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=head1 NAME

Closure::Explicit - check coderefs for variable capture

=head1 VERSION

version 0.002


 use Closure::Explicit qw(callback);

   package Example;
   sub new { my $class = shift; bless {}, $class }
   sub method { my $self = shift; print "In method\n" }
 my $self = Example->new;
 # This will raise an exception due to the reference to $self
 eval {
   my $code = callback {
 # This will not raise the exception because $self is whitelisted
 my $code = callback {
 } [qw($self)];
 # This will wrap the coderef so we can pass a weakened copy of $self
 my $code = callback {
   my $self = shift;
 } weaken => [qw($self)];


Attempts to provide some very basic protection against unintentional
capturing of lexicals in a closure.

For example, code such as the following risks creating cycles which
mean the top-level object is never freed:

 sub some_method {
   my $self = shift;
   $self->{callback} = sub { $self->other_method }

and this can in turn lead to memory leaks.


The main L</callback> function is not expected to change in future versions,
so as long as you use this:

 use Closure::Explicit qw(callback);

to import the function into your local namespace, or fully-qualify it using

 Closure::Explicit::callback { ... }

then you should have no problems with future versions of this module.

However, it is highly likely that a future version will also start exporting
a differently-named function with a better interface.

=head1 EXPORTS

=head2 callback

Checks the given coderef for potential closure issues, raising an exception if any
are found and returning the coderef (or a wrapped version of it) if everything is

The first parameter is the block of code to run. This is protoyped as C< & > so
you can replace the usual 'sub { ... }' with 'callback { ... }'. If you already
have a coderef, you can pass that using C< &callback($code, ...) >, but please

Remaining parameters are optional - you can either pass a single array, containing
a list of the B<names> of the variables that are safe to capture:

 callback { print "$x\n" } [qw($x)];

or a list of named parameters:

=over 4

=item * weaken => [...] - list of B<variable names> which will be copied, weakened
via L<Scalar::Util/weaken>, then prepended to the parameter list available in @_
in your code block

=item * allowed => [...] - list of B<variable names> to ignore if used in the code,
same behaviour as passing a single arrayref


For example, a method call might look like this:

 my $code = callback {
   my $self = shift;
 } weaken => [qw($self)];

although L<curry::weak> would be a much cleaner alternative there:

 my $code = $self->curry::weak::method;

You can mix C<weaken> and C<allowed>:

 my $x = 1;
 my $code = callback {
 } weaken => [qw($self)], allowed => [qw($x)];

=head2 lint

Runs checks on the given coderef. This is used internally and not exported,
but if you just want to get a list of potential problems for a coderef,
call this:

 my @errors = lint($code, allowed => [qw($x)]);

It's unlikely that the C<weaken> parameter will work when calling this
function directly - this may be fixed in a future version.

=head1 SEE ALSO

=over 4

=item * L<curry> - provides a convenient interface for creating callbacks

=item * L<PadWalker> - does most of the real work behind this module

=item * L<Test::RefCount> - convenient testing for reference counts, makes
cycles easier to detect in test code

=item * L<Devel::Cycle> - reports whether cycles exist and provides useful
diagnostics when any are found



=over 4

=item L<Exporter>

L<as_heavy|Exporter/as_heavy>, L<export|Exporter/export>, L<export_fail|Exporter/export_fail>, L<export_ok_tags|Exporter/export_ok_tags>, L<export_tags|Exporter/export_tags>, L<export_to_level|Exporter/export_to_level>, L<import|Exporter/import>, L<require_version|Exporter/require_version>


=head1 AUTHOR

Tom Molesworth <cpan@entitymodel.com>

=head1 LICENSE

Copyright Tom Molesworth 2012-2013. Licensed under the same terms as Perl itself.