Re: A case where Ada defaults to unsafe?

[dewar@gnat.com (Robert Dewar)]

Hyman Rosen <hyrosen@mail.com> wrote in message news:<3C35E733.6030603@mail.com>...
> Robert Dewar wrote:
> 
> > Well any vaguely competent Ada programmer should equally
> > well know that in Ada "and then" and "or else" short circuit
>  
>  > and "and" and "or" do not.
> 
> Sure. But what do you gain from the non-short-circuit ones,
> except for the risk that you've used them incorrectly?
> 
> 
> > Exactly the same as in C, where you use either & or &&.
> 
> 
> In C, you cannot use & and | in the same way that you use
> && and ||, because the former accept only integer operands.
> For example, if you have a pair of pointers, p and q, and
> wish to test whether both are not null, you must use p&&q.
> If you were to use p&q, the compiler would reject it.
> 
> C++ does have the horrible trap that user-defined && and ||
> operators don't short-circuit. Expert advice is to never
> define these except in the rarest of circumstances.

No, I am talking about the case of boolean operands. With
non-boolean operands you are in a completely different
domain. I personally find the usage p&&q here frightful,
and think it much better to introduce the explicit null
tests. But anyway I am only talking about the boolean case.

Why use AND instead of AND THEN, or OR instead of OR ELSE

The argument on this side goes like this. We only want to
use AND THEN if we specifically want to imply that there
is a serial dependence, which we think should be pointed
out. If there is no serial dependence, then some people
find it cleaner to use AND.

After all, if we write

      A = B and C > D

as opposed to

      A = B and then C > D

it is clear that these are semantically equivalent, so a
compiler can freely convert one into another, so there is
no efficiency issue in many cases.

Where there is an efficiency issue, it is often hard for
a programmer to tell what is more efficient.

Pascal has a more general rule. In Pascal,

   A and B

can be evaluated left to right, right to left, with or
without short circuiting, giving the compiler maximum
flexibility.

P.S. this is a peculiar thread, there is an extraordinary
number of very bizarre and misleading posts. I am only
responding to posts that are not in this category, but
beware :-)