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

Question

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 Paul Hudak, and found that the domain specific language "FAL" (for Functional Animation Language) presented there actually works quite pleasently for my simple toy systems (although some functions, notably integrate, seemed to be a bit too lazy for an efficient use, but easily fixable).

My question is, what's the more mature, up-to-date, well-maintained, performance-tuned alternative for more advanced, or even practical applications today?

This wiki page lists several options for Haskell, but I'm not clear about the following respects:

1. The status of "reactive", the project from Conal Eliott who is (as I understand it) one of the inventers of this programming paradigm, looks a bit stale. I love his code, but maybe I should try other more up-to-date alternatives? What's the primary difference between them, in terms of syntax/performance/runtime-stability?

2. To quote from a survey in 2011, Section 6, "... FRP implementations are still not efficient enough or predictable enough in performance to be used effectively in domains which require latency guarantees ...". Alghough the survey suggests some interesting possible optimizations, given the fact that FRP is there for more than 15 years, I get the impression that this performance problem might be something very or even inherently difficult to solve at least within a few years. Is this true?

3. The same author of the survey talks about "time leaks" in his blog. Is the problem unique to FRP, or something we are generally having when programming in a pure, non-strict language? Have you ever found it just too difficult to stabilize an FRP-based system over time, if not performant enough?

4. Is this still a research level project? Are the people like plant engineers, robotics engineers, financial engineers, etc. actually using them (in whaterver language that suits their needs)?

Although I personally prefer a Haskell implementation, I'm open to other suggestions. For example, it would be particularly fun to have an Erlang implementation --- it would then be very easy to have an intelligent, adaptive, self-learning server process!

Answer 1

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 S?ylemez (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.