Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!elroy.jpl.nasa.gov!jpl-devvax!lwall
From: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: Subroutines Returning Null Arrays
Message-ID: <6990@jpl-devvax.JPL.NASA.GOV>
Date: 5 Feb 90 23:31:00 GMT
References: <13395@cit-vax.Caltech.Edu> <6712@ogicse.ogc.edu>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 71

In article <6712@ogicse.ogc.edu> hakanson@ogicse.UUCP (Marion Hakanson) writes:
: In article <13395@cit-vax.Caltech.Edu> joe@cit-750.cs.caltech.edu (Joe Beckenbach) writes:
: >
: >	Is there a way to force a subroutine to return a null array?
: 
: Now I just tracked it down further.  Here's a test program which
: demonstrates the problem (perl 3.0, pl6, anyway -- I didn't notice
: a patch in 7&8):
: 
: ==========cut here===========
: #!/usr/bin/perl
: 
: sub nsub1
: {
:   ();
: }
: 
: sub nsub2
: {
:   return ();
: }
: 
: 
: @nr1 = do nsub1();
: print "nr1 has $#nr1 elements.\n";
: 
: @nr2 = do nsub2();
: print "nr2 has $#nr2 elements.\n";
: ==========cut here===========
: 
: Here's the output:
: 
: nr1 has -1 elements.
: nr2 has 0 elements.
: 
: It turns out that $nr2[0] is empty, which is why my "workaround"
: works.  Definitely looks like a bug to me, though.  Larry?

This turned out to be a syntactic problem.  I had made return be a list
operator so that you can say

	return 1,2,3;

Unfortunately, like other list operators, if the next token was a left
paren, that and the matching paren were taken to delimit the entire function.
So
	return ()

transmogrified itself into

	(return);

and there went the null array.

This suggest several workarounds.  You can say any of

	return (());
	return ((),());
	return +();			# !!!
	return @nullarray;
	return 1..0;

The best solution, since

	return (123),456;

makes no sense, is to exempt "return" from the parenthesis rule for other
list operators, and have it ignore the parens.  It then will do what is
expected in almost every case.

Larry