How does Swift's for-in loop work?

Question

With Java's Stream API it is possible to use functional internal iteration over collections, just like

collection.forEach(out::println)

Is Swift's following for-each loop construct...

for i in names {
    println(i)
}

...merely syntactic sugar for an internal (functional) iteration that could also be paraphrased as the following imperative for loop?

for var i = 0; i < names.count; i++ {
    println(names[i])
}

Answer 1

The language reference states that for a for-in loop,

The generate() method is called on the collection expression to obtain a value of a generator type—that is, a type that conforms to the Generator protocol. The program begins executing a loop by calling the next() method on the stream. If the value returned is not None, it is assigned to the item pattern, the program executes the statements, and then continues execution at the beginning of the loop. Otherwise, the program does not perform assignment or execute the statements, and it is finished executing the for-in statement.

So, we can say that

for i in names {
    println(i)
}

is roughly equivalent to

var g = names.generate()  // "var" because next() is a mutating function
while let i = g.next() {  // "let" pattern because next() returns an optional
    println(i)
}

As for functional iteration, well, we have SequenceType's func map<T>(_ transform: (Element) -> T) -> [T].