Sunday, April 13, 2008

Annotations in Javascript

Last post, I talked about how to implement crude aspect advices. I say crude because the implementation would need to be beefed up a bit for wider use. The ideas were more important. I wanted to spend the article on the tricky bits of implementation. I'll take the same tactic with this article.

Let's start with what our code will look like. In fact, let's implement something everyone is familiar with: a transaction. Now, our transaction will have a blank implementation (we'll print out the important bits). It's the API that we will care about later. Here's the definition:
function Transaction() {
}
Transaction.prototype.run(function() {
this.define(
"begin",
{log: true},
function() {
print("begin");
});
this.define(
"commit",
{log: true},
function() {
print("commit");
});
this.define(
"rollback",
{log: true},
function() {
print("rollback");
});
this.toString=function() {
return "i am transaction";
}
});

OK. There's some new functions named "run" and "define". I'll show their implementations soon. You can probably guess the implementation of "run". It's simply so that my hands don't get tired typing "Transaction.prototype". I could use "with {}", but I have found its implementation inconsistent among Javascript dialects. The next new function, "define", is how we're going to define our annotations. It's a shorthand for better readability and to reduce duplication. All it will do is add the function to our object and add the annotations as properties. Annotations are simple in Javascript because functions are just objects and we can add anything we want to them. How cool.

Here's the implementations:
Object.prototype.run=function(func) {
func.call(this);
}

Object.prototype.define=function(name, annotations, func) {
this[name]=func;
annotations.each(function(each, name) {
func[name]=each;
});
}

Both are straightforward. But, I introduced another function: "each". Those familiar with Ruby will notice what it does right away. For the non-Rubyists, it iterates over all the properties of the object. My implementation also sends the name of the property as an extra parameter. Here's how I did "each":
Object.prototype.each=function(everyFunc) {
for (var name in this) if (this.hasOwnProperty(name)) {
everyFunc.call(this, this[name],name);
}
}

It basically allows me not to have a "for" loop. Simple. You might have noticed my use of "call". The reason is because I wanted to pass along "this" to the function passed in.

That is it for implementing annotations. The query language is already built in (albeit a little verbose). We only have to iterate over the functions of an object. Here's some code I would like to use to test my annotations and query implementation:

var every=function(each,name) {
print(name);
}
var doesItLog=function(each) {
return each.log == true;
}
Transaction.prototype.eachFunction(every.onlyWhen(doesItLog));

Oh boy. Can you guess the output from the above? Here it is:
begin
commit
rollback

The annotations I have is to mark a function to be logged on entry and exit. I'll spend the rest of the article on the implementation and bring together the annotations, aspects, and logging in the next article.

Let's talk about the new functions: "onlyWhen" and "eachFunction". I think you can guess what each of them do. "onlyWhen" is defined on a function and takes a function to check the arguments for some condition. If the condition is true, it executes the receiver function. It returns a function that brings all of this together. This is more functional programming at work. "eachFunction" takes a function that will be called for each function that is a value of a property on the object.

Like always, here's how I implemented them:
Function.prototype.onlyWhen=function(ruleFunc) {
var self=this;
return function() {
if (ruleFunc.apply(this, arguments))
return self.apply(this, arguments);
}
}

Object.prototype.eachFunction=function(everyFunc) {
this.each(everyFunc.onlyWhen(function(every) {
return every.constructor == Function;
}));
}

In the "onlyWhen" implementation, I have to bind "this" (the function) to self so that we can use it later in the function that we use a result. "this" will be bound to a different object. "eachFunction" brings together the functions "each" and "onlyWhen".

That's it! You could of course add implementations to define annotations so that we make sure that only pre-defined ones are used in functions. I'll leave that as an exercise to the reader. It wouldn't take much to implement it. The next article will put everything together. All we have to do now is weave based on rules. We have the means to iterate over functions and we have the means to replace implementations through our advices. The next article will mix this batter.

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