Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!van-bc!ubc-cs!phillips
From: phillips@pacific.cs.ubc.ca (George Phillips)
Newsgroups: comp.lang.perl
Subject: Re: undef for lists
Summary: make @a = undef undefine @a
Message-ID: <7731@ubc-cs.UUCP>
Date: 2 May 90 22:12:51 GMT
References: <7676@ubc-cs.UUCP> <1990Apr27.191209.12150@iwarp.intel.com>
Sender: news@cs.ubc.ca
Organization: University of British Columbia, Vancouver, B.C., Canada
Lines: 54

In article <1990Apr27.191209.12150@iwarp.intel.com> merlyn@iwarp.intel.com (Randal Schwartz) writes:
>sub bub { # error return is (undef)
>	return shift ? @_ : (undef);
>}
>
>sub try {
>	local(@ans) = &bub(@_);
>	if ((@ans != 1) || (defined $ans[$[])) {
>		print 0+@ans, " elements in @ans\n";
>	} else {
>		print "an error return\n";
>	}
>}
>
>
>&try(1,"a","normal","list");
>&try(1,"a single element");
>&try(1); # null list passed/returned
>&try(0, "should","be","ignored"); # finally, trigger an error

You don't need undef at all for your solution, just make sure
that a non-empty list is returned for success and an empty list
is returned for failure.  The caller can act on () as failure and
otherwise shift the list before using it.  This is actually simpler
for the caller because it only needs to test "shift @result".
So, the rehacked (and very silly looking) version:

sub bub { # error return is (), ok is (1, resultlist)
	shift ? unshift(@_, 1) : (@_ = ());
	@_;
}

sub try {
	local(@ans) = &bub(@_);
	if (shift @ans) {
		print 0+@ans, " elements in @ans\n";
	} else {
		print "an error return\n";
	}
}


&try(1,"a","normal","list");
&try(1,"a single element");
&try(1); # null list passed/returned
&try(0, "should","be","ignored"); # finally, trigger an error

This works, but it sure would be nice if assignment to undef always
undefines a variable.  Subroutines returning arrays would be much
cleaner and the language would make more sense.  It's a feature
I'd like to see unless it would require some major changes to perl.


George Phillips phillips@cs.ubc.ca {alberta,uw-beaver,uunet}!ubc-cs!phillips