In this thread I compared $hash{'key'} with $hash{key}, suggesting that the former was slightly superstitious. I still hold to the view that it's not something I should feel obsessively obliged to do. (And I don't! Free at last!) But I was interested to read nodes by other monks who felt it was a sensible precautionary step. In particular, I liked mstone's description of it as "defensive programming".

Just now I wrote the following code: 
    for (@$Courses) {
        if ($Course = CheckCourse($_)) {
            print PageCourseInfo($Course);
            $count ++;
        }
        else {
            last;
        }
        last if $count >= 10;
    }

[download]
Now, note the laste line in the loop. Why not just plain old last if $count == 10;? Is it ever possible for $count to be greater than ten without first being equal to ten? No. I can't think of any possible change I might make in the future to this code that wd mean that it would fail to exit with the simpler test.

But that doesn't mean there could never be. It's costless to replace == with >=, so I do it.

Now, why do I think of *this* as being defensive programming, whereas quoting hash keys feels like a superstition? Well, I don't really know. But one thought I have is that the hash key quoting defends against changes made to that line only. If I ever do alter the hash key to add some whitespace, or make it a variable or whatever, then I'll be right there. Nothing I do anywhere else can make any difference. On the other hand in my loop, there's a whole load of different things I could do, either by altering code at other places in the loop, or by cutting and pasting bits into other code, that could make my last test not work.

Perhaps a way of thinking about defensive programming is that it is about making every line or expression extensible. Not just loosely coupled functions, but loosely coupled conditions, evaluations and assignments, that can work in the widest possible range of circumstances.

I should be most interested to know other monks' practices when it comes to defensiveness. If it's costless and it makes the world a safer place, then why not?

§ George Sherston

In reply to Defensive Programming by George_Sherston

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