Re: S-expression I/O in Ada

["Randy Brukardt" <randy@rrsoftware.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:1o6jah15584x1$.1arrrgog9jdk7.dlg@40tude.net...
> On Mon, 9 Aug 2010 20:17:40 -0500, Randy Brukardt wrote:
>
>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
>> news:1y1c8zzqmcer5.po56hkesa968.dlg@40tude.net...
>> ...
>>>> For these, you don't want modular semantics -- you just want
>>>> a data type whose representation matches what you're
>>>> interfacing/communicating with, such as "type Octet is
>>>> range 0..2**8-1;"
>>>
>>> The problem is that these things require both array-of-Boolean view and
>>> arithmetic. I agree that when arithmetic is used, then it has to be 
>>> wide.
>>> E.g. when interpreting sequences of octets as little/big endian numbers,
>>> we
>>> never use modular arithmetic. But integer arithmetic is incompatible 
>>> with
>>> array/set view.
>>
>> What have you done with Dmitry?? You can't be the *real* Dmitry! :-)
>
> Brainwashed me? (:-))
>
>> Array-of-bytes views and arithmetic views are of clearly different types,
>> with different sets of operations. These shouldn't be mixed, logically or
>> any other way. If you need to go between these views, you need some sort 
>> of
>> type conversion (or Unchecked_Conversion or Stream'Read or...). Thus, it 
>> is
>> *never* necessary to do operations on both views at once, and it is
>> irrelevant what the "math" operations for octets is. If Ada gets anything
>> wrong about this, it is that it has math operations at all for
>> stream_elements.
>
> Right, but there is no contradiction because it is more than one type
> involved. What I meant is:
>
>   type Octet is ...;
>
>       -- Array interface to access bits of the octet (not Ada)
>   type Bit_Index is range 1..8;
>   function "()" (Unit : Octet; Bit : Bit_Index) return Boolean;
>   procedure "()" (Unit : in out Octet; Bit : Bit_Index; Value : Boolean);
>
>       -- Arithmetic interface, immediately leads out of octets (not Ada)
>   function "+" (Left, Right : Octet) return Universal_Integer;
>   function "-" (Left, Right : Octet) return Universal_Integer;
>       ...
> So when you write:
>
>   Little_Endian_Value := Octet_1 + Octet_2 * 256;
>
> There result is not an octet, as it would be with a modular type. 
> Presently
> it is just broken.

I wouldn't mess with mixed "+" routines that return other types. I'd just 
convert the Octets to a suitable type and add them. That is, any mess should 
be in type conversions, not in operators.

Since we're inventing things, I would suggest:

   -- Conversion function (Not Ada):
   function "#" (Right : Octet) return Universal_Integer;

   Little_Endian_Value := #Octet_1 + #Octet_2 * 256;

And now you don't need any Octet math.

BTW, the "#" operator was suggested by Jean Ichbiah and others back during 
the design of Ada 83, to be used for conversions between types. It's a pity 
that it or something like it never made it into Ada. (It comes up each time 
and still is rejected.)

                                Randy.