Here's what Rx is: The idea of a list are some items in an order (a Sequence in FP-speak). On top of a Sequence you can implement all kinds of interesting operations like map/filter/reduce.

If you look at an Event like KeyUp, you'd end up with some items, in a particular order. Thus, you realize, that events and lists really are the same thing. In fact, they're not only ostensibly the same, they are mathematically provable to be the same.

Rx therefore, gives you all the operations you can do to a list, and applies it to an event (or a callback, which is just an event that only happens once).