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From: Robert A Duff on 8 Aug 2010 18:22
"(see below)" <yaldnif.w(a)blueyonder.co.uk> writes:

> I don't use non-power-of-2 mod types, but I write emulators, and I would be
> stuffed if I could not declare, e.g.:
>
> type KDF9_word is mod 2**48;

Yes, that's one of the few cases where modular types make sense.

Unfortunately, nothing in the Ada RM requires implementations
to accept that. GNAT does, on all targets.

But for a circular buffer of (say) 100 items? No, I wouldn't
use modular types for the index.

- Bob
From: tmoran on 8 Aug 2010 18:35
> And it would be desirable, because in most cases the explicit
> "mod" (or "%") is more readable.

I := Ring_Indices'succ(I);
vs
I := (I + 1) mod Ring_Size;
or
Bearing := Bearing + Turn_Angle;
vs
Bearing := (Bearing + Turn_Angle) mod 360;
From: Yannick Duchêne (Hibou57) on 9 Aug 2010 00:46
Le Sun, 08 Aug 2010 22:51:37 +0200, Robert A Duff
<bobduff(a)shell01.theworld.com> a écrit:
> The only feature
> that's worse than "modular types" is "modular types with a
> non-power-of-2 modulus". ;-)
Illegal in SPARK

(not that I support the idea that non power of 2 modular types are ugly
beast)
From: J-P. Rosen on 9 Aug 2010 01:52
Robert A Duff a �crit :
> P.S. Can anybody recall the definition of "not" on modular
> types, when the modulus is not a power of 2, without
> looking it up?
Sure. Like any other operation. First you do the operation in the
mathematical sense, then you take the mathematical result mod T'Modulus.

> Hint: It makes no sense.
It makes sense because it is consistent with all other operations on
modular types. Whether it is usefule is another story ;-)

> The only feature
> that's worse than "modular types" is "modular types with a
> non-power-of-2 modulus". ;-)
Hmmm... Hash coded tables?

--
---------------------------------------------------------
J-P. Rosen (rosen(a)adalog.fr)
Visit Adalog's web site at http://www.adalog.fr
From: Robert A Duff on 9 Aug 2010 09:28
"J-P. Rosen" <rosen(a)adalog.fr> writes:

> Robert A Duff a �crit :
>> P.S. Can anybody recall the definition of "not" on modular
>> types, when the modulus is not a power of 2, without
>> looking it up?
> Sure. Like any other operation. First you do the operation in the
> mathematical sense, then you take the mathematical result mod T'Modulus.

"and", "or", and "xor" are defined that way (bit-wise logical operation,
followed by "mod"). "not" is NOT defined as bit-wise "not" followed
by "mod".

type T is mod 3;
X : T := 2;
...
X := not X;

Now X = 0. 2 = 2#10#. Bit-wise "not" of that is 2#01#.
"mod" that by 3, and you get 2#1#.

>> Hint: It makes no sense.
> It makes sense because it is consistent with all other operations on
> modular types. Whether it is usefule is another story ;-)

It is neither consistent nor useful. I'll stick with
"makes no sense".

>> The only feature
>> that's worse than "modular types" is "modular types with a
>> non-power-of-2 modulus". ;-)
> Hmmm... Hash coded tables?

Signed integers work fine for that. Except when you need
that one extra bit -- that's the problem.

- Bob
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