Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!apple!agate!math.berkeley.edu!raymond
From: raymond@math.berkeley.edu (Raymond Chen)
Newsgroups: comp.lang.perl
Subject: defined() on an undef array element created via slice
Message-ID: <1990Nov8.005449.26158@agate.berkeley.edu>
Date: 8 Nov 90 00:54:49 GMT
Sender: usenet@agate.berkeley.edu (USENET Administrator)
Reply-To: raymond@math.berkeley.edu
Organization: U.C. Berkeley
Lines: 41

[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?

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()?)

[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.
--
@japh{split(/:/, "hacker,:Just :erl :another p")} = (); print sort keys %japh;
# Extra credit:  What is $japh{"hacker,"}?  What is defined($japh{"hacker,"})?