You use sum types / optional<T> / Maybe / Option<T> / whatever your language calls them. Such a value can have two states: 'None', where it calls no value, and 'Some(t)',where it carries a value 't'.

For example, a division function could return (1) 'None' for 'div(20, 0)' because division-by-zero is undefined, and return (2) 'Some 10' for 'div(20, 2)' because 20/2 is defined and is equal to '10'.