On 28/12/2016 19:57, Travis Griggs via swift-users wrote:
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
print("\(each)")
} // 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.
The IndexingIterator returned by `source.makeIterator()` is itself a
`Sequence` (you couldn't call `forEach` if it weren't).
When you call `forEach` (which is implemented using a `for ... in`
loop), you effectively call `stream.makeIterator()`, thus making a new
iterator. The state of the original `stream` iterator isn't affected by
this.
You can see more clearly what happens when you deconstruct the call to
`forEach`, first into a for-in loop and then into a while loop (which is
equivalent to the for-in loop). All three variants give the same result
even if called sequentially on the same sequence, because a new iterator
is created for each loop:
// ...
stream.next() // 10
stream.next() // 20
stream.forEach { (each) in
print("forEach \(each)")
} // prints 30, 40 to the console
// for-in loop has the same effect
for element in stream {
print("for in \(element)")
} // prints 30, 40 to the console
// while loop has the same effect
// Here you can see that a new iterator is created.
var iter = stream.makeIterator()
while let element = iter.next() {
print("while \(element)")
}
stream.next() // 30
stream.next() // 40
stream.next() // nil
(The result would be different if the sequence were _destructively
consumed_, i.e. it can only be iterated over once -- which is allowed by
the Sequence protocol).
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