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

Víctor Pimentel Rodríguez vpimentel at tuenti.com
Fri Jun 30 03:05:28 CDT 2017

On Fri, Jun 30, 2017 at 7:24 AM, Brent Royal-Gordon via swift-evolution <
swift-evolution at swift.org> wrote:

> 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
> 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.

+1 to everything.

If terseness is the only clear advantage of a new operator that is meant to
be used sparsely, it's clear to me that the ?? Never form is better for the
Swift community as a whole.

Víctor Pimentel
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