type (_, _) t =
  | [] : ('a, unit) t
  | (::) : 'a * ('a, 'r) t -> ('a, unit -> 'r) t

let rec map : type a b r. (a -> b) -> (a, r) t -> (b, r) t =
  fun f -> function
    | [] -> []
    | h :: t -> f h :: map f t

let [ x ; y ; z ] = map string_of_int [ 1 ; 2 ; 3 ]