Xref: utzoo comp.lang.misc:1462 comp.lang.pascal:816
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From: gast@lanai.cs.ucla.edu (David Gast)
Newsgroups: comp.lang.misc,comp.lang.pascal
Subject: Re: Threatening Pascal Loops
Message-ID: <11369@shemp.CS.UCLA.EDU>
Date: 18 Apr 88 22:52:52 GMT
References: <2827@enea.se> <1557@pasteur.Berkeley.Edu> <2773@mmintl.UUCP> <294@tmsoft.UUCP> <11047@shemp.CS.UCLA.EDU> <3364@omepd>
Sender: news@CS.UCLA.EDU
Reply-To: gast@lanai.UUCP (David Gast)
Organization: UCLA Computer Science Department
Lines: 75

In article <3364@omepd> bobdi@omepd.UUCP (Bob Dietrich) writes:
>In article <11047@shemp.CS.UCLA.EDU> gast@lanai.UUCP (David Gast) writes:

>Sorry, but the existing ANSI/IEEE and ISO Pascal standards do not allow such
>modification.  [of the loop control variable].

Yes, I realize that.  That was exactly my point.  The program name was
illegal.  One might have thought the name was somewhat self-explanatory.
The main comment went on to say:

>>{This program is not legal pascal, but the compiler does not detect the
>> error.  If the scope of the index variable were local to the for loop
>> (ala Algol 68), instead of global, then this error would be detected
>> at compiler time. } 

>There is a concept called "threatening" explained under the
>for-statement (section 6.8.3.9). Basically, if a variable has been
>"threatened", it cannot be used as a for control variable. 

Dietrich goes on to explain what threatened means.  Are there any
compilers that detect every possible instance of a threatened
variable?

Presumably there must be some other verbage to the extent
that an index variable cannot be accessed after the loop ends.
The following program is also illegal, but again, I suspect that
most compilers do not detect the error.  The standard Berkeley 4.3
compiler does not.

	program ILLEGAL (output);
	var
		i : integer;
	procedure p ;
		begin
			writeln(i);   /* Variable I is undefined at this point */
		end;
	
	begin
		for i := 1 to 10 do
			writeln(i); 
		p;
	end.

Essentially, slightly over stated, one will have to guarantee that
a loop control variable is never used outside the scope of the for
loop or do extensive run-time checking.

Of course, you can put extra checks into the compiler to try and detect
these errors.  These checks take time and make the compiler bigger.

As long as you end up (de facto) not allowing the loop control variable
to be used outside the scope of the for loop, why not do the sensible
thing in the first place?  Why not decide *in the language definition*
that the loop control variable is local to the loop?  That is the
decision Algol 68 made and it works.  It is impossible to assign to a
loop control variable in Algol 68.  And no concept like "threatening"
is needed so it is easier to learn.

One could have the following syntax:

	for FOR-CONT-VAR : S-TYPE := LB to UB do STATEMENT

The one argument against this change is that such a change would make
previously valid pascal programs illegal.  But the use threatening also
makes previously, valid pascal programs illegal.  That is, a loop
control variable can be threatened without actually being assigned to.
Such a program would not have been illegal under the old standard, but
it is, as I understand it, under the new standard.

The old standard Pascal had many type insecurities.  Perhaps the new
standard eliminates all of these; probably it doesn't.  As I no longer
have to use Pascal, I do not care to investigate myself.  If the new
Pascal, however, does not have any type insecurities, then it is a far
different language.  The defining document and the compilers and run time
systems are also undoubtedly much bigger as well.