Re: {Pre,Post}conditions and side effects

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Jean François Martinez <darkquark99@gmail.com> writes:

> By too late I meant that once something is allowed it is difficult to
> make it illegal since a number of programs would no longer
> compile.

I can think of ways to do what you want without any incompatibility.

>... Now functions invoked in pre/post consitions are only
> supposed to check parms...

I don't agree with that.  It sometimes makes sense for pre/post to
check values of global variables.  Probably not often -- after all,
it's not often that you should have global variables in the first place.
But surely if the job of a procedure is to update some global,
it makes sense to have a postcondition ensuring that it did so
correctly.

Modifying globals in pre/post/predicate/invariant is another story,
and that's what you (correctly) complained about.

>...so most of the time they should be quite
> simple and sensible programmers would avoid side effects (except a few
> debugging Puts) but I still feel uncomfortable.about this Cish "be
> careful while playing russian roulette" philosphy.  At least the
> Rationale should have brought this point to attention.  Author must
> have assummed we were bright enough to figure it.  :)

Well, apparently YOU were bright enough to figure it out.  ;-)

- Bob