The case against making Array subscript return an optional

I got a lot of interesting feedback from my previous article, regarding the proposal to change array subscripts to return optionals. Some pro, some anti.

The pro came mostly from people who’d been burned by out of bounds exceptions often and wanted Swift to change in a way that would help them not crash.

The anti camp was obviously not pro crashing, but felt that using optionals was too strict – that it would end up being counterproductive because in fighting with the optionals, developers would likely introduce as many bugs as they eliminated. Their arguments are pretty convincing.

A question tied up in this is, what are optionals for? Should nil be rare like an exception, or commonplace? Should you avoid using optionals unless you absolutely have to, or use them often, to represent things like unknown information? For a good discussion on how they should be rare, see this article by @nomothetis.

There’s definitely a lot of optional mis-use out there, in sample code on the internet – optionals for delayed initialization of members (where lazy stored properties would be better), or for representing collections that might not have contents (rather than just returning an empty collection).

For guidance, we could look at the Swift standard library and how it uses optionals:

  • In sequences, optionals are routine and informational – nil means you’ve reached the end of the sequence.

  • In the case of String.toInt(), its more to return an error – you passed in a bad string. Using toInt() to detect whether a string is an integer feels iffy.

  • Array.first seems in-between – is getting back nil an error or information? If you were expecting your array to always have values, the former. If you’re writing something generic that needs to handle empty arrays, it’s the latter, a convenience for combining the check for empty and getting the value.

  • In find, nil means not found, because maybe the collection doesn’t contain that value. This isn’t an error. But there was an alternative – it could have returned Collection.endIndex, which is what the C++ find does. The optional forces you to check, whereas with endIndex you could forget/not bother.

Array subscripts are closest to the last example. You’ve got an index, but maybe it’s out of bounds, so you should be encouraged to check before you use it. So it seems like a good fit.

The problem is when it becomes annoying without benefit. Take this code:

for idx in indices(a) {
    // this is dumb, of course it has a value!
    if let val = a[idx] {
        // do something

Faced with the above case too often, developers would probably start to get unwrap fatigue. They inspect the code, see that there’s no way the index could not be valid (there’s no index arithmetic going on there, just use of an index that is guaranteed to be within bounds), and just force unwrap instead.

This is a slippery slope. Once you start doing that, you do it all the time. Not just with guaranteed-safe cases but others that aren’t, and one time you use a closed rather than half-open range and bang, run-time error. Only it’s not a array bounds error, it’s a force-unwrap nil error. So we’re back to where we started, only with less helpful debug info and a bunch of exclamation marks all over our code.

So a better solution is needed – one that stays out of the way when doing things that can’t go wrong, but helps with handling the cases that can, like at the edges of ranges or when performing arithmetic on indices.

At this point, I’m pinning my hopes on the second option in my original article – a kind of index type that could never not point to a value in the collection. But implementing this is tricky – especially if it might involve changing the index type protocols, which would break everything that builds on top of them.

In the mean-time, if you feel your code would be better off with an optional array index function, you can of course extend array to provide it. The useful swiftz library has one, safeIndex, that you could re-use.

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