Re: [Caml-list] Obj.magic for polymorphic identifiers

[Goswin von Brederlow <goswin-v-b@web.de>]

On Thu, Apr 24, 2014 at 06:30:24PM +0300, Dmitry Grebeniuk wrote:
> 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>}]

How do you encode a

type foo = Foo | Bar

Someone has to implement a ti_foo for it.

> # 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>";;

Here you have to match every possible type, which doesn't scale. Or
you limit yourself to structure, which is unsound for retrieving the
original type.

> val dump : Cdt.ubox -> string = <fun>
> # List.iter (fun ub -> print_endline (dump ub)) hlist;;
> 123
> qwe
> true
> - : unit = ()
> #

So you can print the values. But how to you get them back?
Where is your

val unbox : type a . uti -> Cdt.ubox -> a

(which isn't well typed like that)
 
>   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.

MfG
	Goswin