Pre-condition vs. Post-condition

[cml8@robin.cs.uofs.edu (Chris M. Little)]

Say we have a function CAPITAL which, given a country's name, returns its
capital city.  If the given country does not exist, an exception COUNTRY_ERROR
is raised.  Should the given country's presence be listed as a pre-condition
for this function, or should its absense (it doesn't exist) and the raising
of COUNTRY_ERROR be listed as a post-condition?

I brought this question up in class today and the outcome was a split decision.
I think exception raising and/or handling is as valid an outcome of a function
or procedure as any other outcome, so I'm tempted to cover the issue in the
post-condition comment.  My opponents believe that a function's pre-conditions
should be the conditions under which it would complete "normally", that is,
without any exceptions being raised.

I'd appreciate any insight on the issue.  E-mail please.  Thanks.

-- 
Chris Little, Graduate Asstistant	-     CML8@JAGUAR.UOFS.EDU	(VMS)
Department of Computing Sciences	-     CML8@SCRANTON.BITNET	(VMS)
University of Scranton, Pennsylvania.	-     CML8@ROBIN.CS.UOFS.EDU    (UNIX)