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

[Brent Royal-Gordon]

> On Jun 27, 2017, at 10:16 AM, Erica Sadun via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Using an operator to provide feedback on the context of a failed unwrap has become a commonly implemented approach in the Swift developer Community. What are your thoughts about adopting this widely-used operator into the standard library?
> 
> guard !lastItem.isEmpty else { return }
> let lastItem = array.last !! "Array must be non-empty"
> 
> Details here:  https://gist.github.com/erica/423e4b1c63b95c4c90338cdff4939a9b <https://gist.github.com/erica/423e4b1c63b95c4c90338cdff4939a9b>
> 
> Thank you for your thoughtful feedback, -- E

Finally found a few minutes to read this thread.

I'm a big fan of the `Never`-based approach. (I was before, but I am more so now.) Here are the points I can see in its favor:

1. It is extremely clear about what's happening—`!!` is another random operator to learn, but `fatalError(_:)` or `preconditionFailure(_:)` are fairly self-explanatory, and `??` is something you might already be using.

2. It allows you to control the optimization behavior by using `fatalError`, `preconditionFailure`, or `assertionFailure` as desired.

3. If we later change `throw` from being a statement to being a `Never`-returning expression, you could use `throw` on the right-hand side of `??`.

4. It supports other `Never`-returning operations, like `abort()` or `exit(_:)` or your custom `usage()` function, on the right side of `??`.

5. It supports file-and-line error reporting without having to add any new features; `!!` could not do this because an operator can't have extra, defaulted parameters to carry the file and line.

6. It harmonizes with the eventual idea of making `Never` a universal bottom type, but we don't actually have to implement that today, because we can just overload `??` for now.

Against these advantages, the only one I can see for `!!` is that it is terse. Terseness is good, especially for a feature which is competing with the single-character postfix `!` operator, but I can't help but be drawn to the flexibility and power of `??` with a `Never` expression on the right-hand side.

-- 
Brent Royal-Gordon
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