[Caml-list] Classes and marshaling

[Caml-list] Classes and marshaling

[Frederic Tronel]

Is it possible to serialize an object ??
In the toplevel:

        Objective Caml version 3.04

#  open Marshal ;;
#  class a = object end ;;
class a : object  end
#  to_string (new a) [] ;;
Exception: Invalid_argument "output_value: object value".
#

Best regards,


Frederic Tronel.
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Re: [Caml-list] Classes and marshaling

[Frederic Tronel]

Frederic Tronel wrote:
> 
> Is it possible to serialize an object ??
> In the toplevel:
> 
>         Objective Caml version 3.04
> 
> #  open Marshal ;;
> #  class a = object end ;;
> class a : object  end
> #  to_string (new a) [] ;;
> Exception: Invalid_argument "output_value: object value".
> #
> 
> Best regards,
> 
> Frederic Tronel.
> -------------------
> Bug reports: http://caml.inria.fr/bin/caml-bugs  FAQ: http://caml.inria.fr/FAQ/
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After having taken a look at the source code (byterun/extern.c), it
seems that 
caml value tagged with Object_tag (that is object) are simply ignored
(an
exception is raised). 

Is there any chance to see object serialization being implemented in the
near future ??
By the way I did not find any reference about this problem in the
documentation
(in the book by chailloux, manoury and pagano, they mention that
abstract
types cannot be serialized, but it seems they do not mention objects).

Best regards,

Frederic Tronel.
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Re: [Caml-list] Classes and marshaling

[Xavier Leroy]

> Is it possible to serialize an object ??

Currently not.

> After having taken a look at the source code (byterun/extern.c), it
> seems that caml value tagged with Object_tag (that is object) are
> simply ignored (an exception is raised).

They are not ignored, but rejected :-)

> Is there any chance to see object serialization being implemented in
> the near future ??

Object serialization, like function serialization, is quite a
challenge.  One can either send the code for the object's methods, so
that it can be read back anywhere -- but OCaml's code is not
relocatable enough to allow this; or one send the object's instance
variables only, and add various mechanisms to ensure that the method
code at the receiving end is compatible with that at the sender -- but
no such really good mechanism exist (witness Java's serialization).

So, we're thinking about it, especially in the context of JoCaml (a
distributed programming language based on Caml), but don't hold your
breath.

In the meantime, I'd recommend either serializing objects "by hand"
(via methods that write the object's state), or converting between your
objects and a pure datatype that you can then read and write
efficiently with output_value and input_value.

- Xavier Leroy
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