Re: When a conditional expression is static, where can it be used?

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Wed, 30 Jun 2010 12:12:17 -0700 (PDT), Adam Beneschan wrote:

> On Jun 30, 10:35�am, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
>> On Wed, 30 Jun 2010 07:39:24 -0700 (PDT), Adam Beneschan wrote:
>>> On Jun 30, 3:39 am, Georg Bauhaus <rm.dash-bauh...@futureapps.de>
>>> wrote:
>>>> A totally meaningless example just to illustrate
>>>> the question: What is it that a compiler must report
>>>> for the case statement below, if anything?
>>
>>> The choice "Sa" is not covered by any alternative. �Other than that, I
>>> don't think there's anything wrong with the CASE statement, and if you
>>> had included a "when others =>" alternative I think it would be
>>> legal. �I'm not sure what potential problem you were trying to
>>> illustrate.
>>
>> Let me propose this one instead:
>>
>> � type DOW is (Mo, Tu, We, Th, Fr, Sa, So);
>>
>> � case D is
>> � when (if D = Mo then Tu else Mo) => P;
>> � when (if D = Tu then Tu else Mo) => Q;
>> � when We..So => R;
>> � end case;
>>
>> The above is equivalent to:
>>
>> � case D is
>> � when Tu => P;
>> � when Mo => Q;
>> � when We..So => R;
>> � end case;
>>
>> But as Pascal suggested, it should not compile because D is not static.
> 
> Ummm, not quite, because (1) Pascal didn't say anything about *why* he
> thought it shouldn't compile (his entire statement was "This should
> not compile I would say"), and (2) in the OP's original example, D
> *is* static.  In your example, you're right that it shouldn't compile
> because D is not static, but that's a different issue.  (Well, I
> *assume* D isn't static.  Since your example doesn't include a
> declaration of D, I can't tell.)

So, if D is static, then all choices are defined and do not overlap, hence
it must compile. Right?

>> As for the problem Georg had in mid. Maybe it is this. Let you have some
>> function, say Gamma function. Now,
>>
>> � �x : constant := 0.1;
>> � �Gx : constant := Gamma (1.1); -- Illegal, what a pity
>>
>> Let us open the table of Gamma, scan it, and write something like:
>>
>> � �(if x < 0.0 then ... elsif x < 0.01 then ... )
>>
>> This wonderful static function can then copied and pasted everywhere you
>> wanted to evaluate Gamma at compile time. Is it legal?
> 
> Gamma cannot be a static function (4.9(18-22)).  You cannot write a
> static function (other than an enumeration literal, which is
> technically a static function).

Now I do not understand you. The expression I gave is an if-operator with
only constants in it. I presume it is static. E.g., simplified:

 � �x : constant := 2.0;
 � �Gx : constant := (if x <= 1.0 then 1.0 elsif x <= 2.0 then 2.0 elsif x
<= 6.0 then 9.0 else 24.0);

We could even add linear or quadratic interpolation between the points.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de