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From: Vinzent 'Gadget' Hoefler <ada.rocks@jlfencey.com>
Newsgroups: comp.lang.ada
Subject: Re: Modular integers
Date: Wed, 29 Oct 2003 09:30:41 +0100
Organization: JeLlyFish software
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Date: 2003-10-29T09:30:41+01:00
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Lars wrote:

>>> Isn't that the expected result?
>
>Thank you Vinzent! Oddly enough it seems totally self-explanatory now =
...
>I have totally forgotten which misconception had bothered me.

Well, I thought about what I wrote and became unsure about it. ;)

So to  prove my point I enhanced the little test program:

|with Ada.Text_IO;
|
|procedure t is
|   type    X_Base is mod 2**64;
|   subtype X_1    is X_Base range 0 .. 2**64 - 1;
|   subtype X_2    is X_Base range 0 .. 2**64 - 2;
|
|   A : X_Base;
|   B : X_1;
|   C : X_2;
|
|begin
|   begin
|      Ada.Text_IO.Put ("A'Last + 1: ");
|      A :=3D X_Base'Last;
|      Ada.Text_IO.Put (X_Base'Image (A));
|      A :=3D A + 1;
|      Ada.Text_IO.Put (X_Base'Image (A));
|   exception
|      when Constraint_Error =3D>
|         Ada.Text_IO.Put (" Constraint_Error");
|   end;
|
|   Ada.Text_IO.New_Line;
|
|   begin
|      Ada.Text_IO.Put ("B'Last + 1: ");
|      B :=3D X_1'Last;
|      Ada.Text_IO.Put (X_Base'Image (B));
|      B :=3D B + 1;
|      Ada.Text_IO.Put (X_Base'Image (B));
|   exception
|      when Constraint_Error =3D>
|         Ada.Text_IO.Put (" Constraint_Error");
|   end;
|
|   Ada.Text_IO.New_Line;
|
|   begin
|      Ada.Text_IO.Put ("C'Last + 1: ");
|      C :=3D X_2'Last;
|      Ada.Text_IO.Put (X_Base'Image (C));
|      C :=3D C + 1;
|      Ada.Text_IO.Put (X_Base'Image (C));
|   exception
|      when Constraint_Error =3D>
|         Ada.Text_IO.Put (" Constraint_Error");
|   end;
|
|   Ada.Text_IO.New_Line;
|
|   begin
|      Ada.Text_IO.Put ("C'Last + 2: ");
|      C :=3D X_2'Last;
|      Ada.Text_IO.Put (X_Base'Image (C));
|      C :=3D C + 2;
|      Ada.Text_IO.Put (X_Base'Image (C));
|   exception
|      when Constraint_Error =3D>
|         Ada.Text_IO.Put (" Constraint_Error");
|   end;
|end t;

The output is as I hoped, if not to say "as I expected", but IMO quite
interesting:

|A'Last + 1:  18446744073709551615 0
|B'Last + 1:  18446744073709551615 0
|C'Last + 1:  18446744073709551614 Constraint_Error
|C'Last + 2:  18446744073709551614 0

As you can see, the behaviour of the subtype is absolutely correct,
although it might not be intuitive. I mean, adding 1 raises a
constraint error while adding 2 (or even more, try it) doesn't.

So what we get here is a modular type with range constraints. *That's*
interesting... :-)


Vinzent.