Functional, or applicative, programming languages are based on the application of mathematical functions to computer programming. A function is a statement that returns a single value based on the arguments, or inputs, it is passed. Functional programming languages evaluate expressions that are formed using functions.

To understand functional programming languages it is necessary to contrast them with imperative programming languages. Imperative programming languages are based on the architecture of the computer. With imperative languages a programmer defines a set of executable instructions and the exact sequence in which those instructions are executed. The computer simply obeys the instructions in the sequence it is told and stores the result as a variable in its memory. These elementary instructions are repeated, new values are stored in variables, and these variables are assigned to memory locations until the problem is solved. In contrast, functional programming languages define...