Path: utzoo!mnetor!uunet!mcvax!ukc!its63b!aiva!ken
From: ken@aiva.ed.ac.uk (Ken Johnson)
Newsgroups: comp.lang.misc
Subject: Re: Loops
Message-ID: <341@aiva.ed.ac.uk>
Date: 11 Apr 88 09:31:19 GMT
References: <2827@enea.se> <1557@pasteur.Berkeley.Edu> <2773@mmintl.UUCP> <AWJ9X4y00WAHE53kVi@andrew.cmu.edu> <270@bby-bc.UUCP> <4266@ihlpf.ATT.COM>
Reply-To: ken@uk.ac.ed.aiva (Ken Johnson,E32 SB x212E)
Followup-To: comp.lang.misc
Organization: Dept. of AI, Univ. of Edinburgh, UK
Lines: 37
Summary: catch, throw

How to allow breaking out of arbitrary loops, which you might otherwise
have to do like this:

>> done := false;
>> while not done do begin
>>	do_some;
>>	done := done | done_now;
>>	if not done then begin
>>		do_some_more;
>>		done := done | done_already;
>>		end;
>>		else do_even_more;
>>	end;

In some recent versions of Logo, and also in Lisp, you have the
`throw/catch' construct, which I think looks a lot better:

	to a_program
	    catch 'foo [
			   forever [
				     repeated_bit
				     if <condition> [
						       throw 'foo
						    ]
				     other_repeated_bit
				   ]
			]
	    last_bit

When the `throw 'foo' is executed, execution resumes at `last_bit'.
Catches may be nested.

-- 
------------------------------------------------------------------------------
From Ken Johnson, AI Applications Institute, The University, EDINBURGH
Phone 031-225 4464 ext 212
Email k.johnson@ed.ac.uk