Tuesday, July 08, 2008
Lisp: Hackable by Default
I'm watching the MIT lectures for the Structure and Interpretation of Computer Programs which my buddy Mike Cheponis was nice enough to give to me. I noticed a few interesting things.
In C, it's common to write "x->a". x is a pointer to a struct with a member a. In Lisp, you'd write (a x). a is a function that operates on some object x in order to return a value. That means that, by default, "a" has some "wiggle room" to do something special because it's a function. That's why in Java, the norm is to use getters and setters for everything. It provides "wiggle room" which I also call "hackability". In languages like Python, Ruby, and C#, "x.a" might or might not be an actual function call because those languages support properties. Depending on the details, in languages with inheritance, you might even say there's an extra dimension of hackability. For instance, in Java, x.getA() might be extended by subclasses.
One of the lectures covered symbolic manipulation of algebraic expressions. That means writing a system that can recognize that "(+ x x)" is the same as "(* x 2)". The professor had a procedure make-sum. "(make-sum x y)" is pretty straightforward. Under the covers, it returns "(list '+ x y)". However, he showed that there's no reason make-sum shouldn't be smarter. For instance "(make-sum x 0)" should really just return x. Internally, I was thinking of make-sum as a Java-like constructor for some class named Sum. However, a Java constructor can't just return x. It has to return an instance of the Sum class. Hence, if you were coding this in Java, you wouldn't use a constructor. You would use a static method that returns an Expression, where Expression is some interface. My point is that in Lisp make-sum is just a constructor, and constructors in Lisp by default have the freedom to return an instance of whatever they want. That means they're hackable by default.
I have one more thing I'd like to mention. Look at the image. This was filmed in 1986. First of all, it's amazing to me that something filmed 22 years ago is still very interesting and relevant to me today. It's even more amazing to consider that Lisp started in 1958. However, what's intriguing is that there are several women in the class. Not just one or two, but several. Where'd all the women go? There are so few female programmers these days.