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From: Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Fixed point constants issue
Date: Fri, 24 Sep 2010 14:38:11 -0700
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Date: 2010-09-24T14:38:11-07:00
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On 09/24/2010 07:43 AM, Markus Schöpflin wrote:
>
> Are you sure about that? Because I compiled and ran the following
> program with an Ada 83 compiler (DEC Ada) and I got a different result.
> Note that I modified the delta to 0.5 to avoid any issues resulting from
> delta /= small.
>
> ---%<---
> with TEXT_IO; use TEXT_IO;
>
> procedure FIXED_POINT
> is
> type ALTITUDE is delta 0.5 range 0.0 .. 50_000.0;
>
> package ALTITUDE_IO is new FIXED_IO (ALTITUDE);
>
> FEET_PER_METER : constant := 0.3048;
> HEIGHT : constant := 10; -- Feet
>
> TEN_FEET_1 : constant := HEIGHT * FEET_PER_METER;
> TEN_FEET_2 : constant ALTITUDE := HEIGHT * FEET_PER_METER;
> begin
> ALTITUDE_IO.PUT (TEN_FEET_1); NEW_LINE; -- gives 3.0
> ALTITUDE_IO.PUT (TEN_FEET_2); NEW_LINE; -- gives 5.0 (*)
> end FIXED_POINT;
> --->%---
>
> (*) This certainly looks like FEET_PER_METER is converted to ALTITUDE
> before doing the multiplication.

That is interesting. I learned Ada 83 in 1984; I was explicitly taught that 
static universal (sub)expressions were evaluated exactly by the compiler, using 
universal operations. For example, I was taught that in

Pi : constant := 3.14159;

X : Float;
...
Reduce : loop
    exit Reduce when X < Pi / 2;

    X := X - Pi / 2;
end loop Reduce;

both occurrences of "Pi / 2" would be evaluated exactly by the compiler. Perhaps 
I was taught wrong.

-- 
Jeff Carter
"We use a large, vibrating egg."
Annie Hall
44