let rec map f l =
  match l with
    [] -> []
  | x :: xs -> f x :: (map f xs);;
let rec map f = function
  [] -> []
| x :: xs -> f x :: (map f xs);;
succ;;
succ 3;;
map succ;;
map succ [];;
map succ [6;7;4;5];;
let hd = function
  [] -> failwith "empty"
| h :: t -> h;;
hd ['a';'b'];;
hd [3,4];;
hd [];;
let app xs ys = match xs with
  [] -> ys
| x :: xs -> x :: (app xs ys);;
let rec app xs ys = match xs with
  [] -> ys
| x :: xs -> x :: (app xs ys);;
app [3;4] [6;7];;
let rec range m n = if m >= n then [] else m :: range (m + 1) n;;
range 5 10;;
range 5 1000;;
range 5 1000000;;
let rec foldr f st = function
  [] -> st
| h :: t -> f (foldr f st t) h;;
foldr (+);;
foldr (+) 0 [3;4;5];;
foldr (+) 1 [3;4;5];;
foldr (^) "." ["bla"; "qwe"];;
let space x y = x ^ " " ^ y;;
foldr space "" ["a"; "b"; "c"];;
let rec filter p = function
  [] -> []
| h :: t -> if p h then h :: (filter p t) else filter p t;;
let even n = x mod 2 = 0;;
let even n = n mod 2 = 0;;
even 1;;
even 12;;
filter even [2;3;4;5;6];;
filter even (range 5 30);;
foldr (+) 0 (filter even (range 5 30));;
let rec length = function [] -> 0 | x :: t -> 1 + length t;;
map length;;
map length [[3]; [4;5]];;