Re: S-expression I/O in Ada

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Tue, 10 Aug 2010 05:06:29 -0700 (PDT), Natacha Kerensikova wrote:

> On Aug 10, 12:36�pm, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
>>
>> Which was the question: what was wrong with the first object so that you
>> wanted another instead?
> 
> The first object is the internal memory representation designed for
> actual efficient use. For example, an integer will probably be
> represented by its binary value with machine-defined endianness and
> machine-defined size.
> 
> The other object is a "serialized" representation, in the sense that
> it's designed for communication and storage, for example the same
> integer, in a context where it will be sent over a network, can be for
> example represented as an ASCII-encoded decimal number, or in binary
> but with a predefined size and endianness.

Why don't you send it at once?

> This is really the same
> considerations as when storing or sending an object directly, except
> that is has to reside in memory for a short time. There is no more
> conversions or representations than when S-expression-lessly storing
> or sending objects; the only difference is the memory buffering to
> allow S-expression-specific information to be inserted around the
> stream.

This is impossible in general case, so the question why. As an example
consider a stateful communication protocol (existing in real life) which is
reacts only on changes. When you send your integer nothing happens because
the device reacts only when the bit pattern changes. So if you wanted to
really send it to another side you have to change something in the
representation of integer, e.g. to toggle some extra bit.

>> What was the problem then?
> 
> The problem is to organize different objects inside a single file. S-
> expression standardize the organization and relations between objects,
> while something else has to be done beyond S-expression to turn
> objects into representations suitable to live in a file.
> 
> Or the same thing with, instead of a file, an IPC socket or a network
> socket or whatever, I just don't know how to call it generically
> without resorting to a word derived from "serialize", but I hope you
> get the point anyway.

Yes, I don't see how S-expression might help there. They do not add value,
because the work of serialization or equivalent to serialization is already
done while construction of the expression object.

>> Because it cannot work without a conversion between
>> S-sequence in the memory (object) and S-sequence in the file
>> (representation).
> 
> The S-expression standard describe what conversions are allowed and in
> what they consist. I cannot follow the standard without such a
> conversion anyway, so either I do it or I drop the idea of S-
> expressions, but then I don't have anything to portably store or
> transmit  objects, so back to square one.

There are two questions to discuss:

1. The external storage format: S-expressions vs. other
2. Construction of an intermediate S-expression object in the memory

You justified #1 by an argument to legacy. You cannot [re-]use that
argument for #2. (OK, since Ludovic had already done it, you could (:-))

>> Why do you need S-sequence in the memory, while dumping
>> objects directly into files as S-sequences (if you insist on having them)
>> is simpler, cleaner, thinner, faster.
> 
> Because I need to examine the S-sequence before writing it to disk, in
> order to have enough information to write S-expression metadata. At
> the very lest, I need to know the total size of the atom before
> allowing its first byte to be send into the file.

That does not look like a stream! But this is again about abstraction
layers. Why do you care?

> That sounds like a very nice way of doing it. So in the most common
> case, there will still be a stream, provided by the platform-specific
> socket facilities, which will accept an array-of-octets, and said
> array would have to be created from objects by custom code, right?

Yes, if TCP sockets is what you use. There is a hell of other protocols
even on the Ethernet, some of which are not stream-oriented.

>> In other post Jeffrey Carter described this as low-level. Why not to tell
>> the object: store yourself and all relations you need, I just don't care
>> which and how?
> 
> That's indeed a higher-level question. That's how it will happen at
> some point in my code; however at some other point I will still have
> to actually implement said object storage, and that's when I will
> really care about which and how. I'm aware from the very beginning
> that a S-expression library is low-level and is only used by mid-level
> objects before reaching the application.

This is what caused the questions. Because if the problem is serialization,
then S-expression does not look good.

>> You do not need S-expressions here either. You can
>> store/restore templates as S-sequences. A template in the memory would be
>> an object with some operations like Instantiate_With_Parameters etc. The
>> result of instantiation will be again an object and no S-sequence.
> 
> Well how would solve the problem described above without S-
> expressions? (That's a real question, if something simpler and/or more
> efficient than my way of doing it exists, I'm genuinely interested.)

The PPN, a simple stack machine. Push arguments onto the stack, pop to
execute an operation. Push the results back. Repeat.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de