Summary

This chapter is one of the most important in the book. It didn't cover any new language features. Instead, we learned how to use some of the existing features in ways that might be new, surprising, or challenging. Higher-order programming and the Abstraction Principle are two ideas that will help make you a better programmer in any language, not just OCaml. Of course, languages do vary in the extent to which they support these ideas, with some providing significantly less assistance in writing higher-order code—which is one reason we use OCaml in this course.

Map, filter, fold and other functionals are becoming widely recognized as excellent ways to structure computation. Part of the reason for that is they factor out the iteration over a data structure from the computation done at each element. Languages such as Python, Ruby, and Java 8 now have support for this kind of iteration.

Terms and concepts

• Abstraction Principle
• accumulator
• apply
• associative
• compose
• factor
• filter
• first-order function
• fold
• functional
• generalized fold operation
• higher-order function
• map
• pipeline
• pipelining

Further reading

• Introduction to Objective Caml, chapters 3.1.3, 5.3
• OCaml from the Very Beginning, chapter 6
• More OCaml: Algorithms, Methods, and Diversions, chapter 1, by John Whitington. This book is a sequel to OCaml from the Very Beginning.
• Real World OCaml, chapter 3 (beware that this book's Core library has a different List module than the standard library's List module, with different types for map and fold than those we saw here)
• "Higher Order Functions", chapter 6 of Functional Programming: Practice and Theory. Bruce J. MacLennan, Addison-Wesley, 1990. Our discussion of higher-order functions and the Abstraction Principle is indebted to this chapter.
• "Second-order and Higher-order Logic" in The Stanford Encyclopedia of Philosophy.