Functional Reactive Programming with Bacon.js

Functional Reactive Programming with Bacon.js(blog.flowdock.com)

114 points by trevoro 13 years ago | 36 comments

asolove 13 years ago |

I am not an FRP expert but I've played around with both Elm [1] and Flapjax [2]. With both libraries, I ran into a basic mismatch with what I wanted to do. They both make it very easy to have static controls create dynamic behavior. You create the controls, observe them, and then combine their signals to output whatever dynamic output the program has.

But most interesting web UIs are not static->dynamic; they instead involve dynamic and circular dependencies. Pressing the "add new todo" control doesn't just dynamically affect the display, it also creates new controls that need to get added into the set of observable things that affect the display. This idea, of modifying existing observables to include new observables, is somewhat problematic. In examples I've seen, it ends up leading back to code just as unfortunate as imperative callbacks.

If you can use Bacon to create an entry for TodoMVC [3] showing how to have dynamic->dynamic dependencies, I'll be sold.

[1] http://elm-lang.org/

[2] http://www.flapjax-lang.org/

[3] http://addyosmani.github.com/todomvc/

tikhonj 13 years ago | |

This is where "event switching" comes in. Unfortunately, I have not used Elm or FlapJax, so I am not sure if the terminology is the same there.

Traditional FRP has two main concepts: behaviors, which are values that can change continually, and stream of events. Scanning over this blog post, these ideas correspond roughly to Properties and EventStreams respectively in Bacon.js.

Event switching is just having an event stream that itself contains behaviors. There's a great blog post[1] about this using Reactive Banana and Haskell.

[1]: http://apfelmus.nfshost.com/blog/2012/09/03-frp-dynamic-even...

Most of the post is fairly Haskell-specific, talking about the type system, but the basic idea should be applicable here as well.

The running example is a bar tab. You want to be able to create new entries in the tab, which are just text entries that give you a price. So you would create something like

    entries :: Event [Behavior Price]

That is, you have a stream of events that contains behaviors corresponding to each text entry widget you've added. This makes calculating things like the sum of all the widgets relatively easy.

I'm not sure how other libraries support this dynamic event switching though. My understanding is that it is somewhat tricky to implement efficiently.

RoboTeddy 13 years ago | | |

FWIW, Bacon.js implements switching with flatMap and flatMapLatest

Helpful diagrams: https://github.com/raimohanska/bacon.js/wiki

japsu 13 years ago | |

Have a look at pyykkis' implementation of TodoMVC using Bacon.js, Transparency and Backbone Models. I think he accomplishes just that.

https://github.com/pyykkis/todomvc/blob/bacon-transparency-r...

RoboTeddy 13 years ago | | |

It looks like events from newly added controls are captured here using asEventStream with jquery's event delegation.

More about event delegation here: http://api.jquery.com/on/

$('#container').asEventStream('click', '.someclass')

If an element with someclass is ever added as a child of #container, clicks on it will fire events. This is true even if the someclass element is added after the asEventStream call.

asolove 13 years ago | | |

Thanks! That one file is really instructive.

lautis 13 years ago | |

We're pragmatic about FRP and also rely heavily on Backbone. For some cases, the MVC approach seems better, for others Bacon is a better fit.

In Flowdock, there is a form that automatically validates parts of the data in JS while needs to confirm other parts with server. During server-side validation, a loading indicator is shown and success enables the submit button. Using Bacon, it has been easier to focus on the logic instead of handling state. But the logic is quickly quite hairy, indeed.

Japsu pointed out that there is a TodoMVC implementation with Bacon and Backbone, but it'd certainly be interesting to see/do a pure-FRP implementation.

rybosome 13 years ago | | |

I'm interested to see if using FRP principles in conjunction with Backbone makes sense. I love Backbone, but I feel like there are times I'm worrying too much about the DOM because I don't feel like abstracting out yet another subset of data into a model. If it was as quick and simple as calling a few functions (binding callbacks to existing view functions), then I feel like it would really clean up my code.

jcheng 13 years ago | |

Our functional reactive web framework is specific for the R language but supports what you're describing (if I understand you correctly).

http://rstudio.github.com/shiny/tutorial/#dynamic-ui (scroll down to reactiveUI)

An example: http://glimmer.rstudio.com/winston/dynamic_ui/ https://gist.github.com/4211337

danabramov 13 years ago |

TL;DR: Reactive Programming ≈ Sequence Comprehensions + Futures.

