[swift-evolution] [Draft] Change @noreturn to unconstructible return type

[Xiaodi Wu]

On Tue, Jun 7, 2016 at 2:49 PM, Michael Peternell via swift-evolution <
swift-evolution at swift.org> wrote:

>
> > Am 07.06.2016 um 19:45 schrieb Charles Srstka via swift-evolution <
> swift-evolution at swift.org>:
> >
> >> On Jun 7, 2016, at 11:47 AM, Brent Royal-Gordon via swift-evolution <
> swift-evolution at swift.org> wrote:
> >>
> >>> I disagree. We are discussing how to annotate a function in some way
> so that the compiler knows that the code following it will never be
> executed *and* so a human who reads the declaration knows that it does not
> return. “Never" is a poor choice for that. Never what? Never return? Never
> use this function? Never say never again?
> >>
> >> "Never return". That's why it's in the return type slot, right after
> the `->`. If you read it out loud, you'll read "returns Never", which is
> exactly correct.
> >>
> >> NoReturn, on the other hand, does *not* read well in that slot:
> "returns NoReturn". Huh? I mean, I suppose you won't misunderstand it, but
> it makes no sense whatsoever *as a type name*.
> >
> > But it’s *not* a type. You’ll never have an instance of it.


That's the dictionary definition of a bottom type
<https://en.wikipedia.org/wiki/Bottom_type>, though: a type which has no
values.


> Since it’s not a type name, it doesn’t make sense that it needs to look
> like one. What it is doing is telling you something about the behavior of
> the function itself, not its return value. Its return value, if there were
> one, is irrelevant, since the function, by its very nature, will never even
> get to the point where it would return it.


And that's the dictionary definition of a function where the return type is
bottom. This isn't something being made up just for Swift...


> Either it’s going to kill the app via a fatalError or something, or we
> have something like dispatch_main() which will keep executing until the
> program stops, and one way or another, it won’t return.
> >
> > For that reason, frankly, I don’t understand why we want to change this
> from being an attribute, which seems to me the more natural and logical
> choice to describe this behavior. If we *do* have to change it, though,
> NoReturn conveys the most clearly to the reader what it does.
>
> +1 for @noreturn
> We don't have to change it.
> We have to keep it.
>
> -Michael
>
> >
> > Charles
> >
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