Category Archives: Talks

I’m not helping

I just arrived home from the Realtime Conference in Lyon, France. It was a terrific event full of interesting people talking about a diverse range of topics in beautiful surroundings, and I’m hugely grateful to the organisers for inviting me. I could wax lyrical about some of the technical things I learned there but I’m going to focus on the topic of Adam Brault’s incredibly honest closing speech, and a topic I rarely discuss here: people.

RealtimeConf was just one in a series of events I’ve attended recently where I got to talk to people about their experience of community events and interpersonal relationships. I hope I can write something about community in general in the future, but this post is about me. I apologise in advance for the grotesque level of self-indulgence here but I need to put this on the record. I have not named anyone who informed this article through one-on-one conversation with me; I am tremendously thankful for their input and if you recognize yourself in this story and I have misrepresented you, please let me know and I will make corrections.

So. I don’t know of any way to say this without seeming boastful or pretentious, but: I have somehow become moderately well-known in the JavaScript and Ruby communities. What I mean by that is that it’s not infrequently that I meet people who ‘know me from the internet’, because of my open-source projects, my blog, conference talks, or my Twitter feed. Actually, particularly my Twitter feed. People tell me they like my rants, or ‘diatribes’ as one friend put it. And indeed I’ve met various great people, and through some slightly bizarre circumstances, made some in-real-life friends because people find my verbose streams of incoherence on matters technological entertaining.

But on the other hand, I’ve been called out by several people for being too negative, too angry, for swearing too much, for disparaging other’s work without constructive feedback, for posting too frequently, and generally making people feel bad. And before anyone feels obliged to leap to my defence or tell me you can’t please everyone: these are all valid criticisms. I’m a nasty person sometimes. People see different sides of you, and have differing opinions of those sides, but the aforelisted behaviours are matters of observable fact. I do those things.

Okay, James, enough with the navel-gazing. Why does any of this matter?

The thing about being well-known is it creeps up on you. It’s very hard to tell, when you walk into a conference hall full of strangers, how many people know who you are. It still comes as a surprise to me when people have heard of me or know my work, and I still feel as anonymous walking into a venue as I did at the very first developer conf I went to in 2007. But the illusion of anonymity soon fades when people come and introduce themselves.

I’ve met a lot of new people recently. RealtimeConf was my third conf in as many weeks, I’m going to another next week, and at the beginning of March I did /dev/fort. And several of those people have had a reaction somewhere in the region of: “wow, you’re a nice person in real life”. Now I won’t claim that’s always the case either, but it’s certainly disappointing to find out that people’s expectation, based solely on following me on Twitter, is that I’ll be a sarcastic, negative troll who’s no fun to be around.

While it sucks to find out people are apprehensive about meeting me, the straw that broke the camel’s back is something another one of the speakers told me yesterday: they were nervous about giving their talk knowing I was in the audience. My reputation for mouthing off about the minutiae of software made someone worry their material would risk setting me off. It’s one thing to know I might not get to meet someone because I put them off, it’s another to know they’re having a worse time on stage on my account.

Public speaking is regularly listed as many people’s greatest fear. As Adam Brault said in his talk, “your heroes are all scared, too.” It’s so true. I don’t know anyone who isn’t at least nervous about speaking in public. I know some remarkable speakers who have almost paralysing fears about what will happen on stage. I’ve seen friends pretty much pass out from exhaustion after presenting. Me, I feel nauseous for about an hour beforehand, I have to pee every five minutes, I sweat like crazy and I’m scared I’ll go into panic attack on stage. Oh and I stuttered as a child and it comes back when I’m not confident in what I’m saying. I don’t know why any of us choose to do it sometimes, and I have huge respect for anyone that even tries. I want the person on stage to know that, and not instead feel scared that I’m looking for ways to trip them up.

I’m ecstatic if I get one laugh during a whole talk. It’s the only way you know the audience is on your side.

Anyway, to get to the point. I make software and give it away and answer people’s questions about it because those are the best ways I have of helping people. I want people to expect that I’ll want to be helpful in real life, but that’s not the signal I send out sometimes. I’m sorry about that. Context matters, and I’m sure I’ll still indulge the people I know are comfortable with my snarkiness when I know I’m talking directly to them, just as good friends are allowed to insult each other without getting hurt. But I ought to remember I’ve not met most of the people who follow me, and some of them look up to me for my programming work. If you only know me from Twitter, you’re mostly seeing my angry side.

