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

Tue Jun 7 14:45:16 CDT 2016

On Tue, Jun 7, 2016 at 2:16 PM, Tim Vermeulen <tvermeulen at me.com> wrote:

> > The meaning of the proposed while is not at all a pair for where, since
> where clauses in while loops would do the same thing as while clauses in
> for loops. That's crazy.
>
> It sounds crazy, but it’s the nature of the while loop. A where clause in
> a while loop also has a different result than a where clause in a for loop.
>

I know. And SE-0099 proposed to eliminate it, and I am very supportive of
that. For all of the reasons outlined in that conversation (including the
different behaviors in while and for loops), I arrive at the conclusion
that `where` is unsuitable for use in all condition statements and would
support its complete elimination from the language.

>
> > filter() is and prefix(while:) will be available on all sequences. The
> for...in loop only traverses through sequences.
> >
> > The meaning of the proposed while is not at all a pair for where, since
> where clauses in while loops would do the same thing as while clauses in
> for loops. That's crazy.
> >
> > On Tue, Jun 7, 2016 at 06:20 Vladimir.S via swift-evolution<
> swift-evolution at swift.org(mailto:swift-evolution at swift.org)>wrote:
> > > My +1 to the proposal and for Charlie's opinion. I believe `while` in
> `for`
> > > loop would be very handy and helpful in some situations, it is a pair
> for
> > > existed `where`, its meaning is obvious, and its existence can't
> depend on
> > > existence of any method in collections. I'd like to see a formal
> proposal
> > > for this feature.
> > >
> > > On 07.06.2016 8:18, Charlie Monroe via swift-evolution wrote:
> > > >I strongly disagree.
> > > >
> > > >Exchanging
> > > >
> > > >for result in results where result.value != .Warning while
> result.value !=
> > > >.Error {
> > > >/// ...
> > > >}
> > > >
> > > >for either
> > > >
> > > >for result in results.filter({ $0.value != .Warning }).prefix(while: {
> > > >$0.value != .Error })) {
> > > >/// ...
> > > >}
> > > >
> > > >or
> > > >
> > > >for result in results {
> > > >if result.value == .Warning { continue }
> > > >if result.value == .Error { break }
> > > >
> > > >/// ...
> > > >}
> > > >
> > > >Seems like an absolute step back. Not to mention filter(_:) doesn't
> return
> > > >a lazy collection, but will recreate it, while the `where` will do
> > > >on-the-fly check.
> > > >
> > > >>On Jun 7, 2016, at 1:34 AM, Xiaodi Wu via swift-evolution
> > > >><swift-evolution at swift.org(mailto:swift-evolution at swift.org)<mailto:
> swift-evolution at swift.org>>wrote:
> > > >>
> > > >>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(mailto:
> tvermeulen at me.com)
> > > >><mailto: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)
> > > >><mailto:tvermeulen at me.com>(mailto: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>(mailto:swift-evolution at swift.org
> > > >><mailto: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>(mailto:tseitz42 at icloud.com
> > > >><mailto: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>(mailto:swift-evolution at swift.org
> > > >><mailto: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
> > > >><mailto:swift-evolution at swift.org>)(mailto: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
> > > >><mailto:swift-evolution at swift.org>)(mailto:swift-evolution at swift.org
> > > >><mailto:swift-evolution at swift.org>)(mailto:swift-evolution at swift.org
> > > >><mailto:swift-evolution at swift.org>)
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> > > >>>>>>
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