Re: In-Out Parameters for functions

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

On Tue, 27 Jan 2004 23:48:58 +0300 (MSK), "Alexandre E. Kopilovitch"
<aek@VB1162.spb.edu> wrote:

>Stephen Leake wrote:
>
>> > Look at this formulae:
>> > 
>> >   F(A) + G(A);
>> > 
>> > If F is permitted to change value of A then the result of the formulae may
>> > depend on order of computation. And this is obviously dangerous, because the
>> > formulae looks as typical mathematical expression, where this effect usually
>> > is not anticipated. 
>>
>> Hmm. Let F be Sqrt (Ada.Numerics.Float_Random.Random), and G be
>> Ada.Numerics.Float_Random.Random. Then we see that Ada _requires_ the
>> behavior you are describing. So it's clearly a Good Thing :).
>
>"Random" is very special case, in which an intention is quite opposite to what
>is common in normal "regular" circumstances. In "regular" circumstances we want
>the same result

And we have the *same* result, a realization of a *random* number. It
indeed does not depend on the input parameters (ideally). A program
using random numbers in turn becomes random in the sense that its code
and results are random. So we could treat Random as a constant, in a
random program. Of course Random is pseudo random, so it needs a
parameter, which is an implementation detail.

BTW, amusing A.5.2:

   procedure Reset (Gen : in Generator);  -- Reset an *IN* parameter!

but consistent with the design choice. Interestingly, that a true
Random, taking numbers from a hardware, should indeed have "in
Generator", provided, that it does not destroy the hardware just by
reading the values. (:-))

--
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de