Understanding functional programming

In the previous chapter, you learned the behavior of a function in detail, including when an argument is evaluated (lazy evaluation), what happens when we try to modify an argument (copy-on-modify), and where to look for variables not defined within the function (lexical scoping). These technical terms that describe the behaviors may look more difficult than they actually are. In the following sections, you will learn about two types of functions: functions that are defined in functions and functions that work with other functions.

Creating and using closures

A function defined in a function is called a closure_. It is special because in the function body of the closure, not only the local arguments but also the variables created in the parent function are also available.

For example, suppose we have the following function:

add <- function(x, y) {
  x + y
} 

This function has two arguments. Each time we call add(), we should supply two arguments. If...