One of the biggest lessons I’ve taken from talking to people about community is that it pays to be up-front and explicit about what you expect from people and what they should expect from you, even when (or maybe especially when) those expectations seem obvious. In that spirit, I’m asking anyone that follows me online to tell me when I’m overstepping the line. Don’t hold back because I’m an ‘expert’ on something or because you think I’ll get defensive. Several people have done this in person in recent weeks and I was grateful to everyone one of them. I’m tired of being the angry programmer from the internet, so I have to change people’s expectations. Telling me when I’m getting it wrong helps that.

Hopefully next time I meet one of you, you won’t be so surprised.

Cross-platform JavaScript testing

Last week I gave a talk at the London Ajax User Group on testing JavaScript software across different platforms. I wanted to share the talk with a wider audience so what follows is an essay version of the talk; it’s what I planned on saying before I got nervous, fluffed my lines and spoke too fast.

I want to start with a quick history lesson. Cast your mind back to 2006. We had a few browsers out in the wild, not as many as are in mainstream use as today but enough to give us a headache, especially with IE6 still dominating the stats. The problem we had back then was that all the browsers behaved in slightly (sometimes vastly) different ways and had different scripting APIs. Standards were being slowly rolled out but the overhead of dealing with browser quirks was still very high.

So, around 2005 and 2006 we see two projects that try to fix the situation: Prototype and jQuery. They both aimed to normalize and improve the scripting API across all browsers, so you could be more productive and be more confident that your code would work across the board. They’ve both been really successful and since then we’ve seen a lot more projects that have their own take on how we should tame the browsers; projects like YUI, MooTools and Dojo.

Skip forward five years, and the landscape looks quite different. We’ve got a few more browsers in mainstream use, especially with the mobile web becoming a mainstream platform, but we’ve also got JavaScript being used all over the place. People are finally taking it seriously for server-side work, it’s being embedded in databases, it’s everywhere. Node has kick-started this, but there’s a ton of frameworks based on Rhino.

So what’s the problem this time? Well, it’s not so obvious. These platforms all have different APIs but that’s largely because they’re used for different things; you probably don’t care that your web front-end won’t run on CouchDB. But one of the promises we’ve heard for years and years about server-side JavaScript is that it’ll amplify code reuse. You’ll be able to share business logic between the client and the server. Remember ‘write once, run anywhere’? We were actually going to make that work.

But what we’re actually seeing is needless specialization. Application frameworks for Node, testing libraries for Node, template languages for Node, API clients for Node. I don’t mean to pick on Node; it’s been the same with jQuery, although fortunately jQuery became so pervasive that depending on it wasn’t so much of a problem. But we’re seeing the same pattern all over again with Node and this seems like a wasted opportunity to me. Node’s popularity is bringing client-side developers onto the server, and it would be great of this helped unify our efforts.

Now I didn’t want to single any particular project out for criticism, because really I’m just as guilty as anybody else. I maintain a project called Faye which bills itself as a pub/sub messaging server for Node. But the truth is, most of the internal logic about managing clients and subscriptions is just pure JavaScript, Node is just used to make this logic accessible over the Internet. You could probably tear the Node part out and run the server in a browser, if you felt like it.

So we have a problem with portabilty, we’re wasting time reinventing the same wheels on every platform we migrate to. And to solve this, to help us share more code, I don’t think we need to normalize APIs like we did in the browser. Variety and innovation are good things, and people should have some choice about which platforms they target. But for those of us that want to write cross-platform code, I think we’re going to need testing tools that work everywhere.

Now, some back story. Since 2007 I’ve been working on this project called JS.Class. It’s an object system for JavaScript that’s based on Ruby, so it gives you classes and mixins and a few of Ruby’s object methods. It comes with a class library of common object-oriented idioms and data structures, and it tries to establish some conventions for things like how to compare objects for equality, how to sort them, store them in hash tables, and what-have-you.

OrderedHash = new JS.Class('OrderedHash', {
  include: JS.Enumerable,
  
  initialize: function() {
    // ...
  },
  
  forEach: function(callback, context) {
    // ...
  }
})

It lets you make a class like this. Say I want a new data structure, which we’ll call an ordered hash. I can create it, add the Enumerable methods to it, give an initializer, tell the Enumerable module how to iterate over it, and add other methods. Pretty standard stuff.

