Re: In-Out Parameters for functions

["Alexandre E. Kopilovitch" <aek@VB1162.spb.edu>]

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 for the same input data and we are in trouble when this does
not happen for any reason (for example, after moving to another compiler). But
when we use Random we want and expect some diversity of the results (perhaps
intermediate results), and the changes produced by the effect mentioned above
not necessary will defeat our purpose. Anyway, if you use Random for some stable
results you should always be very careful with all computations as well as with
mathematical background for them; so in such a case this effect is just a minor
(and easy) detail among various issues.




Alexander Kopilovitch                      aek@vib.usr.pu.ru
Saint-Petersburg
Russia