[swift-evolution] Add a while clause to for loops

[Xiaodi Wu]

Personally, given this discussion and the one about `where` in if and while
statements, I would not be opposed to elimination of `where` in control
statements altogether.

My reasoning would be that words like filter and prefix unambiguously
indicate what happens to elements of a sequence for which the predicate
returns false, whereas words like where and while are ambiguous.

On Mon, Jun 6, 2016 at 17:52 Tim Vermeulen <tvermeulen at me.com> wrote:

> I didn’t mean we should really get rid of the `where` clause, it’s great.
> I guess the point I was trying to make is that we can use a `where` clause
> with a `for` loop in Swift, despite the existence of the `filter` method.
> So despite `prefix(while:)` in Swift 3, there might be room for a `while`
> clause. I think it makes the code a lot more readable, much like how
> `where` can make a `for` loop a lot more readable than using `filter`.
>
> > The burden of proof for adding new features is different from that for
> taking away existing features.
> >
> > If a feature doesn't yet exist, a successful proposal will show how it
> provides additional and non-trivial utility. If a feature already exists, a
> successful proposal to remove it will show how it is harmful to the
> language or contrary to the direction in which it is evolving.
> >
> > On Mon, Jun 6, 2016 at 15:38 Tim Vermeulen<tvermeulen at me.com(mailto:
> tvermeulen at me.com)>wrote:
> > > The functionality of the `where` clause in `for` loops also already
> can be mimicked using `filter`. Wouldn’t we have to get ride of the `where`
> clause by that logic?
> > >
> > > >The functionality being asked for here is already accepted for
> inclusion to Swift as a method on Sequence named `prefix(while:)` (SE-0045):
> > > >
> > > >`for element in array.prefix(while: { someCondition($0) }) { ... }`
> > > >On Mon, Jun 6, 2016 at 14:31 T.J. Usiyan via swift-evolution<
> swift-evolution at swift.org(mailto:swift-evolution at swift.org)(mailto:
> swift-evolution at swift.org)>wrote:
> > > >>(As I said, I can live with `while`. I am simply presenting a
> potential point of confusion.)
> > > >>You aren't evaluating the statements in the loop 'while' the
> condition isn't met. The first time that the condition isn't met,
> evaluation of the loop stops. I get that this is technically true for the
> `while` construct but I suggest that the only reason that it works there is
> that 'stopping the first time that the condition isn't met' *is* the
> construct. Here, we have a loop that we execute for each thing and we're
> tacking on/intermingling the `while` construct.
> > > >>
> > > >>
> > > >>
> > > >>On Mon, Jun 6, 2016 at 2:19 PM, Thorsten Seitz<tseitz42 at icloud.com
> (mailto:tseitz42 at icloud.com)(mailto:tseitz42 at icloud.com)>wrote:
> > > >>>
> > > >>>>Am 06.06.2016 um 19:43 schrieb Tim Vermeulen via swift-evolution<
> swift-evolution at swift.org(mailto:swift-evolution at swift.org)(mailto:
> swift-evolution at swift.org)>:
> > > >>>>
> > > >>>>I also considered `until`, but it would be a bit confusing that
> `where` makes sure a condition is met, while `until` makes sure the
> condition isn’t met. I think `while` makes more sense because it
> corresponds to `break` in the same way that `where` corresponds to
> `continue`.
> > > >>>
> > > >>>That's a good argument! The only drawback is that `while` and
> `where` look quite similar at a glance.
> > > >>>
> > > >>>-Thorsten
> > > >>>
> > > >>>>
> > > >>>>>`while`, to me, actually reads like it should do what `where`
> does.
> > > >>>>
> > > >>>>To me, `while` reads like it should stop the loop once the
> condition isn’t met, just like in a while loop.
> > > >>>>
> > > >>>>>I hadn't thought about `while` in this regard but wouldn't
> `until` make more sense? `while`, to me, actually reads like it should do
> what `where` does. In any case, whether it is `while` or `where`, this
> seems like a reasonable feature in my opinion.
> > > >>>>>
> > > >>>>>TJ
> > > >>>>>
> > > >>>>>On Mon, Jun 6, 2016 at 5:15 AM, Tim Vermeulen via swift-evolution<
> swift-evolution at swift.org(mailto:swift-evolution at swift.org)(mailto:
> swift-evolution at swift.org)(mailto:swift-evolution at swift.org)>wrote:
> > > >>>>>>We can already use a where clause in a for loop like this:
> > > >>>>>>
> > > >>>>>>for element in array where someCondition(element) {
> > > >>>>>>// …
> > > >>>>>>}
> > > >>>>>>
> > > >>>>>>which basically acts like
> > > >>>>>>
> > > >>>>>>for element in array {
> > > >>>>>>guard someCondition(element) else { continue }
> > > >>>>>>// …
> > > >>>>>>}
> > > >>>>>>
> > > >>>>>>Sometimes you want to break out of the loop when the condition
> isn’t met instead. I propose a while clause:
> > > >>>>>>
> > > >>>>>>for element in array while someCondition(element) {
> > > >>>>>>// …
> > > >>>>>>}
> > > >>>>>>
> > > >>>>>>which would be syntactic sugar for
> > > >>>>>>
> > > >>>>>>for element in array {
> > > >>>>>>guard someCondition(element) else { break }
> > > >>>>>>…
> > > >>>>>>}
> > > >>>>>>
> > > >>>>>>I can see this particularly being useful if we have a sorted
> array and we already know that once the condition isn’t met, it won’t be
> met either for subsequent elements. Another use case could be an infinite
> sequence that we want to cut off somewhere (which is simply not possible
> using a where clause).
> > > >>>>>>_______________________________________________
> > > >>>>>>swift-evolution mailing list
> > > >>>>>>swift-evolution at swift.org(mailto:swift-evolution at swift.org
> )(mailto:swift-evolution at swift.org)(mailto:swift-evolution at swift.org)
> > > >>>>>>https://lists.swift.org/mailman/listinfo/swift-evolution
> > > >>>>_______________________________________________
> > > >>>>swift-evolution mailing list
> > > >>>>swift-evolution at swift.org(mailto:swift-evolution at swift.org
> )(mailto:swift-evolution at swift.org)
> > > >>>>https://lists.swift.org/mailman/listinfo/swift-evolution
> > > >>
> > > >>_______________________________________________
> > > >>swift-evolution mailing list
> > > >>swift-evolution at swift.org(mailto:swift-evolution at swift.org)(mailto:
> swift-evolution at swift.org)
> > > >>https://lists.swift.org/mailman/listinfo/swift-evolution
> > > >
> > > >
> > > >_______________________________________________
> > swift-evolution mailing list
> > swift-evolution at swift.org
> > https://lists.swift.org/mailman/listinfo/swift-evolution
> >
> >
> >
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160606/5205e67b/attachment.html>