Then some time in around 2009, I added JS.Packages. It’s a package manager, and the aim is to seperate out the logic of how to load code and dependencies from the code itself. So you say I’ve got this file /lib/ordered_hash.js, it defines OrderedHash, and it depends on JS.Class and JS.Enumerable. You can use local files or files off other domains. Then when you need to use an object you just require() it and JS.Packages will make sure it’s loaded then run your code.

JS.Packages(function() { with(this) {
  
  file('/lib/js/ordered_hash.js')
    .provides('OrderedHash')
    .requires('JS.Class', 'JS.Enumerable')
  
  file('http://cdn.google.com/jquery.js')
    .provides('jQuery', '$')
}})

JS.require('jQuery', 'OrderedHash', function() {
  var links = $('a'),
      hash  = new OrderedHash()
  
  // ...
})

The reason it focuses on object names rather than file paths is because I also wanted to use it to load code from libraries with their own loaders. For example, Google has this thing where you include a seed file, and then use the google.load() function to load more components. Using JS.Packages, you can have custom loader functions that bridge to other platforms’ loading systems, so I can use the same system to load all the objects I want to use. This abstraction also means I can use the same configuration to load code on non-browser platforms; JS.Packages can figure out how to load external files based on the environment.

JS.Packages(function() { with(this) {
  
  file('https://www.google.com/jsapi?key=INSERT-YOUR-KEY')
    .provides('google.load')
  
  loader(function(onload) {
    google.load('maps', '2', {callback: onload})
  })
    .provides('google.maps')
    .requires('google.load')
}})

JS.require('google.maps', function() {
  var node = document.getElementById("map"),
      map  = new google.maps.Map2(node)
})

Finally there’s this autoload() function, which lets you say if I require an object whose name matches this regex, try to load it from this directory. It’ll turn the object name into a path and then try to load that file for you. You can also use the matches from the regex to generate a dependency, for example if I’m testing and I load the TwitterSpec, I probably want to load the Twitter class as well, and I can use the match from the regex to specify that.

JS.Packages(function() { with(this) {
  
  autoload(/^(.*)Spec$/, {
           from: 'test/specs',
           require: '$1' })
  
  // e.g. TwitterSpec
  //      defined in test/specs/twitter_spec.js
  //      requires Twitter
}})

So I’d been using this for months and all was good with the world until this landed in my inbox:

Does it work on Node?

And it turned out that it didn’t. There are some environment differences in Node that meant that JS.Class wouldn’t run. I patched them up and had a play around with it and everything looked okay. But still, all I could legitimately say at the time was, “I don’t know.” All my tests at the time were written with Scriptaculous, which I loved because it’s really simple to get going with, but it meant my tests were confined to the browser. I needed something new.

So I had a look around to see what was out there, and there are a few really nice frameworks around that work on a few platforms. But none of them seemed to Just Work out of the box on all the platforms people were asking me to support. A few of them you can hack to override their platform bindings but it’s not a great first-time experience. So I did what any self-respecting nerd with a GitHub account does, and wrote a whole new testing library.

The next version of JS.Class will ship with a package called JS.Test. It’s a testing library, and it looks pretty much like most other testing libraries you’ve seen, but with a few explicit goals. First, it should run everywhere, without modification or configuration. I shouldn’t have to tell it how to load files or print results, it should figure that out based on the environment. It should remove as much boilerplate as possible, because setting up a new test suite is always a drag and I constantly forget how to do it. And, it should hide any platform differences – you should just be able to use a single API to write, load and execute your tests across all supported platforms.

Now rather than pore over the API details, which aren’t that interesting since they’re similar to stuff you’ve already seen, I thought I’d take you through an example. We’re going to build a little Twitter API client that works in Node and in web browsers.

Twitter has a search API, which you don’t need to sign up or do any OAuth leg-work to use, it’s a great place to get started. It looks like this, you make a request to search.twitter.com/search.json with your query and you get back a JSON document with some tweets matching the search.

