Sneaky Internal State

I've boiled what I want out of a programming language down to a single example.  Problems like this bite me in the ass often and I haven't used a language, yet, that helps.

Suppose I'm writing a library for working with vectors.  For simplicity, it's not fancy n-dimensional vectors, etc., let's say just three floats.  So I write some code to perform operations on vectors, like add, subtract, dot product, cross product, rotate, scale... and then I come to "normalize."

Here, I come up with this optimization that vectors could also store a flag indicating whether or not they are normal.  If that flag is set, then normalize has no work to do.  The catch is that I have to decide which operations clear the flag, which operations set it, and which operations leave it unchanged.  So I do the work and end up with a nice, efficient vector library.

I use my library for a long time -- long enough that I stop caring how that optimization worked.  Eventually, months later, I decide to add some new functions to my vector library.  Of course, I've forgotten all about the "normal" flag, and I forget to set/clear it in the new functions.  Of course, there's no compile error or even a run-time error.  I just observe strange behavior when the application runs and I have to spend hours stepping through floating-point math to figure out what's wrong.

How does your language of choice improve this situation?

>>5
>But then the library isn't really doing much for me.
It should do only what you ask it to do. It should be simple and efficient.

>For example, I'd have to know under what conditions a dot-product operation returns a normal vector.
dot product returns a scalar, not a vector. And you wouldn't need to know, it would just need to be documented. Although it would be cool if the compiler could do some algebraic optimizations to your algorithm, and call better library functions. I bet lisp could do it.

>>7
sqrts are supposed to be expensive. But I don't know. If all you are otherwise doing is adding and multiplying fixnums, then you could probably feel the difference with the same thing with sqrts added in a bunch.