There is no way to return an "undefined list" as distinct from an "empty list". Please go read the replies to your last posting. You can only return an empty list. For the following code:

    sub empty { return; }
    @x= empty();
    print defined(@x) ? "defined" : "not", "\n";
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-- Randal L. Schwartz, Perl hacker

I get real nervous wading into these more subtle issues, so flame if you must but I am just trying to understand.

My question is, based on the code below, is the problem in how you test for an empty list ??. Because if I am reading the output from below correctly an undefined @x and a @x = () will both return 0 when @x is tested for length.

I do find it curious that a an undefined array returns 0 when asked scalar @x.

So in conclusion in order to test if @x or an empty list one must test for both definedness and length ??

Of course I may have just wasted your time and mine beating a dead horse if I have I will meditate on perldoc:perldata for an extra hour tonight.

# x not defined
print "X not defiined:\t ",scalar @x,"\n" if (!defined(@x));
# given an empty list
@x = x();
print "X empty list:\t ",scalar @x,"\n";
@x = (1,2,3);
# x given a populated list
@x = x();
print "X populated:\t ",scalar @x,"\n";

sub x () {
  return @x if defined(@x) and warn "\@x was already defined";
  return @x if (scalar @x==0) and warn "\@x is 0";
  return wantarray ? () : undef;
}


Output:
X not defiined:  0
@x is 0 at wa.pl line 14.
X empty list:    0
@x was already defined at wa.pl line 13.
X populated:     3
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MitD -- Made in the Dark
'My favourite colour appears to be grey.'

Well, they both have a length of 0. Perhaps you are confusing the length with what $#ARRAYTHINGY returns, which is one less than the length, but therefore suitable for use with upper index bounds.

-- Randal L. Schwartz, Perl hacker