$ curl 'http://search.twitter.com/search.json?q=@jcoglan'
    
{
    "results": [{
        "id": 23843942428577792,
        "created_at": "Sat, 08 Jan 2011 20:50:13 +0000",
        "from_user_id": 4393058,
        "from_user": "extralogical",
        "to_user_id": 86308,
        "to_user": "jcoglan",
        "text": "@jcoglan I need to write some JS testing code...",
    }
    ...
    ]
}

Now, let’s say I want to access this with a JavaScript API. I make a new client, tell it to search for something, then process the results with a callback function. This is what we’re going to build.

var client = new Twitter()

client.search('@jcoglan', function(tweets) {
  // tweets == [{to_user: 'jcoglan', ...}, ...]
})

So first we need to set up a project structure for this. JS.Test doesn’t require any particular file layout, this is just how I’ve settled on doing things. You see we have a source directory that contains the project’s source code, we have a vendor directory where I’ve installed JS.Class, and we have a test directory. This test directory needs a few items in it.

twitter/
    source/
        twitter.js            : source code
    test/
        browser.html          : runs in browsers
        console.js            : runs on command line
        run.js                : loads and runs all tests
        specs/
            twitter_spec.js   : test definitions
    vendor/
        jsclass/
            core.js           |
            loader.js         | -> Framework code
            test.js           |
            (etc)             |

browser.html is what we’ll load up in a web browser to run the tests, and console.js is a script we’ll run in the terminal. They both do exactly the same thing, which is to load the JS.Class seed file, and load the test runner. The test runner, that’s run.js, is a cross-platform script that loads the project’s code, loads all the tests, and runs them. The tests themselves live in the specs directory, one spec file for each source file. Again, this is just convention, you can change this easily once you’re familiar with the setup.

I’m going to start with the console setup first, because it’s slightly simpler. As I said the job of console.js is to load the JS.Class seed file and then load the test runner. Here we’re using Node’s require() function to load the files, some platforms use load() but it’s easy to detect what’s available and pick the right one.

// test/console.js

JSCLASS_PATH = 'vendor/jsclass'
require('../' + JSCLASS_PATH + '/loader')
require('./run')

So with that done we move onto the runner file, this is the script that’s used in all environments to load the project and execute its tests. Notice we can do away with require() vs. load() here, since we’ve got JS.Packages loaded now we can use it to load everything. We start with an autoload() statement to tell it where to find Spec objects, then we tell it where our source code is: file source/twitter.js provides Twitter and requires JS.Class. Finally we load JS.Test, load all our specs and tell JS.Test to run the test suite.

// test/run.js

JS.Packages(function() { with(this) {
  autoload(/^(.*)Spec$/, {
           from: 'test/specs',
           require: '$1' })
  
  file('source/twitter.js')
    .provides('Twitter')
    .requires('JS.Class')
}})

JS.require('JS.Test', function() {
  JS.require('TwitterSpec',
             function() { JS.Test.autorun() })
})

Now onto the spec itself. In our spec file, we create a spec. This is almost the same API is Jasmine, or JSpec, or any number of things so it should be familiar. We have a before block that creates a new Twitter client, then a test for it: when I call search() with "@jcoglan", the client should yield tweets mentioning me. That resume() business is there because we’re running an asynchronous test; JS.Test passes this function to the test block, and we call it when the test is ready to continue, passing any assertions we want to make. It you leave the resume argument out, JS.Test assumes it’s a synchronous test and won’t suspend running when the outer test block completes.

// test/specs/twitter_spec.js

TwitterSpec = JS.Test.describe("Twitter", function() {
  before(function() {
    this.client = new Twitter()
  })

  it("yields matching tweets", function(resume) {
    client.search("@jcoglan", function(tweets) {
      resume(function() {
        assertEqual( "jcoglan", tweets[0].to_user )
      })
    })
  })
})

Let’s go and run our test suite. We immediately get a helpful error message from Node: it couldn’t find our source code.

$ node test/console.js

Error: Cannot find module './source/twitter'

Great, let’s go and create a blank file to get rid of this error. Now we’ve created the file, Node finds it but JS.Pacakges starts complaining.

$ mkdir source
$ touch source/twitter.js
$ node test/console.js

Error: Expected package at ./source/twitter.js
       to define Twitter

You said twitter.js would define the Twitter class, but it doesn’t! Better go and add that.

// source/twitter.js

Twitter = new JS.Class('Twitter')

Now we’ve made the package loader happy and we start to get some meaningful output.

$ node test/console.js

Loaded suite Twitter
Started
E
Finished in 0.072 seconds.

