with or without reactive programming, and you can use reactive programming without aiming for those ideals.

Both are nice, but they're two different things.

How do you define reactive programming?

As a model in which you define dependencies among different changing variables, and any change in the value of a variable – which can happen at any time, even concurrently with other variable updates – triggers the re-computation of all dependent values.