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

Rob Norback rnorback at gmail.com
Tue Jun 7 08:58:52 CDT 2016 

I give my +1 to this feature, it allows you to be explicit about a for loop escape condition at the outset instead of it being contained within the loops logic making them easier to maintain.  Also it reads extremely well.

> On Jun 7, 2016, at 5:20 AM, Vladimir.S via swift-evolution <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>> 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
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>>>    <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
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>>>    <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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>> 
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