Functional Programming in Scala, Second Edition

5 Strictness and laziness

This chapter covers

• Contrasting strictness and nonstrictness
• Introducing lazy lists
• Separating program description from evaluation

In chapter 3, we talked about purely functional data structures, using singly linked lists as an example. We covered a number of bulk operations on lists: map, filter, foldLeft, foldRight, zipWith, and so on. We noted that each of these operations makes its own pass over the input and constructs a fresh list for the output.

Imagine you had a deck of cards and were asked to remove the odd-numbered cards and then flip over all the queens. Ideally, you’d make a single pass through the deck, looking for queens and odd-numbered cards at the same time. This is more efficient than removing the ...