[swift-evolution] `once` keyword for use with loops
Robert Widmann
devteam.codafi at gmail.com
Tue May 17 01:13:19 CDT 2016
Not to doubt the usefulness of this proposal in general, but your example is subsumed by simply declaring
var result = "\(gen.next()!)"
and proceeding with the rest of the example sans `once`. I think you'll also have to address how this is any different from dispatch_once or a DSL over the same.
Cheers,
~Robert Widmann
2016/05/17 0:03、Nicholas Maccharoli via swift-evolution <swift-evolution at swift.org> のメッセージ:
> Hello Swift Evolution,
>
> Its not uncommon to have to do a piece of work only once or on the first iteration of
> a loop.
> Take for example producing a comma separated string from an Array:
>
> var result = ""
>
> let values = [1, 2, 3, 4, 5]
>
> var gen = values.generate()
>
>
>
> if let first = gen.next() {
>
> result += "\(first)"
>
> while let value = gen.next() {
>
>
> result += ", "
>
> result += "\(value)"
>
> }
>
> }
>
>
>
> Since on the first iteration we want to skip putting a comma in front we use an `if let` to grab the first element and then embed a `while let` inside the `if let` to handle the rest.
>
>
>
> Another way to do this could be using a bool to keep track of the first iteration:
>
>
>
> var first = true
>
> while let value = gen.next() {
>
> if first {
>
> result += "\(value)"
>
> first = false
>
> continue
>
> } else {
>
> result += ", "
>
> result += "\(value)"
>
> }
>
> }
>
>
>
> These approaches work, but I think there may be a way to do this with less noise.
>
> If there was a keyword to execute a block of code only on the first iteration of a loop I think that would make code like this more concise.
>
> If there was a keyword like `once` then the same thing could be achieved with something like:
>
>
>
> while let value = gen.next() {
>
> once {
>
> result += "\(value)"
>
> continue
>
> }
>
> result += ", "
>
> result += "\(value)"
>
> }
>
>
>
> How does it sound?
>
>
>
> - Nick
>
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