let _ = Some 5;;
let _ = Some "Hello";;
let _ = Some (4 + 5);;
let _ = None;;

let my_div (x: int) (y: int) : int option =
  if y <> 0 then Some (x / y)
  else None
;;

let my_div_or_def (x: int) (y: int) (d: int)
 : int =
  match my_div x y with
  Some q -> q
  | None -> d
;;

let zero_or_def (x: int) (y: int) (d: int)
 : int =
  match my_div x y with
  None -> d
  | _ -> 0
;;

let l1 = [];;
let l2 = 5::l1;;
let l3 = [1; 2; 3; 4];;
let l4 = [[1;2]; [3; 4]; []];;
let l5 = [["HI"; "Hello"]; []]

let is_empty (l: 'a list) : bool =
  match l with
  | [] -> true
   (* 5::(4::[]) *)
  | a::b -> false
;;

let is_empty (l: 'a list) : bool =
  if l = [] then true else false
  (* Don't write it that way *)
;;

let l6 = [2; 3]::[];;

let head_of_list (l: 'a list) : 'a option =
  match l with
  | [] -> None
  | h::_ -> Some h
;;

let second_of_list (l: 'a list) : 'a option =
  match l with
  | [] -> None
  | _::[] -> None
  | _::h2::_ -> Some h2
;;

let second_of_list (l: 'a list) : 'a option =
  match l with
  | _::h2::_ -> Some h2
  | _ -> None
;;

let rec length (l: 'a list) : int =
  match l with
  | [] -> 0
  | h::t -> 1 + length t
;;

let my_div_match (x: int) (y: int) : int option =
  match y with
  | 0 -> None
  | _ -> Some (x / y)
;;

let rec sum (l: int list) : int =
  match l with
  | [] -> 0
  | h::t -> h + sum t
;;

let rec last (l: 'a list) : 'a option =
  match l with
  | [] -> None
  | h::[] -> Some h
  | h::t -> last t
;;

let rec range l h : int list =
  if l > h then []
  else l::(range (l + 1) h)
;;

let rec append (l1: 'a list) (l2: 'a list)
: 'a list =
  match l1 with
  | [] -> l2
  | h::t -> h::(append t l2)
;;