Impure Function¶
An impure function is a function that has at least one of the following characteristics:
Non-deterministic: for the same input, an impure function may return different output. For example,
date()andrandom_string()return different values for each call, by definition.Create side-effect: an impure function modify a state: a global variable, a row in a table, a lock, a file or data. This may affect any subsequent call to this function, leading to the previous characteristic.
A function that is not impure is a pure function.
This notion applies to functions, methods, native and custom functions, closures and arrow functions.
<?php
echo random_string(0, 10);
// an impure function:
// the static variable is a state that changes over time
function foo() {
static $x = 0;
return $x++;
}
?>
See also Pure vs Impure Functions in Functional Programming – What’s the Difference?.
Related : Functions, Method, Return, Determinism, Side Effect, Pure Function
