Re: S-expression I/O in Ada

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

Natacha Kerensikova <lithiumcat@gmail.com> writes:
> 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?

In Ada, you normally model blobs with
System.Storage_Elements.Storage_Array; since arrays are first-class
citizens (as opposed to C's void pointers), you do not need to carry the
length of such an array separately.  Thus, a naive approach might be:

type Sexp_Atom is access System.Storage_Elements.Storage_Array;
type Sexp;
type Sexp_Access is access Sexp;
type Sexp is record
  Car : Sexp_Atom;
  Cdr : Sexp_Access;
end record;

However, the purpose of S-Expressions being to be read and written as
text, a blob may not be the most appropriate; you might be better off
with simply:

type Sexp;
type Sexp_Access is access Sexp;
type Sexp is
  Car : Ada.Strings.Unbounded.Unbounded_String;
  Cdr : Sexp_Access;
  Is_List : Boolean;
end record;

To write a sexp to disk and read back, you would leverage the Ada
streams as Dmitry pointed out.

You could then provide a generic package that serializes an arbitrary
type T back and forth to the unbounded_string.

-- 
Ludovic Brenta.