pure_functions.md

Pure Functions

When a method only uses its arguments in order to compute a return value but doesn't otherwise change the outside world, we call those methods "pure functions."

// square just uses x to compute the square of x
int square(int x) {
    return x * x;
}

If a method changes any values non-local to it, such as by assigning values to an array, we consider it to be "impure."

void multiplyAllByTwo(int[] numbers) {
    for (int i = 0; i < numbers.length; i++) {
        // This assignment is "impure" since it changes
        // something outside the method.
        numbers[i] = numbers[i] * 2;
    }
}

The term "pure" comes from the notion of "pure mathematics."

Pure functions are useful in the sense that they can be easier to understand on account of having what they do only depend on their inputs.

They can also be easier to verify. To know if a pure function does what you expect it to, you can give it example arguents and check to see that its return values are what you expect.