Functional Reactive Programming

This is really a neat idea that I've mentioned before, but reactive programming is the idea of setting up a network of values (Elm calls these signals - values that change over time) that affect one another dynamically.  This is an inherently declarative style of programming (as the second link there explicitly states).

On that same site, there's an excellent exposition of how to write a Pong game in Elm using this programming style, and a more recent article on how reactive programming avoids the proliferation of callbacks that characterize API-style and more recent non-blocking frameworks like node.js.

Elm itself is a language that compiles to JavaScript on the browser and (I think) on the server as well.  It seems to have some way to integrate with Haskell on the server, but I honestly haven't spent much time investigating it - and I should. I should investigate all the things.