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)