Re: In-Out Parameters for functions

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

Robert A Duff 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.
>
> That might be a good argument against side effects in functions, but
> Wojtek Narczynski is asking why, given that side effects in functions
> are allowed in Ada, is it damaging to allow side effects on 'in out'
> parameters.  The F and G functions above might both write upon global
> variable X.  How could writing upon parameter A be any worse?

The F and G functions may both update global variable X, but neither of them
uses that X for computing the result, and this is what makes difference.

Commonly, if several functions may be used in the same expression then they
either update some global variable or use it for computing result, but not both.
That is, if one of them updates a global variable then others should not use
this variable for computation of their results.

So, writing upon global variables in functions may be safe enough - while we
honor the above rule.

But parameters of a function are expected to participate in the computation
of the result, and this is common case. So, even a particular parameter in
a function is not used for computation of the result of *this* function, it
easily may be passed to another function within the same expression and
influence its result.

Therefore, IN OUT parameters in functions will not be worse than the ability
of functions to change global variables if we forbid use of such functions in
expressions, that is:

  If at least one parameter of a function is in IN OUT mode then the function
  cannot be used in a compound expression. This function's call must always
  constitute the whole RHS of an assignment.

or less restrictive rule:

  If at least one parameter of a function is in IN OUT mode then in any
  expression in which this function is called all other calls may be for
  parameterless functions only.





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