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(* Exercise: take drop [★★★]

a) Write a function take : int -> 'a list -> 'a list such that take n lst
    returns the first n elements of lst. If lst has fewer than n elements,
    return all of them.
    b) Write a function drop : int -> 'a list -> 'a list such that drop n lst
    returns all but the first n elements of lst. If lst has fewer than n
    elements, return the empty list.

Both have to be tail recursive; to construct large lists use `--` from
    earlier in the book

let rec take n lst =
    match n, lst with
    | 0, _  -> []
    | _, [] -> []
    | _, h :: t -> h :: take (n - 1) t

let rec drop n lst =
    match n, lst with
    | _, [] -> []
    | 0, h :: t -> t
    | _, h :: t -> drop (n - 1) t;;

(** [from i j l] is the list containing the integers from [i] to [j],
    inclusive, followed by the list [l].
    Example: [from 1 3 [0] = [1; 2; 3; 0]] *)
let rec from i j l =
    if i > j then l else from i (j - 1) (j :: l)

(** [i -- j] is the list containing the integers from [i] to [j], inclusive. *)
let ( -- ) i j = from i j []

let largelist = 1 -- 1_000_000;;
let dlist = drop 500_000 largelist;;
let tlist = take 500_000 largelist