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

[Dmitry Grebeniuk <gdsfh1@gmail.com>]

Hello.

>> # 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.

  I do not encode type foo anyhow, if its structure is
not needed, like in Romain's example.  As for ti_foo,
you are right:
  let ti_foo : foo #ti = new ti_simple "foo" ()
is enough to box/unbox values of type foo using
ti_foo.  (Also type name is not needed, but it eases
values introspection, so I always use meaningful
type names here.  Type annotation is not needed
too, in most cases.)

>>   if uti == (ti_string :> uti) then uget_exn ti_string ub else

> Here you have to match every possible type, which doesn't scale.

  It's just Romain's example encoded with my library.
Of course this approach doesn't scale, but second
piece of code ("show" method) scales well.

> Or you limit yourself to structure, which is unsound
> for retrieving the original type.

  Sorry, can't understand what you mean here;
but maybe other parts of my reply answer this.

> Where is your
>
> val unbox : type a . uti -> Cdt.ubox -> a
>
> (which isn't well typed like that)

  Of course it's not typed.  One has to provide
"witness" for this operation.  Both examples had
such "unbox":

uget_exn : #tti 'a -> ubox -> 'a

  So, no magic.  But no Obj too!

  Answering further possible questions:
- What if 'a and 'b, where 'a <> 'b, will have the
same "runtime type information"?  --
It's impossible.  ('a and 'b could _look_ different
due to type aliases or module abstractions,
but they have the same "original type")
- What if one creates two distinct "runtime type
informations" for given 'a, how will this work?  --
It won't work: once a "witness" was used to
create ubox, only this "witness" can be used to
retrieve typed value.
  (however I do some kind of "inheritance" on
"witnesses" and untyped methods dictionaries
that allows me to use some tricks like redefining
"show" or "cmp" methods leaving possibility
to work with uboxes using different typeinfos,
but it's not essential here.)