Re^4: symbol/glob hash explanation

I did try that and it does not work, at least not on my 5.10.1

  DB<2> $s = Symbol::gensym

  DB<3> x ${*$s}{goo} = 'ber'
0  'ber'
*  DB<4> x \%{*$s}{goo}
syntax error at (eval 8)[/usr/share/perl/5.10/perl5db.pl:638] line 2, 
+near "}{goo"
*  DB<5> x \%{*$s}
0  HASH(0x86285e8)
   'goo' => 'ber'
*  DB<6> x \%{*Symbol::GEN0}
0  HASH(0x87144e0)
     empty hash
*  DB<7> x ${*Symbol::GEN0}{goo}
0  undef
  DB<8> x $]
0  5.010001
  DB<9> x $s
0  GLOB(0x86285b8)
   -> *Symbol::GEN0
[download]

...

*  DB<8> x \%{*Symbol::GEN0}
0  HASH(0x8aab260)
     empty hash
*  DB<9> x \%{*{Symbol::GEN0}}
0  HASH(0x8aab260)
     empty hash
*  DB<10> x \%{*{*Symbol::GEN0}}
0  HASH(0x8aab260)
     empty hash
*  DB<11> x \%{'' . *$s}
0  HASH(0x8aab260)
     empty hash
*  DB<12> x \%{*$s}
0  HASH(0x8a5b5e8)
   'goo' => 'ber'
*  DB<13> use Scalar::Util qw(refaddr)

  DB<14> x \%{*refaddr($s)}
syntax error at (eval 20)[/usr/share/perl/5.10/perl5db.pl:638] line 2,
+ near "*refaddr("
*  DB<15> x \%{refaddr($s)}
0  HASH(0x8b6eea0)
     empty hash
*  DB<16> x \%{sprintf "%x", refaddr($s)}
0  HASH(0x8b73ef0)
     empty hash
*  DB<17> x \%{*{"$s"}}
0  HASH(0x8b73c10)
     empty hash
[download]

It does not show up when I dump \%:: (or \%Symbol or \%Symbol:: or \%Symbol::GEN0 or \%Symbol::GEN0:: either (excluded for brevity) at least not that i can see.

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Re^5: symbol/glob hash explanation by BrowserUk (Patriarch) on May 03, 2011 at 15:21 UTC
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.	[reply] [d/l] [select]
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]