Re: In-Out Parameters for functions

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

Wojtek Narczynski wrote:

> This worry is unclear to me:
>
> >> And only one seems still valid: it is about complex 
> >> expressions where a function call may be used as 
> >> an actual argument for another function. 
>
> I don't see how adding IN OUT arguments could make things worse in
> this matter.

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. Then notice that the expressions often may be more 
complicated... something like this:

  F(A, B) * G(C, F(A, D)) + G(A, B) * F(C, G(A, D))

Add to that possible optimization issues, and probably you will see some
reason to worry.

Expressions with functions are too important part of a language like Ada,
where various scientific formulas are used very often in typical applications.
Therefore this part must be made safe enough and must respect mathematical
viewpoint, which is naturally and commonly associated with formulas.

> A function can have side effects on global state, it can
> have side effects on its IN paramerters (by using the Rosen trick).

Yes, but if a programmer produces/uses side effects on global state in a
function and especially if he changes IN parameters by any trick, he at least
knows that he is doing irregular things - and takes responsibilty for that.
Providing IN OUT parameters for functions we remove this mental warning/deterrent.

> I just don't see how documenting those side effects in the function
> specification could make things worse.

The problem is that it may be not enough to document those possible side
effects in the function's specification - and the above point is exactly
about that. By requiring it to be documented in call site also, we ensure
that the caller is aware of possible side effect there and agreed with it.




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