let rec sum = fun n -> if n = 0 then 0 else n + sum (n - 1) in sum 10;;

let rec f = fun x -> x + 1 in f 10;;

let id = fun x -> x in (id id) 1 + id 2;;

(* type error *)
let succ = fun x -> x + 1 in succ succ;;

(* type error *)
let id = fun x -> x in id + id;;