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From: maine@elxsi.dfrf.nasa.gov (Richard Maine)
Newsgroups: comp.lang.pascal
Subject: Re: Gotos are ok (Was Re: IMPLEMENT GOTO ACROSS MODULES IN TURBO PASCAL??)
Message-ID: <MAINE.91Jan24230505@ra.dfrf.nasa.gov>
Date: 25 Jan 91 07:05:05 GMT
References: <2802@oucsace.cs.OHIOU.EDU> <1991Jan21.134342@cs.utwente.nl>
	<2427@bnlux0.bnl.gov> <21489@oolong.la.locus.com>
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In-reply-to: jfr@locus.com's message of 24 Jan 91 17:38:35 GMT

On 24 Jan 91 17:38:35 GMT, jfr@locus.com (Jon Rosen) said:

Jon> can ONLY goto to a SINGLE place... I.e., in the abstract, the programmer
Jon> can't make a mistake... 
Jon>  
Jon> In the following pseudo-code:
Jon>    
Jon>    Repeat...
Jon>     ...
Jon>     if ... then leave;  
Jon>     ...
Jon>    EndRepeat;
Jon>    
Jon> There is only ONE place the leave statement can place the program...
Jon> That is after the EndRepeat statement... The programmer can only
                   ^^^  (as in THE only such statement anywhere?)
Jon> make a mistake in his logic (does he really want to LEAVE this
Jon> loop at this time?)... He can make NO mistake in his syntax... 

I wasn't going to enter this religious discussion, *but* I think
there is a factual error in the above.  The goto and the exit
are subject to simillar problems.  You could just as legitimately
have said "There is only one place the goto statement can place the
program... That is after the label."  Both your statement and my
modification ignore the important point that there may be multiple
EndRepeat statements or multiple labels.

(Actually I don't think "syntax" is the right term for the class of
errors you are talking about, but your meaning is reasonably clear
even if the word isn't quite right).

In some ways (not all, I agree) the above code is *more* prone to
errors than the equivalent using goto.  Suppose, for instance, I
have the above code working but later decide to add a repeat..endrepeat
loop to handle an input error check (or whatever).  If I don't look
carefully, the code becomes

  Repeat (the original one)
   ...
   Repeat (the added one)
    ...
    if ... then leave
    ...
   EndRepeat
   ...
  EndRepeat

Instant bug.  I don't think this is particularly esoteric either.
I've done it and I've seen others do it.  In this particular case,
the goto construct would not have been as prone to the problem.

Now if the original had been in Fortran 90 (hmm, is my religion
showing?) and looked like

  the_loop: do
   ...
   if (...) exit the_loop
   ...
  end do the_loop

then the odds of introducing errors would be lower I'd think.

For my part, well Fortran is my major language (for various reasons,
some of which are just my religious preferences, and others of which
are more objective, but all of which are irrelevant here).  It's
really hard to do ANSI standard Fortran 77 code without gotos.  Not
impossible, but hard and awkward enough that I don't try.  So of
course I use gotos, but I do use them with discipline.  (Well, anyway
I think so).

I Fortran 90, which I've started dabbling with in anticipation of
compilers coming out, I use far fewer gotos because the language
has constructs like the above example.  Yes, I know the above
exit is a goto in disguise, but its a disguise that I find useful.
Fewer is not the same as none.

Yes, I know you Pascal types don't really care much about the
Fortran aspects of the above.

--
Rich Maine
maine@elxsi.dfrf.nasa.gov