Re: Is this expected behavior or not

[Stefan.Lucks@uni-weimar.de]

On Tue, 9 Apr 2013, Dmitry A. Kazakov wrote:

> This is why string types SHALL be designed properly in order to handle this
> without introducing millions of I/O packages.

We are in violent agreement here. The question is how much compatibility 
is needed between a new string type and the gazillions of old and broken 
ones, not that strings SHOULD be designed properly ...

> File name is not a string in first place. Ada.Directory and other I/O
> packages are broken in that respect, but that is irrelevant here.

It was you who came up with the example of Ada.Text_IO.Open, not me.

> You cannot do
>
>   X : String;
>
> either. String is a pseudo-type?

This is a straw man argument. String is not pseudo.

As you know very well, you can do X: String := ... or X: String(A ..B), 
while you can't do anything similar for Universal_Integer.

>> Again, my understanding is that they are converted before any run-time
>> operation is performed.
>
> Wrong but irrelevant as well, because semantically there is no difference.

OK, so look at the semantic. Different semantic means the same operation 
with the same inputs behave differently.

The following program fragment nicely prints "Smaller or Equal":

     if  ((Character'Pos('z') ** Character'Pos('z'))
        > (Character'Pos('z') ** Character'Pos('z')))
     then
        Put_Line("Larger");
     else
        Put_Line("Smaller or Equal");
     end if;

This is really cool -- the sematic of Universal_Integer enables us to 
perform operations over huge integers without overflows.

Now, guess what the following program fragment does?

      S: String := "zzzz";

   begin

     if  ((Character'Pos(S(1)) ** Character'Pos(S(3)))
        > (Character'Pos(S(2)) ** Character'Pos(S(4))))
     then
        Put_Line("More");
     else
        Put_Line("Less or Equal");
     end if;

The output is neither "More" nor "Less or Equal" but
"raised CONSTRAINT_ERROR : arit64.c:52 overflow check failed".

(When using gnat, don't forget "-gnato" to switch on overflow checks.)

Whenever the compiler cannot statically evaluate expressions of type 
Universal_Integer it switches to run time arithmetic over some integer 
type available at run time. This means, the expression behaves according 
to that type's semantic, raising Constraint_Error on overflows included.

If you insist that all the operations (such as the "**") above are over 
Universal_Integer, than that would mean the semantic of Universal_Integer
is allowed to change, depending on compile. versus run time.

That would be even more "pseudo". ;-)


------  I  love  the  taste  of  Cryptanalysis  in  the morning!  ------
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--Stefan.Lucks (at) uni-weimar.de, Bauhaus-Universität Weimar, Germany--