http://qs1969.pair.com?node_id=251127

diotalevi has asked for the wisdom of the Perl Monks concerning the following question:

I'm looking to extend japhy's current YAPE::Regex and need to alter a lexical named %pat. I thought of several different ways of doing this but I wanted to present each and get your input and opinions on the merits of each.

The following code examples show a method that works, others that don't. All advice is welcome.

#1 Alter YAPE::Regex so it is extension friendly
This would store %pat in the YAPE::Regex object which directly exposes it to modification. This is the best solution but it requires japhy's cooperation if I want to avoid forking the source. (and I'd need to modify a method but that's unrelated to this post).

#2 Use PadWalker
This doesn't work because PadWalker doesn't actually dig out lexicals taken in closures and only really exposes lexicals created in the subroutine. Darn!

use YAPE::Regex; my $p = YAPE::Regex->new(qr//); $p->YAPE::Regex::Extensible::steal_pat; package YAPE::Regex::Extensible; use base 'YAPE::Regex'; use PadWalker 'peek_sub'; use Data::Dumper; sub steal_pat { my $self = shift; $self->{'pat'} = peek_sub(\&YAPE::Regex::next)->{'%pat'}; }

#3 Grab it via Devel::LexAlias
Oops! This doesn't work either. In fact while PadWalker at least reported the existance of %pat, Devel::LexAlias doesn't even do that.

use YAPE::Regex; my $p = YAPE::Regex->new(qr//); $p->YAPE::Regex::Extensible::steal_pat; package YAPE::Regex::Extensible; use base 'YAPE::Regex'; use Devel::LexAlias 'lexalias'; sub steal_pat { my $self = shift; $self->{'pat'} = {}; lexalias(\&YAPE::Regex::next, '%pat', $self->{'pat'}); }

#4 Be really evil and muck around with B and Devel::Pointer
Now this works but its exceedingly ugly and I can't help but think that there has got to be a better way than manually trolling around a pad for pointers.

use YAPE::Regex; my $p = YAPE::Regex->new(qr//); $p->YAPE::Regex::Extensible::steal_pat; package YAPE::Regex::Extensible; use base 'YAPE::Regex'; use B qw(svref_2object); use Devel::Pointer; sub steal_pat { my $self = shift; $self->{'pat'} = {}; my $sub = \&YAPE::Regex::next; my $osub = svref_2object( $sub ); # Unpack next()'s pad list into an array of names and values. my ($names, $values) = map [ $_->ARRAY ], $osub->PADLIST->ARRAY; # Fetch the first HV associated with the name '%pat' my $h; for my $i (0 .. $#$names) { if ( $names->[$i]->can('PV') and $names->[$i]->PV eq '%pat') { $h = $values->[$i]; last; } } # $$h is the address of the hash # Use Devel::Pointer to construct a reference to it. $self->{'pat'} = unsmash_hv( $$h ); }