Re: Fixed point constants issue

["Vinzent Hoefler" <nntp-2010-09@t-domaingrabbing.de>]

On Mon, 13 Sep 2010 20:04:32 +0200, Dmitry A. Kazakov  
<mailbox@dmitry-kazakov.de> wrote:

> On Mon, 13 Sep 2010 19:27:30 +0200, Vinzent Hoefler wrote:
>
>> Is this a GNAT bug or am I just stupid?
>>
>> Suppose we have a fixed point type (with a rather large delta):
>>
>> -- 8< --
>> with Ada.Text_IO; use Ada.Text_IO;
>>
>> procedure Fixed_Point
>> is
>>     FEET_PER_METER : constant := 0.3048;
>>     HEIGHT         : constant := 10; -- Feet
>>
>>    type Altitude is delta (2.0**15 / 50_000.0) range 0.0 .. 50_000.0;
>
> [...]
>> Can anyone shed some light on this? This has driven us crazy for weeks  
>> now.
>>
>> [1] Of course, "Altitude'(HEIGHT * FEET_PER_METER)" yields "0.0" again.
>>      At least this is consistent with my understanding so far.
>
> Hmm, what did you expect? 0.3048 (FEET_PER_METER) is 0 when Altitude.  
> That is because 2.0**15/50_000.0=0.65536 > 0.3048.

Yes, of course. Still, it's not quite intuitive why

  TEN_FEET_1 : constant          := HEIGHT * FEET_PER_METER;
  TEN_FEET_2 : constant Altitude := HEIGHT * FEET_PER_METER;

just because a type is given in the second case.

I would expect the compiler to evaluate the expression "HEIGHT *  
FEET_PER_METER"
_before_ converting it to the appropriate fixed point type. And, obviously  
it
does this that when evaluating

   TEN_FEET_4 : constant Altitude := Altitude (HEIGHT * FEET_PER_METER);

In our opinion, this type conversion on the right hand side shouldn't be
necessary, yet it is (with GNAT).


Vinzent.

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