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From: zhou@brazil.psych.purdue.edu (Albert Zhou)
Newsgroups: comp.lang.pascal
Subject: Re: Gotos are ok
Message-ID: <11724@j.cc.purdue.edu>
Date: 25 Jan 91 11:25:53 GMT
Sender: news@j.cc.purdue.edu
Reply-To: zhou@brazil.psych.purdue.edu (Albert Zhou)
Organization: Purdue University
Lines: 43




I certainly agree that exit (leave or whatever) is different from goto
although you can implement exit with goto. When I posted the original
article, I was plagued by the limitation of "exit" implemented on Turbo
Pascal. I wanted exit several nested subroutines altogether, but I
could find a direct way to do it. Before this posting, I virtually had
never touch "goto" in Pascal. Unfortunately I found that goto's couldn't
go across modules (subroutines). I had thought that labels declared
globally could be refered everywhere just like global variable. As I
later realized, global labels can cause disasters for those who use goto
at all.

To further demonstrate the limitation of exit, please see the following
example:
Some suggest to implement an exit functioning like break in C, it will
work for
		repeat
		  ...
                  if c then exit
		until whatever

However, for a multiple-loop structure, one should be enabled to exit
a specified number of nested blocks:

		for i := 1 to 2
		  for j := 1 to 3 do
		  begin
          	     ...
		     if c then exit(2)
		  end

We also need such kind of exits for subroutines. Sorry I already use
the word exit for exiting blocks. So let me use exitsub for this.
So exitsub(3) means to exit from level n (current level) to level n-3.
If n-3 is negative, it will of course stop after exiting to the top level.
This is very useful for error handling procedures.                    

exitsub would be very easy to use and very useful too, as long as your
program is structured (I guess most people programming in pascal do
write structured programs). For example, one can write exitsub(3) when
he/she wants to exit from level 7 to level 4.