Re: S-expression I/O in Ada

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On Mon, 09 Aug 2010 09:48:02 -0400, Robert A Duff wrote:
>
>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
>> 
>>> The implementation or the idea? Would you agree that objects with some
>>> properties of modular integers have place in Ada programs which do not
>>> interface C?
>> 
>> No.  I do not like implicit "mod".  But that's the essence of
>> modular types.
>> 
>> If I ran the circus, then this:
>> 
>>     M : constant := 2**64;
>>     type My_Unsigned is range 0..M-1;
>>     pragma Assert(My_Unsigned'Size = 64);
>> 
>>     X : My_Unsigned := ...;
>>     Y : My_Unsigned := (X + 1) mod M;
>> 
>> would be legal, and would not raise any exceptions, even when
>> X = M-1.  (And I'd want "(X + 1) mod M" to be implemented
>> as a single "add" instruction on a typical 64-bit machine.)
>
> What does "+" above return?

A value of type My_Unsigned.  The values of this type are
the infinite set of all integers -- that's already the
case in Ada, except that the RM allows (not requires)
certain calculations to overflow and raise C_E.

>...Is "mod M" a required part of the notation or
> not?

No, not required -- my idea is that if you want to
perform some operation ("mod" or "+" or "*" or anything),
you write that explicitly in the code.  No change there
from the way signed integers already work.

>>>> Perhaps they _should_ be unordered, but I won't agree or disagree,
>>>> since I think in an ideal world they should be banished.
>>>
>>> I think they could be fixed.
>> 
>> How?  And having fixed them, when would you use them?
>> That is, when would you prefer them over signed integers?
>
> Ring buffer indexing,

I prefer an explicit "mod" there.

>...flags (by-value sets actually),

I prefer array-of-Boolean over modular.  Even better would
be a proper user-defined abstraction, with notations like "in".

>...cryptography,

Explicit "mod".

> interfacing, communication.

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;"

> I think that all these usages require different types of modular types with
> different sets of operations. So I would prefer a language extension which
> would allow me construction of such types rather than built-in ones, which
> satisfy no one.

I certainly agree with that.  But it sounds like you are
agreeing with my point -- that modular types should not
be in the language.

> I agree. But it rather means that C is wrong defining size_t modular. On
> Ada side it must be Natural_32. Why Ada does not support that? Why is it
> impossible to declare 32-bit range 0..2**32-1 without wrapping (=with
> overflow check)?

Good question.

- Bob