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

1
2
3
4

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

def lazyMap[T, U](coll: Iterable[T], f: T => U) = new Iterable[U] {
  def iterator = coll.iterator map f
}

scala> lazyMap(List(1,2,3,4), (x: Int) => x + 1)
res4: java.lang.Object with Iterable[Int] = (2, 3, 4, 5)

scala> List(1,2,3,4).iterator.map((x: Int) => println(x))
res0: Iterator[Unit] = non-empty iterator

// nothing actually happened yet, just remember to do this printing things

scala> res0.toList
1
2
3
4
res1: List[Unit] = List((), (), (), ())

scala> { lazyMap(List(1,2,3,4), {(x: Int) => println(x); x + 1}); 1 }

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

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

scala> mappedItr.next
1

scala> mappedItr.next
2