Path: utzoo!attcan!utgpu!watmath!maytag!watstat!dmurdoch
From: dmurdoch@watstat.waterloo.edu (Duncan Murdoch)
Newsgroups: comp.lang.pascal
Subject: Re:  Funny evaluation of functions
Message-ID: <407@maytag.waterloo.edu>
Date: 24 Aug 89 12:55:16 GMT
References: <20693@adm.BRL.MIL> <2974@mace.cc.purdue.edu>
Sender: daemon@maytag.waterloo.edu
Reply-To: dmurdoch@watstat.waterloo.edu (Duncan Murdoch)
Organization: U. of Waterloo, Ontario
Lines: 19

In article <2974@mace.cc.purdue.edu> ags@mace.cc.purdue.edu (Dave Seaman) writes:
>In an expression like "x*one(x)", it would be possible to skip the
>evaluation of the function "one" if x happens to be zero.  If "one" has
>side effects, then the result is implementation-dependent.  Assuming that
>both terms are evaluated, they may be taken in either order.  

Another example I just saw of a function with side effects causing problems
was in porting dvitty from Unix Pascal to Turbo.  At one point (in an out
of date copy) is a line something like

  do_something(getvalue,getvalue);

where getvalue is a function that reads a file to get a value.  On Unix, the
second getvalue was evaluated first, and the program worked; in Turbo, the
functions were evaluated from left to right, and the procedure ended up
with its parameters reversed.  Similar things could happen in any expression
that tried to read more than one value from a file.

Duncan Murdoch