let rec map f l =
  match l with
  | [] -> []
  | h :: t -> f h :: (map f t);;
succ;;
succ 3;;
map succ;;
map succ [1;2;34;5];;
let rec map f = function
  | [] -> []
  | h :: t -> f h :: (map f t);;
map succ [1;2;34;5];;
let hd = function [] -> failwith "empty" | h :: _ -> h;;
hd [3;4;5];;
hd [];;
[hd []];;
let rec app xs ys = match xs with
 | [] -> ys
 | h :: t -> h :: (app t ys);;
app [3;4;5] [7;8;9];;
let rec range m n = if m >= n then [] else m :: range (m + 1) n;;
range 5 50;;
let rec foldr f s = function
 | [] -> s
 | h :: t -> f (foldr f s h) t;;
let rec foldr f s = function
 | [] -> s
 | h :: t -> f (foldr f s t) h;;
foldr (+);;
foldr (+) 0;;
foldr (+) 0 [3;4;5];;
foldr (+) 0 (range 1 50);;
let space s1 s2 = s1 ^ " " ^ s2;;
foldr space "" ["asd"; "qwe"; "zxc"];;
let rec filter p = function
 | [] -> []
 | h :: t -> if p h then h :: filter p t else filter p t;;
let even n = n mod 2 = 0;;
evan 4;;
even 4;;
even 5;;
filter even (range 1 50);;
let rec len = function [] -> 0 | _ :: t -> 1 + len t;;
len [1;32;43;5;6;7];;
let len2 = foldr (fun l _ -> l + 1) 0;;
len2 [1;2;43;4;5];;