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From: edkay@lehi3b15.csee.Lehigh.EDU (Ed Kay)
Newsgroups: comp.lang.pascal
Subject: Re: Funny evaluation of functions
Summary: What I thought was funny
Message-ID: <616@lehi3b15.csee.Lehigh.EDU>
Date: 28 Aug 89 21:38:31 GMT
References: <20715@adm.BRL.MIL>
Organization: CSEE Dept. Lehigh University, Bethlehem, PA
Lines: 34


    When I originally posed the question about the evaluation
of  
    X:=4;
    WRITELN(OUTPUT,X*ONE(X));
 where
   FUNCTION ONE(VAR X:REAL):REAL;
   BEGIN
    X:=X+1;
    ONE:=X;
   END.

I was mildly surprised that result was 25 rather than 20, because
I thought that the evaluation of X might precede the evaluation of
ONE(X).  I should have more clearly stated that I was curious whether
the STANDARD had anything to say about the issue.  Some writers of
private letters have stated that the STANDARD says that the order of
evaluation is up to the implementor.
 
I encountered the question in the context of wanting to write a procedure
that had a side-effect on the variable X, and I was being cautious about
when I accessed X.  I wanted to be sure that I accessed X after it had
been changed.  I originally wrote  WRITELN(OUTPUT,ONE(X)*X) to be sure
that X was changed and then became curious whether my caution was warranted
in Turbo 4.0.  In practice, I would opt for the version in this paragraph,
which in most (all?) compilers would yield the desired result.  In any
case, I left behind a large caution sign about the trickery I was 
indulging in.  (Please note, that the actual code I wrote was not the
example I originally posted, but the example catches the spirit of the
actual code.)
 
   Edwin Kay
   CSEE
   Lehigh University