I think you can achieve what you’re looking for by using closures as follows:

{
    my ($foo, $bar, $container);

    my %transformations =
       (
            xform1 => sub { ... },
            xform2 => sub { ... },
       );

    sub transform
    {
       (my $self, $container) = @_;

        for my $xform (@{$self->{xforms}})
        {
            $foo = get_foo();
            $bar = get_bar();
            $transformations{$xform}->();
        }
    }
}
[download]

Using this approach, the anonymous subroutines (closures) in %transformations have access to $foo, $bar, and $container without the need for the boilerplate code.

Hope that helps,

Thank you for your reply. I see what you are trying to do, but I think your use of my in lexical scope around the package means ($foo, $bar, $container) are shared between all Transform objects. This will break thread safety.

Those variables are associated with a specific call to the transform() method, which is what I tried to demonstrate with my sample code. They need to remain private to that call and its delegates, but, ideally, without all the repetitive code. (Keep in mind this example is simplified... there will be more going on like alternative traversals, building transforms on top of other transforms, and so forth.)

...($foo, $bar, $container) are shared between all Transform objects. This will break thread safety.

No, it won’t.

The variables are shared between Foo::Transform objects within a thread (which doesn’t matter in this case, as they’re explicitly re-initialised in sub transform before being used). But in Perl, whenever a new process or thread is created, the memory is cloned at the point of creation, and thereafter is (by default) not shared (unless this is done explicitly).

For processes, see fork:

File descriptors (and sometimes locks on those descriptors) are shared, while everything else is copied.

For threads, see threads and threads::shared:

By default, variables are private to each thread, and each newly created thread gets a private copy of each existing variable.

Hope that helps,

Athanasius <°(((>< contra mundum

The trick here is to return a closure from a generator function. Below is a complete working example. However the salient bit is returning refs to $cont and $foo from the generator function, and dereferencing them in the caller to initialize or change them.

    package Foobar;

    sub _setup_xforms {
        my ($cont, $foo);
        return (\$cont, \$foo, {
            xform1 => sub { printf("Level %d,foo=%s\n", $cont, $foo); 
+},
        });
    };

    sub transform {
        my ($self, $cont) = @_;

        my ($cont_ref, $foo_ref, $xforms) = _setup_xforms();

        $$cont_ref //= $cont; # //= to convince you it's not still set
       
        for my $xform (values %$xforms) {
            $$foo_ref //= 0; # //= as above 
            $xform->();
            $$foo_ref++;
            $self->transform($cont + 1) if ($cont < 5);
            print "Level $cont done, foo=$$foo_ref\n";
        }
    }
    
    sub new { bless({},'Foobar') }


    my $foo = new Foobar( id => 1 );

    $foo->transform(0);
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This produces the following output:

    Level 0,foo=0
    Level 1,foo=0
    Level 2,foo=0
    Level 3,foo=0
    Level 4,foo=0
    Level 5,foo=0
    Level 5 done, foo=1
    Level 4 done, foo=1
    Level 3 done, foo=1
    Level 2 done, foo=1
    Level 1 done, foo=1
    Level 0 done, foo=1
[download]

This is exactly the solution I was looking for. It looks like it'll be easy to extend.

The objects are irrelevant in this case.

Maybe it'd be enough to local()ize the variables instead of insisting that they are lexical:

 {
 our ($cont, $foo, $bar);
 my %transformations = (
   xxx => sub { use_the($foo,'and', $bar); as_you_need($cont); },
   ...
 );
 }
 ...
 for my $xform (@{$self->{xforms}}) {
   our ($cont, $foo, $bar);
   local $foo = get_foo();
   local $bar = get_bar();
   local $cont = $container;
   $transformations{$xform}->();
 }
[download]

I would not bother and just access $_[0], $_[1] and $_[2] within the transformations.

The only other option is a source filter and I bet you don't want to go there.