gadt_lang.ml

type _ expr = Int : int -> int expr | Float : float -> float expr

type any_expr = Any : 'a expr -> any_expr

type _ ty = TyInt : int ty | TyFloat : float ty

let rec eval : type a . a expr -> a = function
  | Int i -> i
  | Float f -> f

let rec typeof : type a . a expr -> a ty = function
  | Int i -> TyInt
  | Float f -> TyFloat

let rec print : type a . a -> a ty -> unit =
  fun value ty ->
    match ty with
    | TyInt -> print_int value
    | TyFloat -> print_float value

let rec print_type : type a . a ty -> unit =
  function
  | TyInt -> print_string "int"
  | TyFloat -> print_string "float"

let eval_and_print (Any e) =
  let ty = typeof e in
  print_string "- : ";
  print_type ty;
  print_char ' ';
  print (eval e) ty;
  print_newline ()

let parse str =
  try Any (Int (int_of_string str))
  with Failure _ ->
    Any (Float (float_of_string str))

let test () = begin
  eval_and_print (parse "13.4");
  eval_and_print (parse "1");
  eval_and_print (parse "bad");
end