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

[Ole Begemann]

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).