Open Radar

 
Summary:
`joinWithSeparator` makes multiple passes over sequences. This is both a correctness and a performance issue.


Steps to Reproduce:
1. Compile and run this code:
import Cocoa

func benchmark(label: String, f: () -> ()) {
	let start = NSDate.timeIntervalSinceReferenceDate()
	f()
	print("\(label): \(NSDate.timeIntervalSinceReferenceDate() - start)s")
}

let separator = ", "
benchmark("control") {
	_ = (0..<2).map { sleep(1); return String($0) }.joinWithSeparator(separator)
}

benchmark("lazy") {
	_ = (0..<2).lazy.map { sleep(1); return String($0) }.joinWithSeparator(separator)
}

benchmark("reduce") {
	_ = (0..<2).reduce("") { sleep(1); return $0 + separator + String($1) }
}

benchmark("lazy.reduce") {
	_ = (0..<2).lazy.reduce("") { sleep(1); return $0 + separator + String($1) }
}


Expected Results:
I expected each pass to take about two seconds.

Actual Results:
The lazy joinWithSeparator makes two passes:

rob@Resonance ~/Desktop> swiftc boom.swift; and ./boom
control: 2.00651800632477s
lazy: 4.01631498336792s
reduce: 2.00521194934845s
lazy.reduce: 2.00257503986359s


Regression:
N/A


Notes:
I’ve illustrated this using side-effects purely for convenience. In real life™, this error was the difference between a program completing interactively and it continuing for 15min and counting before I decided just to kill it—so it could have taken much longer, too.