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All the Perl that's Practical to Extract and Report

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  • by chromatic (983) on 2007.02.19 19:37 (#53308) Homepage Journal

    Anyway, after compile-time the calls to the functions are already bound in the code and the actual symbol table entries aren't needed anymore.

    I don't buy it:

    $ perl -MO=Concise,new
    sub double {
        my $number = shift;
        return $number * 2;

    sub new {
        my ($class, $number) = @_;
        bless {number => double($number)}, $class;

    k  <1> leavesub[1 ref] K/REFC,1 ->(end)
    -     <@> lineseq KP ->k
    1        <;> nextstate(main 3 -:7) v ->2
    8        <2> aassign[t5] vKS ->9
    -           <1> ex-list lK ->5
    2              <0> pushmark s ->3
    4              <1> rv2av[t4] lK/1 ->5
    3                 <#> gv[*_] s ->4
    -           <1> ex-list lKPRM*/128 ->8
    5              <0> pushmark sRM*/128 ->6
    6              <0> padsv[$class:3,4] lRM*/LVINTRO ->7
    7              <0> padsv[$number:3,4] lRM*/LVINTRO ->8
    9        <;> nextstate(main 4 -:8) v ->a
    j        <@> bless sK/2 ->k
    -           <0> ex-pushmark s ->a
    h           <1> srefgen sK/1 ->i
    -              <1> ex-list lKRM ->h
    g                 <@> anonhash sKRM/1 ->h
    a                    <0> pushmark s ->b
    b                    <$> const[PV "number"] s/BARE ->c
    f                    <1> entersub[t7] lKS/TARG,1 ->g
    -                       <1> ex-list lK ->f
    c                          <0> pushmark s ->d
    d                          <0> padsv[$number:3,4] lM ->e
    -                          <1> ex-rv2cv sK/1 ->-
    e                             <#> gv[*double] s ->f
    i           <0> padsv[$class:3,4] s ->j
    - syntax OK

    How does that gv lookup find double() if not in the symbol table?

    • Hrm, point for you. Bad wording on my side. What's the thing called methods are looked up from then? :)

      I honestly thought a delete $Foo::{bar} will remove the symbol table entry.

      Thanks again!

      Ordinary morality is for ordinary people. -- Aleister Crowley
      • You misunderstood what chromatic said. He demonstrated that Perl does not have compile-time binding, contrary to what you are saying. Decompiling shows that the invocation of double in the new method is indirected via the package. If you remove the entry for double from the package, the function call will NOT work.

        Sorry, but your approach won’t work.

        Here’s a pattern for you to read carefully and chew on:

        package Foo::Bar::Internals;

        use Carp qw( croak );

        sub double { ... }

        sub Foo::Bar::new

        • So, you're saying what? I can understand that as either

          • Bad wording on my side, and it's not removed from the symbol table, but from some other table used for method lookups.
          • I'm doing voodoo, because the things I do in the test cases can't work.

          And btw: You call that a pattern? I call that a work-around :) And, just FYI, you might want to stay away from patronising phrases like "Here’s a pattern for you to read carefully and chew on." Because it really decreases my motivation to answer.

          Ordinary morality is for ordinary people. -- Aleister Crowley
          • You said:

            So, you’re saying what?

            It’s bad wording on your part to say the functions are bound in the code, because they’re not; they’re always looked up from the symbol table.

            Interestingly, what you’re doing shouldn’t work – but it does! Apparently the %main::-type hashes aren’t actually an interface to the symbol table, they’re just a one-way mirror:

            sub xx {
                my $in_stbl = *main::foo{CODE} ? 1 : 0;
                my $in_hash = main-

            • Interestingly, what you’re doing shouldn’t work – but it does!

              Of course. Did you honestly think I sent some code off to CPAN without even _trying_ first? :)

              Obviously not. Smells strongly like a bug to me (at the very least like a doc bug), not like behaviour that should be relied on. Someone alert the porters…

              Maybe I will post it there if nobody else does.

              Errm, that’s what patterns are []

              That might be what you (and MJD) think, but I clearly see a difference betw

              Ordinary morality is for ordinary people. -- Aleister Crowley
              • Every programm is a pattern and a sum of patterns.

                No, it’s not. A pattern is a common, complex arrangement of language primitives that has to be aligned just so in order to work; which arrangement addresses a particular problem commonly encountered when using the language.

                Far from everything in a program does meets this definition. Most notably, the solution to the problem addressed by the program in its essence is not a pattern, by definition, although if it’s non-trivial you will ofte

                • Well, everything's said I guess and it would be useless to repeat it, especially since we're both drifting off in the ad-hominem area.
                  Ordinary morality is for ordinary people. -- Aleister Crowley
            • Replace delete by undef in your example, and it works. You can't delete subs from a stash in Perl 5.8, only undef them. If you use delete, the sub still exists (you can use "exists &foo" to test for it). That can be considered as a bug.