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Date: Thu, 24 Apr 2014 18:30:24 +0300
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From: Dmitry Grebeniuk <gdsfh1@gmail.com>
To: Romain Bardou <caml-list@inria.fr>
Content-Type: text/plain; charset=UTF-8
Subject: Re: [Caml-list] Obj.magic for polymorphic identifiers

Hello.

  It's perfectly doable without Obj at all.
  I needed "semi-typed" values quite often, so
decided to write reusable library "cadastr" [1]
that handles them, along with "untyped methods",
structural [de]composition (hence [de]serialization
based on value structure) and more.

  Things you need could be written like:

$ ocaml
        Objective Caml version 3.12.1

# #use "topfind";;
[...]
# #require "cadastr";;
[...]
# open Cdt;;
# let hlist = [ubox ti_int 123; ubox ti_string "qwe"; ubox ti_bool true];;
val hlist : Cdt.ubox list =
  [{ub_store = <fun>; ub_uti = <obj>}; {ub_store = <fun>; ub_uti = <obj>};
   {ub_store = <fun>; ub_uti = <obj>}]
# let dump ub =
  let uti = ub.ub_uti in
  if uti == (ti_string :> uti) then uget_exn ti_string ub else
  if uti == (ti_int :> uti) then string_of_int (uget_exn ti_int ub) else
  if uti == (ti_bool :> uti) then string_of_bool (uget_exn ti_bool ub) else
  "<unknown>";;
val dump : Cdt.ubox -> string = <fun>
# List.iter (fun ub -> print_endline (dump ub)) hlist;;
123
qwe
true
- : unit = ()
#

  As for "methods" I 've mentioned above,

# List.iter (fun ub ->
  let ushow = get_meth_untyped "show" ub.ub_uti in
  let ures = u_app ushow ub in
  print_endline (uget_exn ti_string ures)
  ) hlist;;
123
"qwe"
True
- : unit = ()
#

  However there is some runtime cost on such
"untyped" values.


[1] https://bitbucket.org/gds/cadastr , but it doesn't
    compile with OCaml >= 4.  It can be fixed;
    I'll do it if someone will want to use it.