We have seen that functions are, in effect, abstractions that describe compound operations on numbers independent of the particular numbers. For example, when we
(defun cube (x) (* x x x))
we are not talking about the cube of a particular number, but rather about a method for obtaining the cube of any number. Of course we could get along without ever defining this function, by always writing expressions such as
(* 3 3 3)
(* x x x)
(* y y y)
and never mentioning
cube/1 explicitly. This would place us at a serious disadvantage, forcing us to work always at the level of the particular operations that happen to be primitives in the language (multiplication, in this case) rather than in terms of higher-level operations. Our programs would be able to compute cubes, but our language would lack the ability to express the concept of cubing. One of the things we should demand from a powerful programming language is the ability to build abstractions by assigning names to common patterns and then to work in terms of the abstractions directly. Functions provide this ability. This is why all but the most primitive programming languages include mechanisms for defining functions.
Yet even in numerical processing we will be severely limited in our ability to create abstractions if we are restricted to functions whose parameters must be numbers. Often the same programming pattern will be used with a number of different functions. To express such patterns as concepts, we will need to construct functions that can accept functions as arguments or return functions as values. Functions that manipulate functions are called higher-order functions. This section shows how higher-order functionsp can serve as powerful abstraction mechanisms, vastly increasing the expressive power of our language.