Functional Programming

A functional program is one in which:

• all operations are performed by functions

• a function does not modify its arguments or have side-effects (such as printing, setting the value of a global variable, writing to disk).

A subset of Lisp, with no destructive functions, is an example of a functional language.

(defun hypotenuse (x y)
  (sqrt (+ (expt x 2)
           (expt y 2))) )

Functional programming is easily adapted to parallel programming, since the program can be modeled as flow of data through functions that could be on different machines.