What's the status of current Functional Reactive Programming implementations?

I'm trying to visualize some simple automatic physical systems (such things as pendulum, robot arms,etc.) in Haskell. Often those systems can be described by equations like df/dt = c*f(t) + u(t) where u(t) represents some kind of 'intelligent control'. Those systems look to fit very nicely in the Functional Reactive Programming paradigm. So I grabbed the book "The Haskell School of Expression" by