Re^5: symbol/glob hash explanation

Looking into Symbol, that is apparently deliberate:

my $genpkg = "Symbol::";
my $genseq = 0;

sub gensym () {
    my $name = "GEN" . $genseq++;

    ## Instantiate the glob and take a reference to it.
    my $ref = \*{$genpkg . $name}; 

    ## Delete the name, leaving a reference to an anonymous glob
    delete $$genpkg{$name};        

    ## return the ref
    $ref;
}
[download]

Thus, it effectively mirrors the behaviour [] and {} in that you have a reference to a structure that doesn't (no longer) appears in the pad (symbol table).

The few times I've wanted an anonymous glob I've used:

$r = do{ \local *FRED };;
print $r;;
GLOB(0x4055f70)
[download]

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

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Re^6: symbol/glob hash explanation by rwstauner (Acolyte) on May 03, 2011 at 16:55 UTC
Aha! Thank you! I definitely should have looked at Symbol.pm before using it in my examples... Now I see why they suggest that you use it (and why the IO modules use it). So taking into account that Symbol::gensym uses deliberate voodoo, going back to the STDIN example makes it more clear... It is in fact part of the foo{THING} syntax (foo{HASH} in these examples): `DB<13> ${STDIN}{goo} = 'ber' DB<14> x \%{STDIN} 0 HASH(0x9a7d878) 'goo' => 'ber' * DB<15> x \%STDIN 0 HASH(0x9a7d878) 'goo' => 'ber' * DB<17> x STDIN 0 main::STDIN * DB<18> x STDIN{HASH} 0 HASH(0x9a7d878) 'goo' => 'ber' DB<19> x STDIN{IO} 0 IO::Handle=IO(0x97a5980)` [download] And a full example demonstrating gensym's symbol table manipulation: DB<6> $s = \Symbol::GEN1 DB<7> x $s 0 GLOB(0x9cca740) -> Symbol::GEN1 DB<8> x ${$s}{goo} = 'ber' 0 'ber' DB<9> x \%{$s} 0 HASH(0x9d4ace8) 'goo' => 'ber' DB<10> x \%Symbol::GEN1 0 HASH(0x9d4ace8) 'goo' => 'ber' * DB<13> x Symbol::GEN1{HASH} 0 HASH(0x9d4ace8) 'goo' => 'ber' DB<15> x Symbol::GEN1{HASH}{pea} = 'nut' 0 'nut' DB<16> x \%{$s} 0 HASH(0x9d4ace8) 'goo' => 'ber' 'pea' => 'nut' * DB<17> delete $Symbol::{GEN1} DB<18> x \%{$s} 0 HASH(0x9d4ace8) 'goo' => 'ber' 'pea' => 'nut' DB<19> x Symbol::GEN1{HASH} 0 undef DB<20> x \%{$s} 0 HASH(0x9d4ace8) 'goo' => 'ber' 'pea' => 'nut' [download] So while there's definitely some black magic happening there (and with the symbol tables in general) I have a better idea of what is happening behind the curtain in this particular instance... I now see where that hash is going (or where it has been (or something like that)). Thank you very much!	[reply] [d/l] [select]

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Re^6: symbol/glob hash explanation
by rwstauner (Acolyte) on May 03, 2011 at 16:55 UTC

Aha! Thank you!

I definitely should have looked at Symbol.pm before using it in my examples... Now I see why they suggest that you use it (and why the IO modules use it).

So taking into account that Symbol::gensym uses deliberate voodoo, going back to the STDIN example makes it more clear... It is in fact part of the *foo{THING} syntax (*foo{HASH} in these examples):

  DB<13> ${*STDIN}{goo} = 'ber'

  DB<14> x \%{*STDIN}
0  HASH(0x9a7d878)
   'goo' => 'ber'
*  DB<15> x \%STDIN
0  HASH(0x9a7d878)
   'goo' => 'ber'
*  DB<17> x *STDIN
0  *main::STDIN
*  DB<18> x *STDIN{HASH}
0  HASH(0x9a7d878)
   'goo' => 'ber'
*  DB<19> x *STDIN{IO}
0  IO::Handle=IO(0x97a5980)
[download]

And a full example demonstrating gensym's symbol table manipulation:

*  DB<6> $s = \*Symbol::GEN1
  DB<7> x $s
0  GLOB(0x9cca740)
   -> *Symbol::GEN1
  DB<8> x ${*$s}{goo} = 'ber'
0  'ber'
*  DB<9> x \%{*$s}
0  HASH(0x9d4ace8)
   'goo' => 'ber'
*  DB<10> x \%Symbol::GEN1
0  HASH(0x9d4ace8)
   'goo' => 'ber'
*  DB<13> x *Symbol::GEN1{HASH}
0  HASH(0x9d4ace8)
   'goo' => 'ber'
*  DB<15> x *Symbol::GEN1{HASH}{pea} = 'nut'
0  'nut'
  DB<16> x \%{*$s}
0  HASH(0x9d4ace8)
   'goo' => 'ber'
   'pea' => 'nut'
*  DB<17> delete $Symbol::{GEN1}
  DB<18> x \%{*$s}
0  HASH(0x9d4ace8)
   'goo' => 'ber'
   'pea' => 'nut'
*  DB<19> x *Symbol::GEN1{HASH}
0  undef
  DB<20> x \%{*$s}
0  HASH(0x9d4ace8)
   'goo' => 'ber'
   'pea' => 'nut'
[download]

So while there's definitely some black magic happening there (and with the symbol tables in general) I have a better idea of what is happening behind the curtain in this particular instance... I now see where that hash is going (or where it has been (or something like that)). Thank you very much!

[reply]
[d/l]
[select]