Re: Altering subroutine parameters using Hook::LexWrap

[Zaxo]

By altering $_[-1] in before(), you've instructed the pre-wrap to short-circuit and not call the wrapped sub at all.

Access to wrappers is fairly easy, you just need to keep a lexical copy of the wrap call's returned object. You can kill a particular wrapper by undefining that object or letting it go out of scope.

Changing @_ directly with unshift, shift, push pop or splice in a pre- routine doesn't work. It confuses Hook::LexWrap and doesn't confer aliasing to any added variables. If you want to change an argument, work on individual elements of @_. The only way I've found to change the number of arguments is to design the wrapped sub to take a reference to a data structure and work on that.

Here's an example from my scratchpad of using Hook::LexWrap,

#!/usr/bin/perl
use strict;
use warnings;

use Hook::LexWrap;

{
    my $foo;
    sub foo { @_ ? $foo = shift : $foo; } # _not_ an lvalue!

    my $wrapper = wrap *foo,
        pre => sub {
            warn 0+@_, " @_";
            $_[0] = lc $_[0] if @_ > 1;
            warn 0+@_, " @_";
        },
        post => sub {
            $_[-1] = 
                wantarray ?
                [ map {uc} @{$_[-1]} ] :
                uc $_[-1]
        };
    sub wrapper () :lvalue { $wrapper } # keeps the cloistered
                                        # lexwrap alive

    sub _foo () :lvalue { $foo } # inspection hatch
}

my $str = 'Quux';
my $tmp = $str;
printf "Given $str, wrapped setter reports %s, backdoor shows %s, arg 
+is now %s.\n",
    foo($tmp), _foo, $tmp;    # setter
printf "Wrapped getter reports %s, and backdoor shows %s\n",
    foo(), _foo; # getter

__END__
2 Quux ARRAY(0x804b3f8) at hlw.pl line 13.
2 quux ARRAY(0x804b3f8) at hlw.pl line 15.
Given Quux, wrapped setter reports QUUX, backdoor shows Quux, arg is n
+ow Quux.
1 ARRAY(0x804b50c) at hlw.pl line 13.
1 ARRAY(0x804b50c) at hlw.pl line 15.
Wrapped getter reports QUUX, and backdoor shows Quux

You can keep a clean copy of some_sub() around by wrapping a hard reference to it instead of a name-or-glob. Wrap then returns a reference to a newly constructed sub with wrappers, and the original is unmodified. The pre- wrapper would then be free to call the original if you liked, having short-circuited the wrapping mechanism.

You may note the "not an lvalue!" comment. Hook::LexWrap does not set the original's attributes for the wrapped version.