Learning Functional Data Structures and Algorithms

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. ...