Flapjax: A Programming Language for Ajax Applications

I am immensely pleased to report that our paper on Flapjax was accepted to OOPSLA 2009.

This paper presents Flapjax, a language designed for contemporary Web applications. These applications communicate with servers and have rich, interactive interfaces. Flapjax provides two key features that simplify writing these applications. First, it provides event streams, a uniform abstraction for communication within a program as well as with external Web services. Second, the language itself is reactive: it automatically tracks data dependencies and propagates updates along those data?ows. This allows developers to write reactive interfaces in a declarative and compositional style.

Flapjax is built on top of JavaScript. It runs on unmodi?ed browsers and readily interoperates with existing JavaScript code. It is usable as either a programming language (that is compiled to JavaScript) or as a JavaScript library, and is designed for both uses. This paper presents the language, its design decisions, and illustrative examples drawn from several working Flapjax applications.

The real heroes of this story are my co-authors. Leo, Arjun, and Greg were there for the initial, heroic-effort-based implementation. Jacob and Aleks wrote incredible applications with our dog food. Shriram, of course, saw the whole thing through. Very few of my contributions remain: the original compiler is gone (thank goodness); my thesis work is discussed briefly in How many DOMs? on page 15. Here’s to a great team and a great experience (and a great language)!

Debounce and other callback combinators

It is serendipitous that I noticed a blog post about a callback combinator while adding a few drops to the Flapjax bucket.

Flapjax is nothing more than a coherent set of callback combinators. The key insight to this set of callback combinators is the “Event” abstraction — a Node in FJ’s implementation. Once callbacks are Nodes, you get two things:

  1. a handle that allows you to multiply operate on a single (time-varying) data source, and
  2. a whole host of useful abstractions for manipulating handles: mergeE, calmE, switchE, etc.

The last I saw the implementations of Resume and Continue, they were built using this idea. The more I think about it, the more the FJ-language seems like the wrong approach: the FJ-library is an awesome abstraction, in theory and practice.

C# GC Leaks

Reading this experience report from the DARPA challenge via Slashdot, I wondered: if event registration caused object retention, how can we deal with these memory issues in Flapjax?

Worrying about memory leaks in JavaScript is a losing battle — the browsers have different collectors. But given functional reactive programming in other settings (e.g., Java/C#), how can we solve this kind of problem? We looked at deletion briefly, but never had time to address the issue in detail. The complexity in our case is that the event registration encodes the entire application — how do you decide that a certain code path is dead? It may be helpful to view a functional-reactive program as a super-graph of data producing, splitting, merging, and consuming nodes; the application state is the subgraph induced by the active nodes reaching the consumers. Then there’s a certain static overhead for the program, plus the dynamic overhead of its active components.

Most of the Flapjax code I’ve written has been for the user interface, so binding and unbinding isn’t much of an issue: if the interface changes temporarily (e.g., tabs), the older behavior is usually recoverable and shouldn’t be collected. When working with more complex models with longer lived state, a deletion policy is more important. Shriram has been working on larger Flapjax applications with application logic as well as presentation — I wonder if he’s run into serious GC issues.

Lifting in Flapjax

In the Flapjax programming language, ‘lifting’ is the automatic conversion of operations on ordinary values into operations on time-varying values. Lifting gets its name from an explicit operation used with Flapjax-as-a-library; we use the transform method call or the lift_{b,e} functions. To better understand lifting, we’ll walk through a simple implementation of the Flapjax library.

I consider the (excellent) Flapjax tutorial prerequisite reading, so it will be hard to follow along if you’re not vaguely familiar with Flapjax.

The following code is all working JavaScript (in Firefox, at least), so feel free to follow along.

Continue reading

Flapjax Templates

The response to the template language in Flapjax has been mixed, to say the least. The most common complaint is that they mix content with style. True — this can be done with our templates. But nothing stops a developer from putting CSS in the HTML directly. Nothing except good sense, that is.

