gadt.ml

type _ trepr =
  | Int  : int trepr
  | Bool : bool trepr
  | List : 'a trepr -> 'a list trepr

let bool_to_string = function
  | true -> "True"
  | false -> "False"

let int_to_string = Printf.sprintf "%d"

let strings_to_string strings =
  Printf.sprintf "[%s]" (String.concat "; " strings)

let show : type a. a trepr -> a -> string = fun _ x ->
  failwith "failed overloading resolution"

let show : type a. a trepr -> a -> string = function
  | Bool  -> bool_to_string
  | trepr -> show trepr

let show : type a. a trepr -> a -> string = function
  | Int   -> int_to_string
  | trepr -> show trepr

let show : type a. a trepr -> a -> string = function
  | List trepr -> fun x -> strings_to_string (List.map (show trepr) x)
  | trepr -> show trepr

let test_show : string =
  show Bool true

let print : 'a trepr -> 'a -> unit =
  fun trepr x -> print_endline (show trepr x)

let print_ints : unit = 
  print (List Int) [1;2;3]