Path: utzoo!utgpu!tmsoft!mason
From: mason@tmsoft.UUCP (Dave Mason)
Newsgroups: comp.lang.misc
Subject: Re: Loops
Message-ID: <294@tmsoft.UUCP>
Date: 28 Mar 88 04:52:36 GMT
References: <2827@enea.se> <1557@pasteur.Berkeley.Edu> <2773@mmintl.UUCP>
Reply-To: mason@tmsoft.UUCP (Dave Mason)
Followup-To: comp.lang.misc
Organization: TM Software Associates, Toronto / Ryerson Polytechnical Institute
Lines: 72

In article <2773@mmintl.UUCP> franka@mmintl.UUCP (Frank Adams) writes:
>
>The more I program, the more convinced I become that the 'while' loop is a
>mistake.  It just doesn't match the semantics of too many real loops.
>...
>The key points are:
>
>(1) Exit from any point in the loop.
>(2) Multiple exits.
>(3) Code can be associated with any exit point.
>
In a language I designed for my students to implement in my compiler
course loops look like:
	do
	   ...
	   while expr
	   ...
	od
where the while can go anywhere & there can be any number of them (or
until's which are similar but with opposite logic).  This leads to
Pascal style while & repeat loops:
	do while expr
	   ...
	od

	do
	   ...
	until expr od
as well as the more flexible forms.  The do keyword can be followed by
1 or more 'for' clauses, which are stepped in parallel:

	int a[0..29]
	...
	do 
	   for i in 1..20
	   for ap in a
	   ...
	   while i!=ap
	   ...
	od
equivalent to the C:
	int a[30];
	...
	{int i,*ap;
	for (i=1,ap=a;i<=20 && ap<&a[sizeof(a)/sizeof(int)];++i,++ap) {
	   ...
	   if (i!=*ap) break;
	   ...
	 }}
Notes:
1) loop control variables have a scope of the loop
2) loop control variables traversing an array (or list or other linked
	structure) are aliases for the current element
3) all the efficiency of C pointer arithmetic
4) all the safety of Pascal subscript calculation (and more if we were
	to make the aliased array be not directly usable in the loop
	scope, but I want them to be able to implement a reasonable
	subset in a semester :-)
5) controlled break out of outer loops (continue, break) (do, while,
	until, continue, break all have optional labels)

From the sample programs I've written in this language, this seems
like a very reasonable and natural loop structure.

(BTW, Frank's idea already exists in at least Concurrent Euclid, Turing:
	loop
		...
		exit when a<>b
		...
	end loop
)
	../Dave