I actually just started playing around with this today, after seeing the reference via Chucklefish's blog: http://blog.chucklefish.org/?p=154
TL;DR - Apparently they're stepping away from it with their latest game, primarily because of its focus on 2D rendering. It's heavily influencing their newest internal toolset though, called Cove (not opened up yet, as far as I can tell).
I used Helm for a simple 2D simulator/prototype for edge detection and orientation. Link to code below (it was my first Haskell program). While it was not a game in any sense, I found it quite fun and easy to use.
https://github.com/rgleichman/sense/blob/master/AngleGripper...
That's the biggest problem with pure FRP: people like to talk about it and make libraries for it, but non-trivial game implementations are rare. I am still not sure what is supposed to be a unit of re-usability (like components/systems in ECS) in FRP?
They do have some examples of the engine working in the "guide" pages [1], but I don't really know enough about the technology to be impressed. "It's just a black triangle."
TL;DR - Apparently they're stepping away from it with their latest game, primarily because of its focus on 2D rendering. It's heavily influencing their newest internal toolset though, called Cove (not opened up yet, as far as I can tell).