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)
;;

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

let rev_tr (l: 'a list) : 'a list =
  let rec rev_tr_helper l a =
    match l with
    | [] -> a
    | h::t -> rev_tr_helper t (h::a)
  in rev_tr_helper l []
;;

let range_tr l h : int list =
  let rec range_helper l h a =
    if l > h then rev_tr a
    else range_helper (l + 1) h (l::a)
  in
  range_helper l h []
;;

let range_tr2 l h = int list =
  let rec range_helper l h a =
    if l > h then a
    else range_helper l (h - 1) (h::a)
  in
  range_helper l h []
;;

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

(*
(* Mon = 0, Tues = 1*)
let cs440 = (440, "Muller", [1; 3]);;

let print_course c =
  let (num, prof, days) = c in
  Printf.printf "CS%d is taught by %s\n"
    num prof
;;
*)

type course = { num : int;
                prof : string;
                days : int list }
;;

let cs440 = { num = 440; prof = "Muller";
              days = [1;3]};;

let print_course c =
  Printf.printf "CS%d is taught by %s\n"
  c.num c.prof
;;

let print_course {days = d; num; prof} =
  Printf.printf "CS%d is taught by %s\n"
  num prof
;;

type course2 = { num : int };;

type day = Mon | Tue | Wed | Thur | Fri | Sat | Sun;;

type course = { num : int;
                prof : string;
                days : day list }
;;

let cs440 = { num = 440; prof = "Muller";
              days = [Tue; Thur]};;

let is_weekend day =
  match day with
  | Mon | Tue | Wed | Thur | Fri -> false
  | Sat | Sun -> true
;;