Textbook in PDF format Version of September 4, 2024 This pedagogical tutorial book presents the theoretical knowledge needed by practitioners of the functional programming paradigm. The main topics covered are: working with functional collections and recursion; the fundamental type and code constructions in functional programming; reasoning about types and code with the Curry-Howard correspondence; laws, structure theorems, and code derivation for functors, monads, and other important typeclasses; free type constructions; and parametricity theorems. Detailed explanations are logically developed and accompanied by full derivations and worked examples tested in the Scala interpreter, as well as exercises. Readers should have a working knowledge of programming; e.g. be able to write code that reads a small text file and prints the number of words in it. The mathematical complexity of the calculations in this book is at the level of undergraduate calculus