First thoughts on Dart from a GWT developer

I consider myself more of a tools & frameworks guy than a language geek so I’m more likely to groan than gush over a new language for the Web.

However, as I dug into Dart a bit working on a couple of the samples, I have to say I caught a bit of the same fire that I felt when I first encountered Google App Engine and GWT. Dart is fun. Coming from a GWT / Java background, I found it easy to get started.

Here are a few of my favorite things about Dart so far:

1. Closures. You can register an event handler as simply as
   element.on.click.add((Event e) { window.alert('You clicked'); });
   Already, I don’t miss the clutter of anonymous inner classes.
2. Functions can be passed by reference. You can define
   myClickHandler(Event e) { ... }
   and then just write
   button.on.click.add(myClickHandler);
3. Function type aliases are a compact way to create your own callback types.
4. Optional typing. At first I thought I wouldn’t like this. However, as the Ruby community has long pointed out, Java is full of redundancy. Why write
   String[] values = new String[] {"a","b","c"};
   when
values = ['a','b','c'];
   will do?
5. Getters and setters. If you define methods using the get or set keywords, Dart will invoke them when you reference a field using dot notation. That is, some.property automatically invokes some.getProperty();.
6. Constructor initializers. If an argument has this. before it in a constructor argument list, the field with that name will automatically be initialized with that argument’s value. So a constructor that simply assigns arguments to fields can be written on one line:
   Point(this.x, this.y);
7. The Dart DOM and HTML libraries provide convenient and mostly transparent access to the DOM. Thus, you can easily use static layout in an HTML file and call document.getElementById(). The Canvas code in the Sunflower and Spirodraw samples is almost identical to the corresponding Javascript.
8. document.query(id_or_class) and .queryAll() work very similar to jQuery. These are part of the Dart HTML library, which offers convenient access to the DOM.

Thanks to all the syntactic sugar, Dart code is quite a bit smaller than the corresponding GWT app. My GWT sunflower app required very little modification to run in Dart, and the Dart code for the app went from 96 lines to 58 lines. Better still, I replaced the GWT SliderBar widget (almost 1000 lines of Java) with just a few lines of static HTML (my apologies to Firefox users: FF doesn’t support <input type=”range”> yet). Spirodraw was reduced from ~500 lines to ~300 lines, and the control panel layout is completely static HTML, which makes it easier to read.

Of course, it’s very, very early for Dart. GWT is a mature toolkit with a solid IDE (Google Plugin for Eclipse, among others), mature frameworks for RPC, MVP, etc., and a large open source community. But based on first looks, Dart is a promising new way to write Web apps. If browser vendors implement the Dart VM natively, it will rock! Chrome seems pretty likely to do this, at least. Until then, you can use dartc to produce JS for any modern browser. Hopefully, the editor (demo’d in the GOTO conf keynote this morning) will ship soon and make it easy for everyone to get started with Dart.

See also

• Official Dart language site
• Dart code samples (including ported GWT samples Sunflower and Spirodraw)
• Idiomatic Dart (great intro to Dart features)

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