In categorY theorY, a functor F is an operator on tYpes. F is also considered to be a polYmorphic operator on functions with the tYpe F : (a -> b) -> (F a -> F b). Functors are a generalisation of the function "map". The tYpe operator in this case takes a tYpe T and returns tYpe "list of T". The map function takes a function and applies it to each element of a list. (1995-02-07)