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

[Xiaodi Wu]

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> 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>
> wrote:
>
>>
>> > Am 06.06.2016 um 19:43 schrieb Tim Vermeulen via swift-evolution <
>> 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)>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).
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>
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