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?

>>18
If the compiler can prove that u is normalized when f is called on it, at compile time
Ah, I think you're just using awkward terminology...  the time "when f is called on it" is always run-time.  I'm guessing that you mean to say that the compiler might identify some essentially "const" or "idempotent" code and just remove it.

For example:


   int x = 3;
   if (x != 3)
      DoSomething();

A really sophisticated compiler might strip that code out entirely.  Usually, compilers don't even attempt this sort of code analysis, because it's known to be equivalent to solving the halting problem in the general case.

Even so, if you had an ultra-sophisticated compiler that basically executed your code while compiling it, it would not help you in the case of this "normalize" optimization.  The vectors you'd be working with would be generated at run-time, and they could come from anywhere (maybe user input, maybe some 3D model loaded from disk, etc...), so no "const" optimization is possible.