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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: S-expression I/O in Ada
Newsgroups: comp.lang.ada
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Date: Sun, 8 Aug 2010 13:44:48 +0200
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Date: 2010-08-08T13:44:48+02:00
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On Sun, 08 Aug 2010 11:26:11 +0100, Simon Wright wrote:

> The only caution I'd add is that, at some point, when you're reading an
> S-expression from an external file and you expect the next 4 bytes to
> contain an integer in binary little-endian format,

S-expressions are bad, but not that bad. They have binary data encoded as
hexadecimal strings.

> you're going to have
> to trust _something_ to have got it right; if you wrote "*(struct foo
> **)((char *)whatever + bar.offset)" in C you will have to write the
> equivalent in Ada.

Luckily, Ada makes it difficult to write C equivalents. In Ada a
"non-equivalent" could be:

with Ada.Streams;     use Ada.Streams;
with Interfaces;      use Interfaces;
with Ada.Exceptions;  use Ada.Exceptions;

function Get (Stream : access Root_Stream_Type'Class) return Unsigned_16 is
   Result : Unsigned_16 := 0;
begin
   if '#' /= Character'Input (Stream) then
      Raise_Exception (Syntax_Error'Identity, "Opening '#' is expected");
   end if;
   for Octet in 0..3 loop
      Result :=
         Result + Character'Pos (Character'Input (Stream)) * 2**Octet;
   end loop;
   if '#' /= Character'Input (Stream) then
      Raise_Exception (Syntax_Error'Identity, "Closing '#' is expected");
   end if;
   return Result;
end Get;

> Unless you were going to invent a sort of checked
> S-expression? (I don't get that impression!)

I don't think that were an invention. The program in C or Ada that does not
check the format of the input is broken to me.

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