[swift-users] Confused/Surprised by IndexingIterator.forEach behavior

Travis Griggs travisgriggs at gmail.com
Wed Dec 28 12:57:46 CST 2016

The behavior of the following playground snippet surprised me:

var source = [10, 20, 30, 40]
var stream = source.makeIterator()
stream.next()  // 10
stream.next()  // 20
stream.forEach { (each) in
}  // prints 30, 40 to the console
stream.next()  // 30
stream.next()  // 40
stream.next()  // nil

I can move the forEach statement up and down the stack there, and it *appears* that while it respects the the current position of the stream/iterator as a start point, it does not actually consume the elements (as next does). That seems inconsistent to me I guess. I would have expected the forEach to either pass through to the source and just ignore the current position, or to consume the items. Whereas the current behavior is kind of a hybrid. 

Guess I’m just looking for some enlightenment as to why it was designed to work this way. I figure there’s some valuable insight here that I’m missing.

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