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1 point by Pauan 3437 days ago | link | parent

"I do wish there was an easier way to tell whether or not a value was provided as nil, or was left empty and defaults to nil."

I too have sometimes wished for that in JavaScript, but let me tell you a little story. I was writing a syntax highlighter, and got it working fine in Chrome and Firefox 3.5, but there was a bug in Firefox 3.0.

You see, I was using this bit of code here:

  output.push(text.slice(curr.index[1], next && next.index[0]));
If `next` doesn't exist, it will pass the value `undefined` to the `slice` method. In JS, if you don't pass an argument, it defaults to `undefined`, so this is supposed to behave like as if I hadn't passed in the argument at all.

But in Firefox 3.0, the slice method behaves differently depending on whether you pass it `undefined`, or don't pass it any arguments. So, I had to use this instead:

  if (!next) {
  } else {
      output.push(text.slice(curr.index[1], next.index[0]));
This was (thankfully) fixed in 3.5. The moral of the story: most of the time it doesn't matter whether the caller passed nil, or didn't pass anything. You can treat the two situations as the same.

Consider this hypothetical example in Arc:

  (your-func 5 (and x y z))
If x, y, or z are non-nil, it will be passed in as usual. On the other hand, if any of them are nil, it will be like as if you had used (your-func 5 nil).

By behaving differently when nil is passed in vs. not passing in an argument, you might cause the above example to break. Or perhaps it would work, but the behavior would be subtly different... introducing bugs.

By having different behavior depending on whether an argument is passed or not, you force callers to do this, instead:

  (iflet val (and x y z)
    (your-func 5 val)
    (your-func 5))
Note the redundancy. In fact, this is even more important in Arc (compared to JavaScript) because you can use any expression, such as (if), a macro call, etc.

So... let me ask: what situations do you really need to know whether the caller actually passed in nil, or didn't pass anything at all?

1 point by rocketnia 3436 days ago | link

Great point. In fact, I don't check whether an optional argument was passed very often, and the times I do, I usually expect to regret it at some point, for exactly that reason. ^_^