[Caml-list] Question on typing of class/object and optional argument.

[Nobuyuki Tomizawa <n-tomizawa@mua.biglobe.ne.jp>]

Dear list:

I have a question on class/object with optional arguments method.

I could not compile following program:
------------------------------------------------------------
class foo s = object
  val str : string = s
  method to_string ?(opt = "" ) () = opt ^ s
end;;

let l = [new foo "a"; new foo "b"; new foo "c" ];;

List.iter (fun e -> print_endline (e#to_string ())) l;;
------------------------------------------------------------

with folloing message.

------------------------------------------------------------
File "test.ml", line 8, characters 52-53:
This expression has type foo list but is here used with type
  < to_string : unit -> string > list
Type foo = < to_string : ?opt:string -> unit -> string >
is not compatible with type < to_string : unit -> string > 
------------------------------------------------------------

In contrast, I did not get the above message if I rewrote the program
into module style like:

------------------------------------------------------------
module type BAR = sig
  type t
  val create : string -> t
  val to_string : ?opt:string -> t -> string
end;;

module Bar : BAR = struct
  type t = string
  let create t = t
  let to_string ?(opt = "") t = opt ^ t
end;;

let m = [Bar.create "a"; Bar.create "b"; Bar.create "c" ];;

List.iter (fun e -> print_endline (Bar.to_string e)) m;;
------------------------------------------------------------

I can not understand how different these two versions are in Ocaml's
typing systems.

Colud you please tell me the point and what should I know to
understand well about such kind of typing issues?

Thanks in advance.

Nobuyuki Tomizawa
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