Function composition is not only powerful and expressive but also pleasant to work with. It’s used to some extent in any programming style, but in FP, it’s used extensively. For example, have you noticed that when you use LINQ to work with lists, you can get a lot done with only a few lines of code? That’s because LINQ is a functional API, designed with composition in mind.
In this chapter, we’ll cover the basic concept and techniques of function composition and illustrate its use with LINQ. We’ll also implement an end-to-end server-side workflow in which we’ll use the Option API introduced in chapter 6. This example illustrates many of the ideas and benefits of the functional approach, so we’ll end the chapter with a discussion of those.
Let’s start by reviewing function composition and how it relates to method chaining. Function composition is part of any programmer’s implicit knowledge. It’s a mathematical concept you learn in school and then use every day without thinking about it too much. Let’s quickly brush up on the definition.