Right now there are mainly two practical Haskell libraries out there for functional reactive programming. Both are maintained by single persons, but are receiving code contributions from other Haskell programmers as well:
-
Netwire focusses on efficiency, flexibility and predictability. It has its own event paradigm and can be used in areas where traditional FRP does not work, including network services and complex simulations. Style: applicative and/or arrowized. Initial author and maintainer: Ertugrul Söylemez (this is me).
-
reactive-banana builds on the traditional FRP paradigm. While it is practical to use it also serves as ground for classic FRP research. Its main focus is on user interfaces and there is a ready-made interface to wx. Style: applicative. Initial author and maintainer: Heinrich Apfelmus.
You should try both of them, but depending on your application you will likely find one or the other to be a better fit.
For games, networking, robot control and simulations you will find Netwire to be useful. It comes with ready-made wires for those applications, including various useful differentials, integrals and lots of functionality for transparent event handling. For a tutorial visit the documentation of the Control.Wire module on the page I linked.
For graphical user interfaces currently your best choice is reactive-banana. It already has a wx interface (as a separate library reactive-banana-wx) and Heinrich blogs a lot about FRP in this context including code samples.
To answer your other questions: FRP isn’t suitable in scenarios where you need real-time predictability. This is largely due to Haskell, but unfortunately FRP is difficult to realize in lower level languages. As soon as Haskell itself becomes real-time-ready, FRP will get there, too. Conceptually Netwire is ready for real-time applications.
Time leaks aren’t really a problem anymore, because they are largely related to the monadic framework. Practical FRP implementations simply don’t offer a monadic interface. Yampa has started this and Netwire and reactive-banana both build on that.
I know of no commercial or otherwise large scale projects using FRP right now. The libraries are ready, but I think the people aren’t – yet.