1) Error:
test: Twitter returns tweets matching the search:
TypeError: Object #<Twitter> has no method 'search'

1 tests, 0 assertions, 0 failures, 1 errors

We get an error because our Twitter class doesn’t have the method we need. So, let’s go and implement it. I won’t go through the whole TDD cycle here, let’s just assume I prepared some Node code earlier that does what we want.

// source/twitter.js

Twitter = new JS.Class('Twitter', {
  search: function(query, callback) {
    var http   = require('http'),
        host   = 'search.twitter.com',
        client = http.createClient(80, host)

    var request = client.request('GET',
                  '/search.json?q=' + query,
                  {host: host})
    
    request.addListener('response', function(response) {
      var data = ''
      response.addListener('data', function(c) { data += c })
      response.addListener('end', function() {
        var tweets = JSON.parse(data).results
        callback(tweets)
      })
    })
    request.end()
  }
})

And run the test again:

$ node test/console.js 

Loaded suite Twitter
Started
.
Finished in 2.684 seconds.

1 tests, 1 assertions, 0 failures, 0 errors

We’re all good! Except… this won’t run in a browser. All the network code we wrote only works on Node, and we want this to work client-side too. We’re going to need tests for this. Thankfully, JS.Test makes this easy: all we need is a web page that, just like our terminal script, loads the JS.Class seed file, and loads the test runner. All the test code we wrote earlier will work just fine in the browser.

<html>
  <head>
    <meta http-equiv="Content-type" content="text/html">
    <title>Twitter test suite</title>
  </head>
  <body>
    <script src="../vendor/jsclass/loader.js"></script>
    <script src="../test/run.js"></script>
  </body>
</html>

If we load this up in a browser, we see something like this. “ReferenceError: require is not defined”. Okay, it’s hitting our Node implementation where we load the Node HTTP library, we want to avoid that. So what do we do? Easy, just detect whether we’re in a DOM environment and switch to using JSONP to talk to Twitter instead of Node’s HTTP libraries. Again, here’s one I made earlier, this is just the usual JSONP hackery, nothing fancy, and we’ve moved the Node version into the nodeSearch() method that will be called if we’re not in a DOM environment.

Twitter = new JS.Class('Twitter', {
  search: function(query, callback) {
    if (typeof document === 'object')
      this.jsonpSearch(query, callback)
    else
      this.nodeSearch(query, callback)
  },
  
  jsonpSearch: function(query, callback) {
    var script  = document.createElement('script')
    script.type = 'text/javascript'
    script.src  = 'http://search.twitter.com/search.json?' +
                  'callback=__twitterCB__&' +
                  'q=' + query
    
    window.__twitterCB__ = function(tweets) {
      window.__twitterCB__ = undefined
      callback(tweets.results)
    }
    var head = document.getElementsByTagName('head')[0]
    head.appendChild(script)
  },
  
  nodeSearch: function(query, callback) {
    // ...
  }
})

Reload the page, and fantastic – we’ve got a green build. Quick side-note, in the browser UI, JS.Test will print out a tree of all your nested context blocks that you can browse, which can be more useful than the terminal UI for finding errors. It’ll also notify TestSwarm if that’s where you’re running your tests, so you can use it for continuous integration.

The final piece of the process is to refactor. We’ve got a bunch of networking code gunking up our API client. Maybe we should move the networking code into its own module that’s a generic interface for making HTTP calls in any environment we support. Then we could call it like this:

// source/twitter.js

Twitter = new JS.Class('Twitter', {
  search: function(query, callback) {
    var resource = 'http://search.twitter.com' +
                   '/search.json?q=' + query
    
    Twitter.Net.getJSON(resource, callback)
  }
})

Because the logic for how to do networking is now isolated in one module, it’s easier for a user to replace if they want to make the Twitter client run in another environment: they just have to replace the implementation of Twitter.Net.getJSON() with the HTTP code for their platform. It also means that the network is easier to stub out, since we don’t want to rely on the real Internet during testing:

// test/specs/twitter_spec.js

TwitterSpec = JS.Test.describe("Twitter", function() {
  before(function() {
    this.client = new Twitter()
    
    stub(Twitter.Net, "getJSON")
        .given("http://search.twitter.com/...")
        .yields([{to_user: "jcoglan"}])
  })
  
  it("yields matching tweets", function() {
    // ...
  })
})

This approach also means we can run the test in any platform because we don’t need to actually talk to the network. We’d then write unit tests for the Twitter.Net module to make sure it worked on the right platforms.

