comp f next x y | r == 0    = next x y
                | otherwise = r
                where r = f x y

(* функция, создающая один итератор *)
let file_iter n = Enum.init 10 (fun i -> Printf.sprintf "file%d line%d" n i)

(* список таких итераторов *)
let files = List.init 10 (fun n -> file_iter n)

(* вывод строк по одной *)
let rec show_lines lst =
  let continue = List.fold_left (fun not_end it -> (* проходимся по списку итераторов *)
    match Enum.get it with (* берем из каждого по одной строке *)
    | None -> not_end 
    | Some line -> print_endline line; true) false lst  in (* выводим ее, и определяем, нужно ли продолжать *)
  if continue then show_lines lst else ();; (* пока все не кончились, повторяем *)

show_lines files (* запускаем *)

DM>Простите за мой французский, но я напишу на Окамле

fold :: (a -> b -> b) -> b -> IntMap a -> b
fold f z coll  (f an ...(f a2 (f a1 z)))
prod = fold (*) 1 coll
       (an * ...(a2 * (a1 * 1)))
prodbut n = snd (fold iteratee (n,1) coll)
  where iteratee a (n,s) =
      if n <= 0 then (n,a*s) else (n-1,s)

DM>Простите за мой французский, но я напишу на Окамле

DM>(* функция, создающая один итератор *)
DM>let file_iter n = Enum.init 10 (fun i -> Printf.sprintf "file%d line%d" n i)

DM>(* список таких итераторов *)
DM>let files = List.init 10 (fun n -> file_iter n)

DM>(* вывод строк по одной *)
DM>let rec show_lines lst =
DM>  let continue = List.fold_left (fun not_end it -> (* проходимся по списку итераторов *)
DM>    match Enum.get it with (* берем из каждого по одной строке *)
DM>    | None -> not_end 
DM>    | Some line -> print_endline line; true) false lst  in (* выводим ее, и определяем, нужно ли продолжать *)
DM>  if continue then show_lines lst else ();; (* пока все не кончились, повторяем *)

DM>show_lines files (* запускаем *)
DM>

let lst = [ [ 1;2;3 ]; [ 4;5;6 ]; [ 7;8;9 ]; ];;                  

let rec iter = function
    | x::xs -> ( Printf.printf "%d\n" x; `Some (xs) )
    | []    -> `None;;
            
let rec filter f lst = 
    let rec filter' = function
        | x::xs -> (match f x with
                   | `Some(v) -> v :: filter' xs
                   | `None    -> filter' xs)
        | []    -> []
    in
    match filter' lst with
    | []  -> ()
    | nl  -> filter f nl;;
        
filter iter lst;;

open ExtLib;;

let lst = [ [ 1;2;3 ]; [ 4;5;6 ]; [ 7;8;9 ]; ];;                  

let rec iter action = function
  | x::xs -> action x; Some xs 
  | []    -> None;;
            
let rec filter f lst = 
  match List.filter_map f lst with
  | []  -> ()
  | nl  -> filter f nl;;
        
filter (iter (Printf.printf "%d ")) lst;;

let rec iter arr i act =
    act arr.(i);
    if i < Array.length arr - 1 then
        let next = iter arr (i + 1)
        in `Some next
    else
        `None;;
        
let (~~) arr = iter arr 0;; (* just a construction helper *)

let lst = [ ~~[| 1;2;3 |]; ~~[| 4;5;6 |]; ~~[| 7;8;9 |]; ];; 

let rec filter f lst = 
    let rec filter' = function
        | x::xs -> (match x f with
                   | `Some v -> v :: filter' xs
                   | `None   -> filter' xs)
        | []    -> []
    in
    match filter' lst with
    | []  -> ()
    | nl  -> filter f nl;;
        
filter (Printf.printf "%d\n") lst;;

let rec iter arr i act =
    act arr.(i);
    if i < Array.length arr - 1 then
        let next = iter arr (i + 1)
        in `Some next
    else
        `None;;
        
let rec iter2 lst act =
    match lst with
    | x::xs -> act x; `Some (iter2 xs)
    | []    -> `None;;
        
let (~~) arr = iter arr 0;; (* just a construction helper *)

let lst = [ ~~[| 1;2;3 |]; ~~[| 4;5;6 |]; ~~[| 7;8;9 |]; ];; 

let lst2 = [ iter2 [ 1;2;3 ]; iter2 [ 4;5;6 ]; iter2 [ 7;8;9 ]; ];; 

let rec filter f lst = 
    let rec filter' = function
        | x::xs -> (match x f with
                   | `Some v -> v :: filter' xs
                   | `None   -> filter' xs)
        | []    -> []
    in
    match filter' lst with
    | []  -> ()
    | nl  -> filter f nl;;
        
let _ = filter (Printf.printf "%d\n") lst;;

let _ = filter (Printf.printf "%d\n") lst2;;

let rec iter arr i act =
    act arr.(i);
    if i < Array.length arr - 1 then
        let next = iter arr (i + 1)
        in `Some next
    else
        `None;;
                
let rec iterOther mact other act = 
    let v = other (fun x -> act (mact x)) in
    match v with
    | `Some v -> `Some (iterOther mact v)
    | `None   -> `None;;
        
let (~~) arr = iter arr 0;; (* just a construction helper *)

(* That's the function you've asked :) *)
let rec map' f it = iterOther f it;;
        
let res = map' string_of_int ~~[| 1;2;3 |];;

(* we've already constructed a new iterator, just checking what's inside *)
let rec each' f it = 
    match it f with
    | `Some v -> each' f v
    | `None   -> ()
    in each' (Printf.printf "%s\n") res;;