in reply to The Null Mull (or, when OO needs more O)

Since the undef SV is shared between all undefined values, one could modify undef using a feature in Perl 5.8. (I only thought of this because of the solution to #13 of How's your Perl? (II)).

At the beginning of your program (possibly in a BEGIN block) (completely untested):

&Internals::SvREADONLY(\undef, 0);
undef = Object::EveryMethod->new;
&Internals::SvREADONLY(\undef, 1);

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And override Object::EveryMethod for string/num/bool to behave like undef normally does.

"There is no shame in being self-taught, only in not trying to learn in the first place." -- Atrus, Myst: The Book of D'ni.

Replies are listed 'Best First'.
Re^2: The Null Mull (or, when OO needs more O)
by dragonchild (Archbishop) on Nov 29, 2004 at 18:55 UTC
    Hrmm ... 5.8.4/Solaris doesn't seem to do the right thing. 
    BEGIN {
    &Internals::SvREADONLY(\undef, 0);
    undef = 42;
    &Internals::SvREADONLY(\undef, 1);
}

my $x = undef;

print( ((undef) ? 'True' : 'False'), $/);
print( ((defined undef) ? 'True' : 'False'), $/);
print( (($x) ? 'True' : 'False'), $/);
print( ((defined $x) ? 'True' : 'False'), $/);

--------------

True
True
False
False

    [download]
    Maybe the code isn't actually affecting SV_UNDEF as you think ...

    Being right, does not endow the right to be rude; politeness costs nothing.
    Being unknowing, is not the same as being stupid.
    Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence.
    Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.

[reply]
[d/l]

      I just ran your code on 5.8.4 i686-Linux with the same result you did. I'm not sure what is going on here--someone with more understanding of the internals is needed. My guess is that undef isn't staying shared like I expected it would.

      It does work with taking a reference to undef, but this breaks the transparency I was hoping for:

      BEGIN {
    &Internals::SvREADONLY(\undef, 0);
    undef = 42;
    &Internals::SvREADONLY(\undef, 1);
}

my $x = ${ \undef };
print( ((undef) ? 'True' : 'False'), $/);
print( ((defined undef) ? 'True' : 'False'), $/);
print( (($x) ? 'True' : 'False'), $/);
print( ((defined $x) ? 'True' : 'False'), $/);
print $x, $/;

__OUTPUT__
True
True
True
True
42

      [download]

      "There is no shame in being self-taught, only in not trying to learn in the first place." -- Atrus, Myst: The Book of D'ni.

[reply]
[d/l]
        BEGIN {
    &Internals::SvREADONLY(\undef, 0);
    undef = 42;
    &Internals::SvREADONLY(\undef, 1);
}

use constant UNDEF => ${ \undef };

my $x = UNDEF;

        [download]

        Which is probably better because you're commenting that you did something funky with undef. *shrugs*

        Being right, does not endow the right to be rude; politeness costs nothing.
        Being unknowing, is not the same as being stupid.
        Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence.
        Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.

[reply]
[d/l]
Re^2: The Null Mull (or, when OO needs more O)
by diotalevi (Canon) on Nov 29, 2004 at 21:49 UTC
    undef isn't normally an lvalue. How about trying that as ${ \ undef } = ... instead?
[reply]
[d/l]

      Still no:

      BEGIN {
    &Internals::SvREADONLY(\undef, 0);
    ${ \undef } = 42;
 &Internals::SvREADONLY(\undef, 1);
}

my $x = undef;

print( ((undef) ? 'True' : 'False'), $/);
print( ((defined undef) ? 'True' : 'False'), $/);
print( (($x) ? 'True' : 'False'), $/);
print( ((defined $x) ? 'True' : 'False'), $/);
print $x;

__OUTPUT__
True
True
False
False

      [download]

      "There is no shame in being self-taught, only in not trying to learn in the first place." -- Atrus, Myst: The Book of D'ni.

[reply]
[d/l]
        Aha. my $x = undef wasn't as simple as we thought. $x was being cleared without PL_undef actually being assigned. Swap the right side for something more complicated like $hash{'non-existant'} and you'll find that $x is now 42.
[reply]
[d/l]
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In Section Meditations