Re: S-expression I/O in Ada

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Tue, 10 Aug 2010 16:42:08 -0500, Randy Brukardt wrote:

> "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.

Conversion mess is what we already have right now. The point is that "+" is
well-defined and meaningful for octets, but it is not closed in there. Why

   function "**" (Left : T; Right : Natural) return T;
   function S'Pos(Arg : S'Base) return universal_integer;
   ...

are OK and "+" is not?

> 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.

1. What is this else?

2. I see no advantage over

   Little_Endian_Value := Integer (Octet_1) + Integer (Octet_2) * 256;

> 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.)

It would be nice to have more operators  than now ("^", "|", "..", "in",
":=", "<>", "()", "[]", "{}", "<=>", "@", "~" and so on. Ah, Ada is Unicode
now, here is the list: http://www.unicode.org/charts/PDF/U2200.pdf ).

But what is so special in type conversions, aren't they just functions?

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