Introduction

Clojure is a functional programming language, and functions are of primordial importance to the Clojure programmer. In functional programming, we avoid mutating the state of a program, which, as we have seen in the previous chapter, is greatly facilitated by Clojure's immutable data structures. We also tend to do everything with functions, such that we need functions to be able to do pretty much everything. We say that Clojure functions are first-class citizens because we can pass them to other functions, store them in variables, or return them from other functions: we also call them first-class functions. Consider an e-commerce application, for example, where a user is presented with a list of items with different search filters and sorting options. Developing such a filtering engine with flags and conditions in an imperative programming way can quickly become unnecessarily complex; however, it can be elegantly expressed with functional programming. Functional composition...