Julia is first and foremost a functional language because computations and data transformations are done through functions; they are first-class citizens in Julia. Programs are structured around defining functions and to overload them for different combinations of argument types. This chapter discusses this keystone concept, covering the following topics:

• Defining functions
• Optional and keyword arguments
• Anonymous functions
• First-class functions and closures
• Recursive functions
• Broadcasting
• Map, filter, and list comprehensions
• Generic functions and multiple dispatch