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

Tue, 07 Jun 2016 04:20:48 -0700

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
<[email protected] <mailto:[email protected]>> 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 <[email protected]
<mailto:[email protected]>> 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<[email protected]
    <mailto:[email protected]>(mailto:[email protected]
    <mailto:[email protected]>)>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<[email protected]
    <mailto:[email protected]>(mailto:[email protected]
    <mailto:[email protected]>)(mailto:[email protected]
    <mailto:[email protected]>)>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<[email protected]
    <mailto:[email protected]>(mailto:[email protected]
    <mailto:[email protected]>)(mailto:[email protected]
    <mailto:[email protected]>)>wrote:
    > > >>>
    > > >>>>Am 06.06.2016 um 19:43 schrieb Tim Vermeulen via
    swift-evolution<[email protected]
    <mailto:[email protected]>(mailto:[email protected]
    <mailto:[email protected]>)(mailto:[email protected]
    <mailto:[email protected]>)>:
    > > >>>>
    > > >>>>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<[email protected]
    <mailto:[email protected]>(mailto:[email protected]
    <mailto:[email protected]>)(mailto:[email protected]
    <mailto:[email protected]>)(mailto:[email protected]
    <mailto:[email protected]>)>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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