Pure function #

Side effects #

Definition. A method has side effects if it modifies resources (variable, object, array, etc.) that are accessible outside of its scope.

Definition #

A pure function is a method that largely behaves like a mathematical function. More precisely:

Definition. A pure function is a method that:

1. returns a value,
2. returns the same value if called twice with the same arguments, and
3. has no side-effect.

Example #

public class MyClass {

    String[] names;
    User[] users;

    public MyClass(){
        String[] names = parseNames();
        users = new User[names.length];
        createUsers();
    }

    ...

    void createUsers() {
        for(int i = 0; i < names.length; i++){
            users[i] = new User(i, names[i]);
        }
    }
}

public class MyClass {

    String[] names;
    User[] users;

    public MyClass(){
        String[] names = parseNames();
        users = createUsers(names);
    }

    ...

    Users[] createUsers(String[] names) {
        User[] users = new User[ids.length];
        for(int i = 0; i < ids.length; i++){
            users[i] = new User(i, names[i]);
        }
        return users;
    }
}

Benefits #

Easier debugging #

Because it has no side effect, a pure function may cause a bug only if it returns an incorrect value.

Readability #

The behavior of a pure function can be fully documented by describing its signature (arguments and return value).

Easier parallelization #

Consider two pure functions $f$ and $g$. If the input of $f$ (resp. $g$) does not depend on the output of $g$ (resp. $f$), then they can be executed in any order (therefore also in parallel).

Example. If $f$ and $g$ are pure functions, the two programs below (written in pseudocode) are equivalent:

   x = f(a)
   y = g(b)
   result = x + y

   y = g(b)
   x = f(a)
   result = x + y