Scala map on iterator does not produce side effects

Question

Why is it that,

scala> List(1,2,3,4).iterator.map((x: Int) => println(x))

does not print out

1
2
3
4

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Answer 1

The point of an Iterator is laziness. In other words, when you create an Iterator, it will not evaluate anything until you go to read the data. Here's what that looks like:

scala> val itr = List(1,2,3).iterator
itr: Iterator[Int] = non-empty iterator

Ok, we've got an iterator now. But it hasn't actually looked at the list yet.

scala> val mappedItr = itr.map((x: Int) => println(x))
mappedItr: Iterator[Unit] = non-empty iterator

Now we have a new Iterator. This one will, when data is accessed, apply the function that has been mapped. But we still haven't actually looked at the original list.

scala> mappedItr.next
1

This is the first time we have accessed data, so it is the first time that the Iterator has looked into the list. We called next, so we got the first element. Since our iterator has a map queued up, it applies the mapped function when we access that element. So we see the result of the function applied to the next item.

We can do it again to get the next element:

scala> mappedItr.next
2

And, again, it evaluates the function only when it needs to, in order to give us the final result.