> On May 16, 2016, at 11:03 PM, Nicholas Maccharoli via swift-evolution
> <[email protected]> wrote:
>
> ​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?
IMO, a better approach would be to write an 'interleave' combinator, like this:
func interleave<S: Sequence>(_ x: S, between: () -> (), each:
S.GeneratorType.Element -> ()) {
var generator = x.generate()
if let first = generator.next() {
each(first)
while let rest = generator.next() {
between()
each(rest)
}
}
}
-Joe
>
>
>
> - Nick
>
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