November | 2006 | Missives

I caught this article on Reddit this weekend and had definite mixed feelings about it. Many of their suggestions were obviously good ideas—calling SetTimeout() with a string is obviously stupid, as are 90% of the possible uses of eval() and Function(). The advice on what causes the DOM to repaint and reflow was interesting and informative.

It would have been a really good article except that some of their advice seemed weak. Some of what was masquerading as optimization “advice” seemed more like work-arounds for a substandard javascript implementation.

Take this for example:

var min = Math.min(a,b);
A.push(v);
These alternatives provide the same functionality, while performing better:
var min = a < b ? a : b;
A[A.length] = v;

Yes, I can see in someone’s world that might perform better. But come on, the compiler/scanner should be able to produce the equivalent code. Anything less is a failing of your browser and not an “unoptimized” program. Do you really expect people to crappify their programs because your browser can’t properly optimize javascript for you?

“Well,” I hear you say, “it’s interpreted, not compiled.” To which I say, yeah, well there’s your first problem. Fix that before telling me that I need to use stupid constructs to make my program go fast.

Here’s another example:

This example requires the script engine to create 21 new string objects, once for each time the length property is accessed, and once each time the charAt method is called:
var s = '0123456789';
for( var i = 0; i < s.length; i++ ) {
  s.charAt(i);
}
This equivalent example creates just a single object, and will perform better as a result:
var s = new String('0123456789');
for( var i = 0; i < s.length; i++ ) {
  s.charAt(i);
}

Again, this strikes me as being a failing of the compiler. I see no reason why the browser should instantiate a string object just to call its length(). A little bit of hard coding in your implementation can optimize that to a strlen() call or better—No object instantiation at all. What if someone overrode String.prototype.length()? That’s one if statement in C and some fallback code. Again, I don’t think my program should suffer because your browser doesn’t properly optimize things.

But then again, maybe I’m just crazy.

First off, I don’t like Applescript. In fact, I hate it with a passion. You would think the Apple would learn from Cobol that natural language programming languages just suck. But no, Apple created the abomination called Applescript anyway and foisted it upon us and now sometimes you just have to suck it up and use it. Preferably from a distance with gloves on (and maybe a 10 foot pole) so that it doesn’t dirty your soul too much. But I digress.

I needed to call some Applescript from perl and was quite proud of my end result:

sub osascript($) { system 'osascript', map { ('-e', $_) } split(/\n/, $_[0]); }

The usage looks like this:

osascript <<END;
 tell application "Finder"
 display dialog "Hello"
 end tell
END

So basically the Applescript sits right inside your perl program and gets launched without needing any temporary files on the disk.

How does it work?

If the perl line is incomprehensible to you, let me break it down. The inner most expression is:

split(/\n/, $_[0]);

That takes the first argument to the function, breaks it apart at the line endings and puts the parts into a list. If we were to dump the result of the split using the dialog example above it would look like this:

(' tell application "Finder"', ' display dialog "Hello"', ' end tell')

Next, it uses the map function to insert “-” before every item in the list. It does that by using the fact that perl flattens arrays. The dump of the output of the map function conceptually looks like:

(('-e', ' tell application "Finder"'), ('-e', ' display dialog "Hello"'), ('-e', ' end tell'))

but really perl immediately flattens it to:

('-e', ' tell application "Finder"', '-e', ' display dialog "Hello"', '-e', ' end tell')

At that point it calls the system function with “osascript” and that last array as arguments. When system is passed a list instead of a string it runs the command with the arguments directly instead of launching a shell. This way all the arguments are effectively quoted correctly but without ever having to worry about quotes. This is an extremely clean way of launching commands. You really should never pass system (or open) a string: (a) it needlessly launches a shell, and (b) you have to worry about quotes.

In the end, I like that it fits on one line and that it lets the Applescript sit in the program in its native form.

Edit: Also check out my Ruby version.

Month: November 2006

Efficient JavaScript or Inefficient Browser?

Calling Applescript from Perl

How does it work?