[swift-evolution] repeat loop enhancement

Jordan Rose jordan_rose at apple.com
Tue Dec 8 18:21:38 CST 2015

I've also thought of this idea before, but how often in code do you actually repeat something a given number of times? There are only a few use cases I can think of:

- retries before giving up on something
- emitting padding when doing formatted output of some kind
- reading formatted data when you have a given number of rows/records to read

If it's that rare, I'm not sure it's worth dedicating syntax to.

> On Dec 8, 2015, at 13:54, Jason Pollack via swift-evolution <swift-evolution at swift.org> wrote:
> I'd like to propose a small enhancement to the repeat loop.
> Currently if we want to perform 'count' iterations in a loop, we need to do something like:
> for _ in 0 ..< count {
>    //Do something here
> }
> It looks and feels a little awkward. We need to create an unnamed variable, and it's easy to forget (especially for language newcomers) that the loop starts with 0 and doesn't include count.
> We can also do:
> var i = 0
> repeat {
>     //Some code
>     i += 1
> } while i < 10
> This is worse, in that it introduces a variable in the outer scope, and puts the repeat count at the end. Plus, if the expression inside the repeat is complex, the value of i may never be incremented, or may be incremented more than once.
> I propose the following:
> repeat count {
>     //Do something here
> }
> It's cleaner, and IMO clearer what this code is intended to do.
> Naturally 'count' should be non-negative. A while clause is not needed, although I could imagine it being supplied, to create a construct such as:
> var ok = true
> repeat numberOfTimes {
>     //Do something, possibly set ok to false
> } while ok
> This would repeat the loop a maximum of  'numberOfTimes', but could be ended early if some signal 'ok' is set to false (or, of course, by a break statement).
> Thoughts?
> Thanks!
> -Jason-
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