Path: utzoo!mnetor!uunet!husc6!cmcl2!brl-adm!adm!wsmith@b.cs.uiuc.EDU
From: wsmith@b.cs.uiuc.EDU
Newsgroups: comp.lang.pascal
Subject: Pascal User Manual and Report
Message-ID: <11899@brl-adm.ARPA>
Date: 19 Feb 88 00:31:28 GMT
Sender: news@brl-adm.ARPA
Lines: 61

>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.

I tried this on my system where the second part is in fact NOT evaluated!
I believe it points out quite clearly the problem that you stated.  We are
running VS Pascal under VM.

pas side
The SIDE LISTING and TEXT files will be put on the A minidisk
 INVOKING PASCAL/VS R2.2
 NO COMPILER DETECTED ERRORS

 Source lines:    16;  Total time:   0.05 seconds;  Total rate:  19200 LPM
EXECUTION BEGINS...
     FALSE           2
      TRUE           1
R;
type side pascal

program side;
 var x:integer;
function side_effect:boolean;
begin
   x:=1;
   side_effect:=true
end;
begin
  termout(output);
  x:=2;
  if (x<2) and side_effect
    then writeln(True,x) else writeln(False,x);
  x:=2;
  if (x<3) and side_effect
    then writeln(True,x) else writeln(False,x);
end.


>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.

I suppose that this program would have different results via your compiler.
---------------------------------------------------------------------
Tim Margush                                    R1TMARG@AKRONVM.BITNET
(216) 375-7109
Department of Mathematical Sciences
University of Akron
Akron, OH 44325