[swift-evolution] `once` keyword for use with loops

[Daryle Walker]

Aren’t there already methods that stick elements between others (and only between, neither before the first nor after the last)?

//=====
fun toCSV2(values: [CustomStringConvertible]) -> String {
    return values.map { $0.description }.joinWithSeparator(“, “)
}
//=====

— 
Daryle Walker
Mac, Internet, and Video Game Junkie
darylew AT mac DOT com 

> On May 17, 2016, at 11:49 AM, Haravikk via swift-evolution <swift-evolution at swift.org> wrote:
> 
> This only works with the example exactly as-is, I think a fairer re-write would be something like the following:
> 
> func toCSV(values:[CustomStringConvertible]) -> String {
>     var gen = values.generate()
> 
>     var result = gen.next()?.description ?? ""
>     while let value = gen.next() { result += ", \(value)" }
>     return result
> }
> 
> toCSV([1, 2, 3, 4, 5])
> 
> For brevity I opted to use nil-coalescing and a default value to handle the first (potentially nil) element cleanly.
> 
> I’m undecided about whether I want the feature or not; as shown the example can actually be handled very neatly already, so it doesn’t really highlight a need for the feature. One thing I like about it though is the ability to possibly eliminate the need for a generator and use a for in loop instead, like so:
> 
> func toCSV(values:[CustomStringConvertible]) -> String {
>     var result = ""
>     for value in values {
>         once { result += "\(value)"; continue }
>         result += ", \(value)"
>     }
>     return result
> }
> 
> But this isn’t really much of a saving. I can’t think of a real-world example offhand that can better demonstrate a use for this; ideally you’d want to use several once {} blocks, or do something in the loop that requires other statements in the loop, i.e- you use the once to avoid duplicating statements in the loop while processing the first element.
> 
>> On 17 May 2016, at 07:13, Robert Widmann via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> 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 <mailto: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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> 
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