[swift-evolution] [Pitch] Introducing the "Unwrap or Die" operator to the standard library

Ben Cohen ben_cohen at apple.com
Wed Jun 28 20:18:19 CDT 2017

> On Jun 28, 2017, at 5:27 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
> In the initial example, repeated here in largely identical form, the desired comment is "array must be non-empty." In what way does that provide more information than a bare `!`?

By the same token, why does precondition have an optional string? Why would you ever want to write more than:


It’s self evident – the array must not be empty! Why the second argument? 

Because you might want a meaningful string to be output when the app traps. And it gives you somewhere to document why it mustn’t be empty – the explanation for the precondition rather than the precondition itself. Similarly, when your app traps because of an unwrapped nil, it’s desirable to get some output in your debug console telling you why straight away, rather than have to hunt down the code, read the line that failed, then read the context around the line, to understand the reason. This is even more important if you didn’t write this code yourself.

Finally, there’s a woolier justification: there’s an often-touted misconception out there that force unwraps are bad, that they were only created to accommodate legacy apps, and that force-unwrapping is always bad and you should never do it. This isn’t true – there are many good reasons to use force unwrap (though if you reaching for it constantly it’s a bad sign). Force-unwrapping is often better than just whacking in a default value or optional chaining when the presence of nil would indicate a serious failure. Introduction of the `!!` operator could help endorse/encourage the use of “thoughtful” force-unwrapping, which often comes with a comment of the reasoning why it’s safe (of why the array can’t be empty at this point, not just that it is empty). And if you’re going to write a comment, why not make that comment useful for debugging at the same time.

In cases where it’s really not necessary, ! would remain just like, when you’re not really too fussed, you can leave off the string from a precondition.

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