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From: rm55+@andrew.cmu.edu (Rudolph T. Maceyko)
Newsgroups: comp.lang.perl
Subject: Re: null? or not null?
Message-ID: <obaCBy600VozM6l0pF@andrew.cmu.edu>
Date: 20 Jan 91 00:25:34 GMT
References: <YbZGbb600VpQ82Jl1T@andrew.cmu.edu>,
	<zm7G&lq1@cs.psu.edu>
Organization: Carnegie Mellon, Pittsburgh, PA
Lines: 49
In-Reply-To: <zm7G&lq1@cs.psu.edu>

flee@cs.psu.edu (Felix Lee) writes:
> > print 2 if ($a,$b) = undef;               # prints 2 -- why?
> 
> "undef" is not a null list.  "()" is a null list.  "undef" is the same
> as "(undef)".  If you try

That's true, and I see why the assignment of "undef" to the list
returns true, but I still would like the the following while loop to
terminate if the function returns "()" (not "undef," as I stated
previously) instead of iterating indefinitely.

while (($a,$b) = (&function())[7,8]) {
 ...
}

I understand that "mentioning" non-existent array elements causes
their creation, but is that entirely necessary when the array is empty
and only referred to?

Unless it causes trouble that I don't forsee, I would consider
"ignoring" such array slices of "()" both a feature and an
optimization.  It's a feature because it makes my scripts work ;-> and
an optimization because creating new elements for the null list in
this case accomplishes nothing.

References to arrays (shift, pop) apparently always ignore null
elements (while references to lists [for, grep] do not), and
conditionality is apparently processed as a list (in an "array"
context).  Hmmm....  

A kludge which does what I want is:

sub function { (); }

while (($a,$b) = (@_ = &function())[7,8], @_) {
  print "yes\n";
}

but that involves the extra assignment and testing.  Do I have to live
with the kludge, or am I talking about a viable addition to the
language?

Rudy

+---------+
: +-----+ : Rudy Maceyko
: : +-+ : : rm55+@andrew.cmu.edu
: : : +-+ : rtmst@cis.pitt.edu
+-+ +-+-+-+