Strict versus lazy

Consider the operation of zipping up two lists. The zip method pairs elements from the first list with the elements from the second list. Here is a sample run:

scala> List(1,2,3).zip(List(4,5,6,7)) 
res8: List[(Int, Int)] = List((1,4), (2,5), (3,6)) 
 
scala> List(1,2,3).zip(Nil) 
res9: List[(Int, Nothing)] = List() 

All the elements of both the lists are visited to create a zipped list. The following figure shows the zip operation in action:

As another example of strict evaluation, consider the reverse method of List:

scala> List(1,2,3).reverse 
res11: List[Int] = List(3, 2, 1) 

The reverse method also visits all the elements of the list. On the other hand, consider the following:

scala> val q = List.range(1, 1000000).view.reverse 
q: scala.collection.SeqView[Int,List[Int]] = SeqViewR(...) 
 
scala> (q take 10) foreach println 
999999 
999998 
999997 
... 

Only the first 10 elements of the lazy lists are computed on demand.