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

[Xiaodi Wu]

You're describing a while loop:
`while let element = sequence.next() where condition {...}`

Which as we've discussed can already be re-written with a for loop (which,
yes, can be lazy):
`for element in sequence.lazy.filter({ condition }) {...}`

And it can be explicitly spelled out inside the loop, a definite
readability gain for the same reason guard always requires an explicit else
block.

What do you gain with a new keyword?

On Tue, Jun 7, 2016 at 05:02 Haravikk via swift-evolution <
swift-evolution at swift.org> wrote:

> I’m a +1 for this idea. Like Thorsten I was initially a little concerned
> that while and where may look too similar, but actually I find them
> visually distinct enough, and actually in my code I’m probably more likely
> to use while than where on for loops, although both are useful.
>
> > On 6 Jun 2016, at 11:15, Tim Vermeulen via swift-evolution <
> 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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