Tail Call Optimization attribute and modifier

[griotspeak]

Add a @tailrec attribute and tail modifier allowing the compiler to emit errors when tail call optimization will not be used.

[kostiakoval]

Is there are See swift-evolution thread for where this features was discussed? Can you include it ?

"Swift-evolution thread: link to the discussion thread for that proposal" in template.md

[griotspeak]

Thanks for pointing that out.

[LarsJK]

Could you add newlines above and below the Alternatives considered section?

Also i'm not sure I understand the Proposed Solution.

It was my understanding that the tail modifier would be a modifier on the return statement where you do the tail call.

func sumRecursively(numbers: [Int], _ total: Int = 0) -> Int {
    guard let head = numbers.first else {
        return total
    }

    let tail = Array(numbers.dropFirst())

    tail return sumRecursively(tail, head + total)
}

Example taken from natashatherobot.com..

And i thought the @tailrec function attribute was only needed on a public API that wants to be externally tail-callable.

Can`t think of an example of that..

[lattner]

@griotspeak It was great to meet you TJ! I'm sorry that this is not in scope for Swift 3, but there will certainly be another release...

[DougGregor]

I'm closing out PRs for proposals that are out of scope for Swift 4 stage 1. When we open up the process to the point where this proposal can be in scope, please re-open the pull request.