Re: S-expression I/O in Ada

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

Natacha Kerensikova wrote on comp.lang.ada:
[...]
> Sexp_Stream is supposed to perform S-expression I/O over streams,
> without ever constructing a S-expression into memory. It is supposed
> to do only the encoding and decoding of S-expression format, to expose
> its clients only atoms and relations.

But how can it expose atoms and relations without an in-memory tree
representation? Honestly, I do not think that such a concept is
viable. Hence, the "S-Expression stream" must actually depend on the
in-memory tree representation, both for reading and for writing. That
is whay my package does.

> A further unnamed yet package would handle the memory representation
> of S-expressions, which involve object creation, memory management and
> things like that. It would be a client of Sexp_Stream for I/O, so the
> parsing would only be done at one place (namely Sexp_Stream). As I
> said Ludovic Brenta's code might take this place.

No, it replaces both of your hypothetical packages and I don't think
you can have the "stream" package without the "S-Expression" package.
You could, however, have an S-Expression package without any I/O.

> There are situations when a memory representation of S-expressions is
> not needed, and the tcp-connect example here seems to be such a case.
> That's why I imagined TCP_Info as a client of Sexp_Stream instead of a
> client of the second package.

If no S-Expressions are needed in memory, then your entire data is
just one big unstructured String, and you cannot parse it into a
TCP_Info record unless you re-introduce the S-Expression tree in
memory.

>> I think the client should just do
>>
>> S : S_Expression := Some_Initialization;
>>
>> Put (File => F, Expression => S);
>
> To achieve such an interface, the client has to build an in-memory S-
> expression object. In the tcp-connect example, there are eight nodes
> (five atoms and three lists). They have to be somehow built, and it
> doesn't look like a simple initialization.
>
> The second interface I proposed, with a lot of nested calls, builds
> the S-expression object with functions looking like:
> function NewList(Contents, Next: S_node_access) return S_node_access;
> function AtomFromWhatever(Contents: whatever, Next: S_node_access)
> return S_node_access;

That's what I do in the test.adb example (using Cons () and To_Atom ()
respectively).

> However, unless I'm building other higher-level functions, to build 8
> nodes I need 8 function calls, exactly like the second example.
>
> That 8-function-call example looks very C-ish to me, and honestly I
> don't like it, I prefer the sequence of procedure calls from
> Sexp_Stream example, but that's just a personal taste.
>
> The problem is that I don't see how to do it otherwise. Any idea would
> be welcome.

Here is one idea: embed the S-Expression "language" in Ada:

function To_Sexp (S : in String) return S_Expression.T is
  String_Stream : constant Ada.Streams.Stream_IO.Stream_Access :=
     Stream_Reading_From (S); -- implementation left as an exercise :)
begin
   return Result : S_Expression.T do
      S_Expression.T'Read (String_Stream, Result);
   end return;
end To_Sexp;

TCP_Info : constant String := "(tcp-connect (host foo.bar) (port
80))";
TCP_Info_Structured := constant To_TCP_Info (To_Sexp (TCP_Info));

(this is based on my implementation).

>> Your TCP_Info-handling pkg would convert the record into an S-expression, and
>> call a single operation from your S-expression pkg to output the S-expression.
>
> That's the tricky part. At least so tricky that I can't imagine how to
> do it properly.

Yes, the S_Expression.Read operation is quite involved. But it can be
done, and has been done :)

--
Ludovic Brenta.