let rec foldr f base list =
  match list with
  [] -> base
  | x::xs -> f x (foldr f base xs)

let rec unfoldr p f g seed =
  if p seed then []
  else f seed :: unfoldr p f g (g seed)

(*
let rec map f list =
  match list with
  [] -> []
  | x::xs -> f x :: (map f xs)
*)
let map f list = foldr (fun x l -> f x :: l) [] list

let rec range min max =
  if min < max then min :: (range (min + 1) max)
  else []

let sum list = foldr (+) 0 list
let prod list = foldr ( * ) 1 list