Re: In-Out Parameters for functions

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

Hyman Rosen wrote:

> I believe that once code is written, tested, and found to work, then
> having the evaluation order chosen by the language means that it will
> continue to work even when the environment changes. If the order is
> selected by the compiler, then you have no such assurance. Surely
> this is obvious?

You must be surprised, but the right answer is "generally, NO". The situation
depends on kind of applications for which the language is aimed.

When you rely upon fixed (in the language) evaluation order you are risking
to make things non-commutative. And if those things are commutative in the
underlying theory then you are subjected to unexpected failure when some part
of your "environment" changes. And if evaluation order is fixed in the
language then you lose an opportunity to caught that potentially dangerous
non-commutativity by testing with various compilers and various optimization
levels. Moreover, those failures that happen because of this difference -
between a theory, which is based on commutativity, and a non-commutative
implementation, may be extremely hard to investigate, just because the reason
is located neither in theory nor in software implemention, but somehow between
them. So, by fixing order of evaluation you enhance the overall situation
for theoretically trivial applications, but worsen it for applications that
are substantially based on non-trivial mathethical/physical theories.

So, both decisions - to fix evaluation order in Java and not to do that in Ada
- are right, these languageas are extreme cases: Java is used primarily for
theoreticaly trivial applications, while Ada is often used for applications,
based on mathematically non-trivial theories.

> Nevertheless, in conventional programming languages overspecification
> is the rule, and it is what programmers are used to.

That's true about commodity, but Ada language too often is used for programming
things that aren't commodities in any reasonable sense, and this is Ada's
native land. Ada isn't a popular language, can't be, and don't strive to be
too popular; what is desired is to get as much as possible of proper audience,
but not much more.




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