The name "Functor" can be confusing; it's a thing you apply a function to, not a function itself.
A Functor is a type t parameterized by some other type a, such that if you have a function from type a to type b, you can apply that function to a t of a and get a t of b.
Most commonly, a functor will be a container of values of type a, and the mapping consists of applying the function to each value, resulting in a container of values of type b.
Other common functors include a computation that returns type a (use composition to apply the function to the result to get a computation that returns type b), or a possibly null pointer to type a (map null to null, map pointer to a to pointer to b).
A Functor is a type t parameterized by some other type a, such that if you have a function from type a to type b, you can apply that function to a t of a and get a t of b.
Most commonly, a functor will be a container of values of type a, and the mapping consists of applying the function to each value, resulting in a container of values of type b.
Other common functors include a computation that returns type a (use composition to apply the function to the result to get a computation that returns type b), or a possibly null pointer to type a (map null to null, map pointer to a to pointer to b).