Path: utzoo!attcan!uunet!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: defined() on an undef array element created via slice
Message-ID: <10334@jpl-devvax.JPL.NASA.GOV>
Date: 12 Nov 90 19:22:14 GMT
References: <1990Nov8.005449.26158@agate.berkeley.edu>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 83

In article <1990Nov8.005449.26158@agate.berkeley.edu> raymond@math.berkeley.edu writes:
: [The question:]
: 
: Consider the following script:
: 
:     @foo{"bar"} = ();
:     print keys(%foo), "\n"; # See, it's listed as one of the keys
:     if (defined($foo{"bar"})) { print "Patch level 18 prints this\n"; }
: 		        else  { print "Patch level 36 prints this\n"; }
: 
: Observe that $foo{"bar"} was created via an associative array slice.
: 
: Who is right, Patch level 18 or Patch level 36?

It's a judgement call, but I think 36 is righter.  It's nice to be able
to differentiate

	@foo{"bar"} = ();

from

	@foo{"bar"} = ("");

Or look at it another way.  You can have an existing variable with an
undefined value, why not an existing array element with an undefined
value?

If the x operator worked on array values (it doesn't), then you could say

	@foo{"foo", "bar"} = (1) x 2;

(I don't think I can make x work on array values without breaking scripts,
alas.  Hmm, maybe I can, if I require the parens.  Hmm...  Still possible
to break scripts, I think...)

: The problem seems to be that, although $foo{"bar"} is defined,
: its value is undef.  This schizophrenic state of affairs seems to
: lead to different interpretations depending on your patch level.
: 
: (Maybe because PL16 initialized $foo{"bar"} = ""?  Is this related
: to the change in semantics of local()?)

Yes, it is.  It's actually a difference in the semantics of list assignment,
of which local() assignment is a subset.

: [How I noticed this:]
: 
: My current code initializes a list of commands via
: 
:    @commands{ split(/ /, "Scale Height Width Borders etc...") } = ();
: 
: and tests if a particular token is a command via
: 
:    if (defined($commands{$token})) {...}
: 
: This breaks under PL36 as described above.
: 
: I know I can write
: 
:   for (split(/ /, "Scale Height Width etc...")) { $commands{$_} = 1; }
: 
: but somehow, the slice looked spiffier to me at the time.  Ah, the
: price one pays for a moment of folly.

How 'bout something like this:

	%commands = &set("Scale Height Width Borders etc...");

	sub set {
	    local($string, @result) = @_;
	    for (split(' ', $string)) {
		push(@result, $_, 1);
	    }
	    @result;
	}

	if ($commands{$token}) {...}

It would be nice if there was a way to generate arbitrarily long lists of
some specified value on demand, but I haven't (yet) got up the gumption to
turn Perl into Icon.  :-)

Larry