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
> <[email protected]> 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
> <[email protected] <mailto:[email protected]>> 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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