1.ml

let rec take n l =
  match l with
    [] ->
    (
        match n with
          0 -> []
        | _ -> raise (Invalid_argument "take")
    )
  | h::t ->
    if n < 0 then raise (Invalid_argument "take")
    else if n = 0 then []
    else h :: take (n - 1) t;;
take 2 [1; 2; 3; 4];;
take 2 [];;