Chapter 14 discusses the benefits of lazy evaluation and how lazy computations can be composed. This is a general pattern for which you’ll see several practical uses.
Chapter 15 shows how you can implement stateful programs without state mutation and how stateful computations can also be composed.
Chapter 16 deals with computations that deliver a single value or a stream of values asynchronously, an important part of modern computing.
Chapter 17 shows how to combine different effects discussed throughout the book—still an open topic of research in FP.
Finally, chapter 19 introduces message-passing concurrency, a style of lock-free concurrency that can be used when writing stateful concurrent programs.
Each chapter in part 4 introduces important techniques that have the potential to completely alter the way you think about writing software. Many of these topics are too vast to be discussed comprehensively, so these chapters aim to provide an introduction and a starting point for further exploration.