[swift-evolution] [Pitch] Can we make `default` on switches optional?

[Xiaodi Wu]

There is a plausible argument for `switch!`, because it is not possible for
the compiler to prove exhaustiveness in all circumstances where you might
know it to be the case.

However, I'd be very against `switch?`: it undermines the exhaustiveness
guarantee of the switch statement and is wholly inconsistent with Swift
usage of `?`, which indicates the possibility of an Optional. We simply
don't need a new spelling for `default: break`.
On Tue, Oct 4, 2016 at 3:35 AM Tim Vermeulen via swift-evolution <
swift-evolution at swift.org> wrote:

> I agree about exhaustiveness checks being very useful. There was a
> suggestion for `switch?` implicitly adding `default: break`, `switch!`
> implicitly adding `default: fatalError()`, and `switch` remaining as it is
> now. I think that’s a great compromise, would you be in favor of that?
>
> > -1. The “default: break” is not only not difficult to write, it clearly
> communicates the programmer’s intent to only handle a subset of the cases.
> Without it, it is impossible to know whether that was intended, or by
> accident. Furthermore, the exhaustiveness by default can catch many
> mistakes, including after an additional case is added to an enum.
> >
> > Charles
> >
> > > On Oct 3, 2016, at 5:14 AM, Adrian Zubarev via
> swift-evolution<swift-evolution at swift.org>wrote:
> > >
> > > I know that there is this note in Commonly Rejected Changes<
> https://github.com/apple/swift-evolution/blob/master/commonly_proposed.md
> >:
> > >
> > > Remove support for default: in switch and just use case _:: default is
> widely used, case _ is too magical, and default is widely precedented in
> many C family languages.
> > > I really like to use the switch instead of if case for pattern
> matching, just because it’s neat block design. I do not want to remove
> default from switches because it’s a must have and powerful feature.
> > >
> > > I’d like to know why switches must be exhaustive.
> > >
> > > switch someValue {
> > >
> > > case …:
> > > // Do something
> > >
> > > case …:
> > > // Do something else
> > >
> > > default:
> > > () // useless nop; do nothing when no pattern matched
> > > }
> > >
> > > // VS:
> > >
> > > if case … {
> > >
> > > } else if case … {
> > >
> > > } else if case … {
> > >
> > > } // No need for `else`
> > > Can’t we make default optional, or at least on non-enum values?
> > >
> > >
> > >
> > >
> > > --
> > > Adrian Zubarev
> > > Sent with Airmail
> > >
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> >
> >
> >
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