## Functional programming languages

Every programming language has functions, so what makes a programming language functional? There are many definitions for precisely what makes a language functional, but there are two common threads.

Firstly, functional languages have first-class functions, functions that behave like any other data structure. In R, this means that you can do many of the things with a function that you can do with a vector: you can assign them to variables, store them in lists, pass them as arguments to other functions, create them inside functions, and even return them as the result of a function.

Secondly, many functional languages require functions to be pure. A function is pure if it satisfies two properties:

• The output only depends on the inputs, i.e. if you call it again with the same inputs, you get the same outputs. This excludes functions like runif(), read.csv(), or Sys.time() that can return different values.

• The function has no side-effects, like changing the value of a global variable, writing to disk, or displaying to the screen. This excludes functions like print(), write.csv() and <-.

Pure functions are much easier to reason about, but obviously have significant downsides: imagine doing a data analysis where you couldn’t generate random numbers or read files from disk.

Strictly speaking, R isn’t a functional programming language because it doesn’t require that you write pure functions. However, you can certainly adopt a functional style in parts of your code: you don’t have to write pure functions, but you often should. In my experience, partitioning code into functions that are either extremely pure or extremely impure tends to lead to code that is easier to understand and extends to new situations.