In response to a recent post on our discussion group for Flapjax, I wrote about this briefly:

…the only compiler specific syntax is the templates: curly-bang — {!, !} and triple-stick — |||. But these are parsed out of HTML PCDATA and attribute nodes, so a JavaScript/Flapjax programmer needn’t consider them. We introduced the syntax as a way to reduce the clutter of “insertDomB” and “insertValueB” calls. For example, the template language is:

It's been {! currentSeconds ||| (loading...) !} seconds since the beginning of 1970, and it's all been downhill.

while we must write the following without it:


It's been (loading...) seconds ... .

This can become a huge mess, so the templates simplify it. We wouldn't want to encourage doing all of your computation in the display layer! I'd compare it to Smarty or JSP -- while you can include complex computations in the tags, it's meant only to ease the interface between models in code and presentations on screen.

I think that soundly characterizes the template language. Shriram wants that in the tutorial, so that'll show up there eventually. (Some people take classes during the semester, but the Flapjax team has much more important things to do. Or so we're told.) But it would be a big mistake to see Flapjax as providing just that -- the functional reactivity is the interesting bit.


Tonight we're presenting the language to the undergraduates in the DUG; there's an exciting contest announcement coming up, as well. Who has time for midterms?

Flapjax

Since I got back from Israel, I’ve been working on a top secret project: a programming language for the Web. Well, for the Web 4.0 — we gave 3.0 a miss. The language is Flapjax. As you’ll note on the homepage linked above and on the Flapjax development blog, it’s multifaceted. I’ll mention the salient major features here.

The main feature is functional reactivity, found in flapjax.js. Functional reactive programming (FRP) has been around for a while in the Haskell community. The PhD lead on our project, Greg Cooper, wrote FrTime (read Father Time), which embeds FRP in Scheme.

To learn more about FRP, you might as well walk through our tutorial. It’s a callbackless world in which values vary over time and whole expressions are appropriately re-evaluated. For example, the text in a textbox can be computed with over time — no need to register callbacks for onfocus, onblur, onthis, or onthat.

In essence, FRP is a monad. But in practice, this means that FRP is a driver/callback manipulator and CPS-ed code. In FrTime, CPS-ing isn’t done directly, but instead all of the language’s primitives (+, cons, etc.) are lifted. In Flapjax, either the programmer does it manually or the compiler (my work) translates the code to CPS. There are arguments in either direction — the compiler’s aggressiveness can make it hard to use.

While on the topic of the compiler, we also introduced a templating language for in-lining Javascript/Flapjax in HTML elements and attributes. More on this and it’s utility later.

But the Web 4.0 has to subsume all of Web 2.0. Which is where the -jax morpheme comes in. Given a functionally reactive language, we can deal with values from a server — via AJAX — as they vary over time, without having to fight with request scheduling and callback handling, and so on. In a few dozen lines (and with a Flash proxy, written by Aleks Bromfield, to get around Javascript’s outmoded security model) you can hook up, say, Google Maps and Yahoo! Local. Seriously, we did that: Yaggle. So that’s pretty cool.

If AJAX without the callback mess wasn’t enough, we also wrote a generalized object store. It’s accessed via AJAX (really AJAJ, since we use JSON extensively), and was built to allow quick and easy sharing. We don’t have a demo as cool as Yaggle yet, but it’s certainly in the works.

So that’s it. Future blogging topics are: the templating syntax, compiler internals, client API internals, basic howtos. The whole project was immensely fun. Shriram got me on board by asking me what would happen if PL people actually got together and wrote something for real — that is, fully implemented an idea and sent it out at the world in a language the world can use. We both chuckled for a moment, thinking how funny it would be to actually apply PL. And then he pointed out that there’s nothing funny about that at all.


A quick addendum: Flapjax is an experiment. Shriram will kill me for saying this, but the truth has to get out: Flapjax is a functional programming language. You can’t write loops, you can’t write if — you can use fold and map and expression-if (also called ternary if: test ? consequent : alternate). Can the world handle it? We promise, fame, riches, glory, callback elimination — the stuff of dreams! …but at what cost? All hyperbole, of course. You can write loops and if statements and so on, but we require a separation of the functional, declarative Flapjax language and the procedural, imperative world of Javascript. The real question is: do programmers know the difference?