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

[Adam Beneschan <adam@irvine.com>]

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.)


> 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).

                           -- Adam