Pure function

A function is pure if it returns the same output on the same input. This is a component of referential transparency.

Examples of pure functions

Arithmetic functions are pure, and they are the archetype.

today() is impure, as at different times it will yield different results: it refers to some global state.

Similarly, any function that uses global state or a static variable is potentially impure.

random() is impure, as each call potentially yields a different value.