Re: Ada safety road Was: Which is right ...

[Keith Thompson <kst@cts.com>]

Robert A Duff <bobduff@world.std.com> writes:
[...]
> There are few enough erroneous cases in Ada, that it would have been
> reasonable to spell it out in a long phrase every time.  Something like,
> "If <RM-ese for dangling pointer>, the program is incorrect.  The
> implementation is not required to detect this error, neither at compile
> time nor at run time.  If this error occurs, execution is totally
> unpredictable."  And then put some verbiage in chapter 1 about how awful
> "totally unpredictable" really is.

The folks over in comp.std.c have a good term term for this: "nasal
demons".  This originated when someone observed that when the compiler
encounters a construct with undefined behavior, it is legal for it to
make demons fly out of your nose.  See also the entry for "nasal
demons" in The Jargon File and/or _The New Hacker's Dictionary_.

Note that there's no implication that the compiler is *required* to
make demons fly out of your nose.

It's important to remember that one possible consequence of undefined
behavior (or erroneous execution) is for the implementation to
silently do exactly what you naively expected it to do.  This is often
the nastiest possibility, since any problems may not show up until
years later.

This is why the authors of Ada (both 83 and 95) made such an effort to
minimize the number of cases where this can happen, and to clearly
document the cases that couldn't reasonably be avoided.

-- 
Keith Thompson (The_Other_Keith) kst@cts.com  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center  <http://www.sdsc.edu>                 <*>
One of the great tragedies of ancient history is that Helen of Troy
lived before the invention of the champagne bottle.