Usually we speaking only about sum and product (because article usually refers to ADT, so Algebraic Data type). A function is not really Data, so it is not incl

Usually we speaking only about sum and product (because article usually refers to ADT, so Algebraic Data type). A function is not really Data, so it is not included. But you can use the same tricks (ie: a -> b has arity b^a) to compute the number of potential inhabitant