This looks pretty similar to a .NET library called Rx (Reactive Extensions). We're using it in a MonoTouch iOS app and I found it to solve some problems in a very convenient way. If you used functional style in your code, you know it shines in mapping/filtering/aggregating/reducing sequences of data. In .NET LINQ sequences have type of IEnumerable. You can think of Rx's IObservable as of IEnumerable's odd twin brother. The main difference is you IEnumerable is “cold” by its nature: it produces new items when you ask for them by calling MoveNext(). It doesn't do anything by its own. It's also synchronous.

IObservables, however, can both be cold and hot, i.e. they can yield items without being asked to. It's like IEnumerable but with “don't call me—I'll call you” approach. Because of this, it doesn't have to be synchronous. Long processes that emit one item at a time, which may later be transformed, processed in batches, throttled, etc, benefit from being expressed via IObservable. The benefit I found is you can easily switch between different asynchronous data source implementations.

For example, it's trivial to write an IObservable wrapper around iOS Photo Library that will yield one item at a time. Then you can write a function that compares cached items in array with new items that arrive from photo library, and return IObservable of “differences” between cached and new data. Then you group those differences using built-in methods into batches—and feed them to UICollectionView so it can animate the updates as the application receives them from system API.

Then you just write another IObservable implementation for getting photos from Facebook, and wrap it in a function that will pre-fetch their thumbnails before yielding them. It is also an IObservable. You already implemented updating collection view, so you just plug in your new asynchronous data source and that's it.

Need to implement a Dropbox file picker? You already implemented collection view logic, diff logic and prefetching images, so you just need to wrap Dropbox folder fetch API in an IObservable, and merge IObservable-s from nested folders.

On top of that, you can add some caching, and magically it will work for any data source.

stu_k 13 years ago |

Anyone who's interested in this should also take a look at Functional Reactive Bindings: http://documentup.com/montagejs/frb/

Boasts of realtime, 2-way bindings between any JS object.

nullzzz 13 years ago | |

That's interesting! Do you have an idea how they implemented bindings between arbitrary JS objects? Polling?

stu_k 13 years ago | | |

It uses ES5 getters and setters to replace the property and react to changes. This is what makes it realtime, after the set all bound properties will have their new values.

msutherl 13 years ago |

It really disappoints me when good projects are given names that trigger bad mental associations.

Sure, everybody loves bacon – except vegans, vegetarians, Jews, and other swine-shunning cultures – but I really don't want to be thinking about bacon when I'm programming. Nor do I want to evoke the smell, taste, and unhealthiness of bacon, nor the dirtiness of pigs, in the minds of people I talk to about Bacon.

Largely for this reason I will probably not try, use, or try to get my friends to use Bacon. Please consider changing the name, perhaps to something that evokes the conceptual spirit of the project, because I'd like to see more people adopting FRP techniques.

robertfw 13 years ago |

Does anyone know of anything similar to this in python? I did a quick look around but came up empty

davvid 13 years ago | |

Trellis comes to mind. There's also Twisted if you're interested in event-driven programming.

batgaijin 13 years ago |

I think this is related but not sure: http://www.cs.umd.edu/projects/PL/arrowlets/

frankus 13 years ago |

It strikes me that this would lend itself well to a graphical programming interface (a la Quartz Composer).

ilaksh 13 years ago |

Have you guys seen LiveScript?

maxcan 13 years ago | |

I have and I love it although it's fairly orthogonal to bacon.js. The problem with livescript is that although its absolutely beautiful to a haskeller like me, almost no one uses it so the tooling is weak and building a big project in it is a massive risk.