The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

Go figure.

Saturday, February 24, 2007

More fun with static types

What else can you do with typing systems? And what about that relentless drive towards multiple processor machines?

One thing, better parallelization of code. One such method is the map function, where some function (or code) is applied to every element in a list, and each such application is independent of each other. In the current crop of langauges, there's an actual function or keyword that is used:

(DEFINE INCR (LAMBDA (X) (+ X 1)))
(MAPCAR #'INCR '(1 2 3 4 5))

This bit of Lisp code increments the elements of a list by 1. Other languages have a similar function or library variation on map. But why? Couldn't we have the compiler figure it out? At least in a few instances?

For instance, in C, the following will give a compiler error:

{
  double x[100];
  double y[100];

  y = sin(x);
}

since sin() takes a single double parameter, not an array. But since I'm talking hypothetical compilers here, couldn't the compiler recognize that x is an array of doubles, and that sin() takes a single double, and since the result is going to another array of doubles, why not just map sin() over the array x? On a uniprocessor machine the code generated could be something like:

{
  double x[100];
  double y[100];
  size_t i;

  for (i = 0 ; i < 100 ; i++)
    y[i] = sin(x[i]);
}

Given a routine that expects a parameter of type X, and a variable array of type X, passing said variable array into said routine should not produce an error, but a mapping instead. No fence post errors. No having to type out for loops, or while loops, or loops of any kind. Or even of having to type out “map.”

Or is this too much DWIM?

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