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From: wsmith@uiucdcsb.cs.uiuc.edu
Newsgroups: comp.lang.pascal
Subject: Pascal User Manual and Report
Message-ID: <169900010@uiucdcsb>
Date: 17 Feb 88 17:22:00 GMT
Lines: 28
Nf-ID: #N:uiucdcsb:169900010:000:1281
Nf-From: uiucdcsb.cs.uiuc.edu!wsmith    Feb 17 11:22:00 1988


I was just looking at Jensen & Wirth's _Pascal User Manual and Report_.  
In the section on the AND and OR operators, it says:

"Assume for example, that x=0.  Then 
	(x>0) and (x<10)
is already known to be false after computation of the first factor, and the
scond need not be evaluated.  The rules of Pascal neither require nor forbid
						  ^^^^^^^^^^^^^^^
the evaluation of the second part in such cases."

Is this still true in the "current definition" of Pascal.  All of the
implementations I've found will evaluate the second part.  In fact, I believe
most people think that it the evaluation of the second part _is_ required.
However, the statement goes on:

"This means that the programmer must assure that the second factor is
well-defined."   (and goes on to give a standard example when the second part
of an AND expression might access an array out of bounds.)

What if the second part of the AND expression has side effects?  If my
reading of Jensen & Wirth is correct, the result of any program with such 
side effects is undefined in Pascal!  Is this conclusion true?  It certainly 
isn't a pleasant one since there is probably plenty of code that depends on 
the evaluation of the second part being required.

Bill Smith
ihnp4!uiucdcs!wsmith
wsmith@a.cs.uiuc.edu