So what I wanted to get across here isn’t that you should all go and use my code, but try to follow some of the same patterns. If we want highly reusable software we’re going to need to test it everywhere. If you’re building libraries to support your work, and it looks like the abstraction would be useful in other contexts, consider making it available to users of other platforms. If you do have platform-specific code, try to isolate it in one place and hide it behind abstractions. Remember how in Faye I’ve isolated the Node bindings to make it easy to run and test the internal components in other environments. Make it easy to replace the platform bindings, so if someone wants to run it somewhere you didn’t expect it’s easy to swap in new bindings.

And finally, write usage documentation. If someone’s trying to get your code running somewhere new, step-by-step tutorials are great for showing people the ropes and getting them comfortable with how your stuff works, so they feel more confident hacking it to their needs. You’ll be amazed what people do with your code when you make it easy to use and write nice docs for it.

Speaking at RubyConf 2010

A quick bit of self-promotional guff: I’m thrilled to announce I’ll be presenting at RubyConf 2010 in New Orleans this November. I’ll be speaking about the Ruby incarnation of Faye, and more broadly about doing asynchronous programming and testing using Ruby and EventMachine.

I gave a similar talk based on the JavaScript/Node version at the London Ajax User Group a couple months back, if you want a preview.

You can grab a ticket from the conference site, assuming they’ve not sold out by the time you read this. It’s a pretty daunting prospect sharing a bill with Matz and DHH, so fingers-crossed my first proper conference spot goes well.

Evented programming patterns: Object lifecycle

This post is part of a series on event-driven programming. The complete series is:

Earlier in this series I covered a very common pattern in event-driven programming: the Observable object. This technique lets one object notify many others when interesting things happen. JavaScript developers will be very familiar with this: it’s the same pattern that underlies the DOM event model.

I while ago I rewrote the JS.Class package loader and noticed a variation of this pattern emerge, which I’m going to call the object lifecycle. The typical use case is when some part of your code needs to execute once, as soon as some condition becomes true. In the package loader, this looks something like:

thePackage.when('loaded', function() {
  // Run code that relies on thePackage
});

This says: if thePackage is already loaded, then run this callback immediately. Otherwise, wait until thePackage is loaded and then run the callback. The implication is that the package will become loaded, only once, at some point in its life, and as soon as that happens we want to be notified. (I tend to use when for one-shot lifecycle events, and on for multi-fire events.) The implementation is quite similar to the Observable pattern, so you might want to revisit that before reading on.

Obviously, our lifecycle object is going to need to store lists of callbacks, indexed by event name as before. But in this case, if we know the event has already been triggered on that object, we can run the callback immediately and forget about it. When we trigger events, we also want to remove all the old pending callbacks after running them, since they don’t need to be called again.

LifeCycle = {
  when: function(eventType, listener, scope) {
    this._firedEvents = this._firedEvents || {};
    if (this._firedEvents.hasOwnProperty(eventType))
      return listener.call(scope);

    this._listeners = this._listeners || {};
    var list = this._listeners[eventType] = this._listeners[eventType] || [];
    list.push([listener, scope]);
  },
  
  trigger: function(eventType) {
    this._firedEvents = this._firedEvents || {};
    
    if (this._firedEvents.hasOwnProperty(eventType)) return false;
    this._firedEvents[eventName] = true;
    
    if (!this._listeners) return true;
    var list = this._listeners[eventType];
    if (!list) return true;
    list.forEach(function(listener) {
      listener[0].apply(listener[1], args);
    });
    delete this._listeners[eventType];
    return true;
  }
};

Note how the trigger() method checks to see if the event has already been fired: we don’t want the same stage in the lifecycle to be triggered multiple times. It also removes the listeners from the object after calling them, and returns true or false to indicate whether the event fired. This makes it easy to tell whether some action that should only be done once has already happened; for example in my package system I do something like this:

JS.Package = new JS.Class({
  include: LifeCycle,
  
  // various methods
  
  load: function() {
    if (!this.trigger('request')) return;
    // perform download logic...
  }
});

This kills two birds with one stone: it lets other listeners know that the package has been requested and checks whether it’s already been requested, so we don’t try to download the package multiple times.

