An introduction to higher-order functions
In essence, a higher-order function is any function that either takes a function as the input or returns a function as the output. Recall from the previous chapter that both of these things are made possible through the support for functions as “first-class citizens.” Although it’s perhaps uncommon to call them “higher-order functions,” many programming languages do support these functions out of the box. For example, in Java and Python, the map, filter, and reduce functions are all examples of higher-order functions.
Let’s create a simple example in Go. We’ll have a function, A, that returns hello, and a function, B, that takes A as an input parameter. This is a higher-order function, as the A function is used as input to B:
func A() string {
return "hello"
}
func B(a A) string {
return A() + " world"
}
...