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Journal of phaylon (6115)

Monday February 19, 2007
06:00 PM

namespace::clean off to the CPAN

[ #32442 ]

Yesterday (at least I think it was yesterday, excuse the laziness) I released a pragma called namespace::clean. The short explanation for what this module does is this: When use'd, it will build a list of current functions in the package, and install a handler to call after the requesting modules compile time (via Filter::EOF) in which it removes those entries from the symbol table.

The long explanation is that you can import functions into your namespace, or even declare your own functions, that won't show up as methods on calls on your classes and instances. Previously, it always felt ugly to me to import functions (read: Carp, aliased, Scalar::Util & Co, ...) into a packages namespace, so I called them by their full name, for example, Scalar::Util::weaken(). The same ugliness overwhelms me everytime I put (proper documented) functions in OO packages, so a lot of them end up as methods, even if they don't need to. That's where this module joins the game.

Here is a simple demonstration: Let's define a usual object oriented class:

package Foo;
use warnings;
use strict;

use Carp qw( croak );

sub double {
    my $number = shift;
    return $number * 2;
}

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

1;

This package will contain three functions: croak, double and new. But we actually just want new available as a method. Not because of coworkers, since they should refer to the hopefully complete and well written documentation, but to prevent conflicts with sub- and future base-classes.

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. So we put a line reading use namespace::clean; in our code at the position where all the functions to remove are defined:

package Foo;
use warnings;
use strict;

use Carp qw( croak );

sub double {
    my $number = shift;
    return $number * 2;
}

use namespace::clean;

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

1;

After this, the following will work correctly:

my $foo = Foo->new(23);
print $foo->{number}; # prints 46

But these will both return undef:

my $can_double = $foo->can('double');
my $can_croak  = $foo->can('croak');

So, that's it. I'd very much appreciate some comments. I'm currently thinking about no namespace::clean; and unimport magick for being able to build excludable parts.

Update: 0.02 has just entered the PAUSE and will be available soon.

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  • 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;
    }

    main::new:
    k  <1> leavesub[1 ref] K/REFC,1 ->(end)
    -     <@> lineseq KP ->k
    1     

    • 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.
  • I wish you had given this normal capitalization and not called it a pragma. Pragmas by definition are shipped with core perl. This may be a useful module, but calling it a pragma only muddies the waters.
    • It hadn't occured to me yet and, in fact, I had to look for a pretty long time before I found the term "default module" in perlglossary for pragma. I think you mean that sentence. That was a fault on my part, so sorry for that.

      But there seems to be oil all over the ocean already: autorequire [cpan.org], version [cpan.org] (will be ok then in 5.9 I guess), fake [cpan.org], all [cpan.org],

      --
      Ordinary morality is for ordinary people. -- Aleister Crowley
      • I don't mean to single you out. You're certainly not the only one to do it. I think it's better to stick with the standard though, and avoid calling CPAN modules pragmas or naming them all lowercase.
        • No worries, I'm not feeling singled out :) Though in my opinion it would have been more confusing to name it Namespace::Clean and affect the local package the way it does. I, and I had the impression that many others too, understand the term 'pragma' more as 'altering behaviour or environment' vs. the 'providing code' situation of library modules.

          --
          Ordinary morality is for ordinary people. -- Aleister Crowley
  • This is a bug that it works. It's only semi-unlikely to get fixed. I've things I'd like to do that need this bug to get fixed so I may spend some time to ensure that this bug "works" appropriately.

    There's two related bugs here. If you localize or delete the stash a function exists in, the GV pointed to by the pp_gv opcode has a pointer to the original stash. This makes it oblivious to wholesale replacement of the stash.

    It appears the other bug is that that the pp_gv opcode has a pointer to the GV and retain