Naturally, one thing a package system has to deal with is dependencies. Dependencies are just prerequisites: you can’t load a package until all its dependencies are loaded. More precisely, a package is loaded once the browser has downloaded its source code, and it is complete once it is loaded and all its dependencies are complete. To fill in some more of the load() method, this is easily expressed as:

JS.Package.prototype.load = function() {
  if (!this.trigger('request')) return;
  
  when({complete: this._dependencies, load: [this]}, function() {
    this.trigger('complete');
  }, this);

  when({loaded: this._dependencies}, function() {
    loadFile(this._path, function() { this.trigger('load') }, this);
  }, this);
};

This reads quite naturally: if the package has already been requested, do nothing. When the dependencies are complete and this package is loaded, then this package is complete. When the dependencies are loaded, load this package and then trigger its load event. Note how the load event will trigger a complete event if there are no dependencies, and this will ripple down the tree and trigger dependent packages to load.

I’ve used when() above to express groups of prerequisites in a natural way, but we don’t have an implementation for that yet – we only have the when() method for individual objects. So let’s write one. This when() function will need to gather up the list of preconditions, keep a tally of how many have triggered, and when they’re all done we can fire our callback. The first step in the function converts preconditions, which maps event names to lists of objects, into a simple list of object-event pairs. That is it turns {complete: [foo, bar], load: [this]} into [[foo, 'complete'], [bar, 'complete'], [this, 'load']].

var when = function(preconditions, listener, scope) {
  var eventList = [];
  for (var eventType in preconditions) {
    for (var i = 0, n = preconditions[eventType].length; i < n; i++) {
      var object = preconditions[eventType][i];
      eventList.push([object, eventType]);
    }
  }
  
  var pending = eventList.length;
  if (pending === 0) return listener.call(scope);
  
  for (var i = 0, n = pending; i < n; i++) {
    eventList[i][0].when(eventList[i][1], function() {
      pending -= 1;
      if (pending === 0) listener.call(scope);
    });
  }
};

If there are no pending events, we can just call the listener immediately. Otherwise, we set up listeners for all the events, and when each one fires (and remember: some of them may have fired already) we count down how many events we’re waiting for. When this reaches zero, we can carry on with the work we wanted to do.

This pattern is essentially a cross between Observable and Deferrable: we’re deferring an action, but the deferred items – the events – aren’t complex enough to merit their own objects so the implementation is closer to an observable object. The technique lends itself really well to expressing prerequisites in a natural way, even if the work you’re doing is not asynchronous.

I’ll have a couple more articles on event-driven programming next week, and you can catch me speaking at the London Ajax User Group on August 10th where I’ll be talking about Faye, event-driven code and testing.

Talk: Writing a language in 15 minutes

I gave a talk at London Ruby User Group yesterday, based on the work I’ve been doing on Heist, my Scheme interpreter project. I wrote the core of a basic Scheme interpreter in about 15 minutes as a live-coded demo (well, kind of – the coding was pre-recorded so I could focus on talking), which seemed to go down pretty well. If you missed it (or if you were there and want to watch it again in slow motion), here’s the slides and the video (just code, no narrative (sorry)). (Side note: I think Lisp may be affecting my writing style.)

The slides first: lrug-scheme-15.zip. They are S5-format HTML, introducing the Scheme language features I implement during the talk. The video shown below is available at higher resolution from Vimeo.

Scheme interpreter in 15 minutes from James Coglan on Vimeo.

Video is also available from Skills Matter if you want the narrative. The code’s not really visible in this version so combine the audio from this with the above video and you should just about piece things together.

Some relevant links:

  • Heist is my main Scheme interpreter project. It has macros, tail recursion, continuations, and a reasonable chunk of the R5RS spec and its REPL auto-indents your code. It’s about 1000 lines of well-commented Ruby with a few hundred lines of Scheme, including macros for most of the syntax.
  • Stickup is a tiny interpreter for a small subset of Scheme, about 150 lines long. Closer to what I present in the talk, and easier to get your teeth into.
  • Treetop is what I use to generate parsers, it’s super-simple to use and lets you write a parser in no time at all.

Thanks to everyone who came along and had nice things to say about the talks, especially to whoever was telling me about about the trie data structure; Heist’s tab-completion code is now much prettier.