Purely functional programming

In computer science, purely functional programming usually designates a programming paradigm—a style of building the structure and elements of computer programs—that treats all computation as the evaluation of mathematical functions. Purely functional programming may also be defined by forbidding changing-state and mutable data.

Purely functional programing consists in restricting programming to its functional paradigm.

The exact difference between pure and impure functional programing is a matter of controversy.

A program is usually said to be functional when it uses some concepts of functional programming, such as first-class functions and higher-order functions. However, a first-class function may use techniques from the imperative paradigm, such as arrays or input/output methods are not purely functional programs, therefore the first-class function needs not be purely functional. In fact, the earliest programming languages cited as being functional, IPL and Lisp, were both "impure" functional languages by the current definition.

Purely functional data structures are persistent. Persistency is required for functional programming; without it, the same computation could return different results. Functional programming may use persistent non-purely functional data structures, while those data structures may not be used in purely functional programs.

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