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From: Natacha Kerensikova <lithiumcat@gmail.com>
Newsgroups: comp.lang.ada
Subject: S-expression I/O in Ada
Date: Sun, 1 Aug 2010 05:17:45 -0700 (PDT)
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Date: 2010-08-01T05:17:45-07:00
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Hi,

I'm trying to learn Ada, coming from a C background. The first thing I
planned to code is a S-expression parser, because it's quite easy (at
least in C, less than 1000 lines including comments and memory
handling (dynamic arrays reinvented)) and very useful considering
almost all my existing programs use S-expressions as a serialization
format.

To describe briefly S-expressions, I consider them to be the simplest
existing data organization beyond raw sequences of bits. They are
basically lists of elements, each element being either a list or an
atom, and atoms being raw sequences of bits.

While I'm still not deep enough into Ada to know how to represent the
lists, I guess there won't be major issues, I think I can handle it
myself (though pointers and hints would still be welcome).

My question here is about how to represent the atoms in Ada. In C it
was merely a void pointer and a size, but it seems more difficult in
Ada because of the strong typing. Because it's up to the application
to make sense (i.e. type) out of the raw sequences of bits in atoms,
the S-expression library has to handle them as a sort of untyped
memory chunk. Do you know of a way to handle that?

Please correct me if I'm wrong, but my guess would be that the S-
expression library would provide a Sexp_Atom type, which refers to the
untyped memory chunks, and the application would have procedures to
convert back and forth between Sexp_Atom and whatever types it
internally uses (i.e. serialization and deserialization procedures).
However the library would provide these procedures for the most common
types (e.g. strings and numeric types).

Though it looks like a fine Ada API (at least to my eyes), I have
absolutely no idea about how to implement the library. How to define
the application-opaque Sexp_Atom type? How to read Sexp_Atom objects
from a file? How to build them from Ada types? How to write them back
to disk?

Thanks for your help,
Natacha