Difference between pure and impure functional programming[edit]

The exact difference between pure and impure functional programming is a matter of controversy. Sabry's proposed definition of purity is that all common evaluation strategies (call-by-name, call-by-value, and call-by-need) produce the same result, ignoring strategies that error or diverge.^[1]


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.^[2] However, a first-class function need not be purely functional, as it may use techniques from the imperative paradigm, such as arrays or input/output methods that use mutable cells, which update their state as side effects. In fact, the earliest programming languages cited as being functional, IPL and Lisp,^[3][4] are both "impure" functional languages by Sabry's definition.