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From: sampson@cod.NOSC.MIL (Charles H. Sampson)
Newsgroups: comp.lang.ada
Subject: Re: Pre-condition vs. Post-condition
Keywords: pre-condition, post-condition, exception
Message-ID: <2938@cod.NOSC.MIL>
Date: 19 Mar 91 20:38:57 GMT
References: <2865@sparko.gwu.edu> <97779@tut.cis.ohio-state.edu> <311@nic.cerf.net>
Organization: Computer Sciences Corporation
List-Id: <comp.lang.ada>

In article <311@nic.cerf.net> sss@nic.cerf.net (Marlene M. Eckert) writes:
>How about exceptions should be raised only in _EXCEPTIONAL_ 
>situations?  Reaching the end-of-file or trying to POP off an
>empty stack are NOT exceptional conditions.

     O. K., but that just pushes the issue off to deciding what the word
_exceptional_ means.  The definition I prefer to use is: "An exceptional
condition is one that occurs infrequently or unexpectedly."  (Not original,
but I forgot whom I stole it from.  I think it was John Barnes.)  When
teaching Ada, immediately after giving that definition I point out to the
students that it does not require all exceptional conditions to be handled
by exceptions.

     What's happening here is that we're in an area where a lot of design
decisions have to be made.  I'm not sure that there is a rule that can be
applied to all cases and I am sure that if there is one we haven't found
it yet.  Unlike Mike Feldman, I have no philosophical problem with using
End_error to detect end-of-file or stack underflow to determine that the
stack is now empty, both satisfying the _infrequent_ criterion.  (I doubt
that I would ever use the latter myself, but my reasons are more esthetic
than anything else.)  Before I would condemn these uses in any particular
situation, I would want to hear the reasons for them.

     This has been an interesting thread to follow as we grapple with
this problem.  I particularly appreciate the fact that very few dogmatic
positions have been put forward.

                              Charlie