Monday, April 14, 2008

Missing Explanations

I was looking over my last post and forgot that I didn't discuss these two methods below:
Function.prototype.everyTimeButFirst=function() {
var self=this;
var toCall=function() {
toCall=self;
return null;
};
return function() {
return toCall.apply(this, arguments);
};
}

Array.prototype.toString=function() {
var result="[";
var addComma=function() { result = result + ", " };
var addCommaOnlyAfterFirst=addComma.everyTimeButFirst();
Array.prototype.each.call(this, function(every) {
addCommaOnlyAfterFirst();
result = result + every;
});
return result + "]";
}

It might seem a little overkill to have the everytimeButFirst function, but I think it reads better in the method and communicates the intent. Normally, you could implement putting in the comma in between items in the collection like so:

Array.prototype.toString=function() {
var result="[";
var shouldAddComma=false;
Array.prototype.each.call(this, function(every) {
if (shouldAddComma) {
result = result + ","
} else {
shouldAddComma=true;
}
result = result + every;
});
return result + "]";
}

It's a little bit longer. I hate the boolean variable "shouldAddComma". I really do. It just doesn't read well. It's hard to see what's going on. The naming makes it better. But, it's just adding unnecessary noise.

But, all is not well in the first version that I used either. Not all programmers are versed in functional programming, but even a functional programmer would scoff at the first version. Why? It changes state. Plus, the implementation of everytimeButFirst while clever would not be obvious. Basically, what you're seeing is my experiment in using nothing but functions instead of a boolean. Fun for blogging and brain stretching, but not for production systems. Clever is the enemy of the maintenance programmer. But, what if we did this:
Function.prototype.everyTimeButFirst=function() {
var shouldExecute=false;
return function() {
if (shouldExecute) {
return toCall.apply(this, arguments);
} else {
shouldExecute=true;
return null;
}
};
}

You might say I just displaced the boolean to somewhere lese and you would be right. But, I got it out of the method toString which is where I didn't like it. It's safely behind a wall where it makes sense to have. The internal implementation of everyTimeButFirst was too clever the first time around. It is still using higher-order functions, but one is easier to understand than nested ones that change themselves out.

Remember I said the original implementation would make a functional programmer gag because of the side-effects. We could make it side-effect free by making it recursive. But, Javascript is not optimized for tail-recursion. So, I did what I thought was best.

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