Functor (functional programming)

A functor is a special kind of thing in functional programming. It can be thought of like an object or box that holds a value inside it. In the same way that a box can be opened and closed to access the things inside it, a functor can be opened to get the value inside and closed again afterwards. With a functor, you can also apply a special type of 'function' to modify what's inside the functor or operate on it in different ways.