This chapter is dedicated to functional programming and how to use this paradigm with ReactiveX. It is composed of three parts. The first part is a rapid introduction to functional programming. The aim of this part is to introduce the few notions that are needed to start writing functional code in Python. The second part presents one of the main issues of real ReactiveX applications: observable cycles. This part describes when this problem occurs and several ways to solve it. In particular, it explains how to use functional programming to solve the issue while isolating side effects from pure code. The last part goes in detail about a new implementation of the echo server, but using ReactiveX and coded in a functional way.

The following topics will be covered in this chapter:

• Functional programming
• Observable cycles
• Structuring functional and reactive code
• The HTTP echo server with an HTTP driver