This book gives an introduction to functions in R and how to write functional programs. Functional programming is a style of programming, like object-oriented programming, but one that focuses on data transformations and calculations rather than objects and state. In functional programming, you model programs by describing how functions translate input data to output data. Functions themselves are data you can manipulate them, building more complex functions by combining simpler functions.The R programming language is primarily a functional language, although with some impurities. This book will teach you fundamental functional programming concepts such as pure functions, higher-order functions, scopes and closures, continuations, and point-free programming, and how to apply these techniques effectively in R.