Functional programming

Functional programming, time and state
A functional programming language is a language modeled after mathematical functions. Non-functional computer programs allow state changes. State changes are represented by changes in the values of variables over time. In strict functional programming languages variables are not allowed to change durring a single invocation of a function.

State may not change during a function call. Fixed state allows for the control of side effects(bad distant changes caused by local changes). When state is fixed, functions can be treated similar to mathematical functions and facts can be proved about such functions. Some functional programs can be proved to equal to each other or proved to be stable under certain conditions. Such proofs can aid the efforts of software engineers and computer scientists. See (Hutton, 1999) for an example.

Ref 1: Graham Hutton, 1999, A tutorial on the universality and expressiveness of fold J. Functional Programming 9 (4): 355–372, July 1999, Cambridge University Press.