poly.ml

   1
   2 type t = [`A of int | `B of string | `Z of t]
   3
   4 let rec use_t = function
   5   | `A i -> i
   6   | `B s -> String.length s
   7   | `Z x -> use_t x
   8
   9 let rec show_t
  10   : t -> unit (* comment this *)
  11   = function
  12   | `A i -> print_endline "A"
  13   | `B s -> print_endline "B"
  14   | `Z x -> (print_string "Z"; show_t x)
  15
  16 let rec make_narrow = function
  17   | `A i -> `A i
  18   | `B s -> `A (String.length s)
  19   | `Z x -> `Z (make_narrow x)
  20
  21 let rec use_narrow = function
  22   | `A i -> i
  23   | `Z x -> (use_narrow x)
  24
  25 let tee f x = f x; x
  26
  27 let final x =
  28   let y = make_narrow x in
  29   show_t y;
  30   use_narrow y
  31
  32 let final x = use_narrow (make_narrow (tee show_t x))
  33
  34 let final x =
  35   use_narrow (tee (fun z -> show_t z) (make_narrow x))
  36