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

Vladimir.S svabox at gmail.com
Tue Jun 7 06:20:31 CDT 2016 

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>> 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>> 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>)>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>)>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>)>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>)>:
>>     > > >>>>
>>     > > >>>>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>)>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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>>     > > >>>>>>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>)
>>     > > >>>>>>https://lists.swift.org/mailman/listinfo/swift-evolution
>>     > > >>>>_______________________________________________
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>>     > > >>>>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>)
>>     > > >>>>https://lists.swift.org/mailman/listinfo/swift-evolution
>>     > > >>
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>>     > > >
>>     > > >
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