Re: Fixed point constants issue

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

Vinzent Hoefler wrote:
> 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 think these cases are clear from the language rules:

The declaration of TEN_FEET_1 is a named-number declaration, so the 
expression is evaluated using the "root" multiplication operator "*" 
(Left : root_integer; Right : root_real) return root_real, which is done 
at compile-time and exactly (with unlimited precision).

 > I would expect the compiler to evaluate the expression
 > "HEIGHT * FEET_PER_METER" [for TEN_FEET_2] _before_
 > converting it to the appropriate fixed point type.

The declaration of TEN_FEET_2 is an object declaration with type 
Altitude, therefore the right-hand-side expression is expected to be of 
type Altitude and so the "*" will be resolved to the Altitude 
multiplication operator "*" (Left, Right : Altitude) return Altitude, 
with result zero.

I agree that this is a bit nasty and counter-intuitive. Perhaps the 
compiler should warn that the conversion of the universal_real number 
0.3048 to Altitude is seriously inexact, but then, what is "seriously"?

> And, obviously it does this that when evaluating
> 
>   TEN_FEET_4 : constant Altitude := Altitude (HEIGHT * FEET_PER_METER);

Here the right-hand side expression is a type conversion, in fact a 
value conversion, and so the operand HEIGHT * FEET_PER_METER is expected 
to be of any numeric type. At first sight the multiplication could be 
resolved to the root "*" or to the Altitude "*", which would thus be 
ambiguous. However, RM 8.6(29) says that the ambiguity is resolved by 
using the root "*", giving the more precise result.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .