functor
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)