>>>>> "sj" == Scott J <jscott@planetinternet.be> writes:

    sj> What is meant by first first-class polymorphism?

First class polymorphism in this case lets you define values with
polymorphic types at levels other than the top level, like so:

type foo = { test : 'a 'b . ('a -> 'b) -> 'a list -> 'b list }
let a = { test = List.map }

# a.test;;
- : ('a -> 'b) -> 'a list -> 'b list = <fun>
# a.test ((+) 1) [3; 5; 7];;
- : int list = [4; 6; 8]
# a.test ((^) "a") ["a"; "b"; "c"];;
- : string list = ["aa"; "ab"; "ac"]

For objects, this allows you to define a list type, complete with map
method, as an object.  For example:

class ['a] clist x =
  object
    method map : 'b . ('a -> 'b) -> 'b list = fun f -> List.map f x
  end
let a = new clist [1;2;3]
let b = new clist ["a";"b";"c"]

# a #map (fun x -> x + 1);;
- : int list = [2; 3; 4]
# b #map (fun x -> x ^ "-");;
- : string list = ["a-"; "b-"; "c-"]

(Note: this is a very uninteresting definition if a list class, but it
should show you the kind of thing you can now do.)

John.
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