Hook::LexWrap doesn't capture return value of subroutine in post() wrapper

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

#!/usr/bin/perl -w
use strict;
use Hook::LexWrap;

sub get { return "Foo!" }
wrap get, post => sub { warn $_[-1] };
get();
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This code should print 'Foo! at wrap.pl line 7.', but instead, outputs:

Use of uninitialized value in warn at wrap.pl line 6.
Warning: something's wrong at wrap.pl line 6.
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Anyone know why?

---
"A Jedi uses the Force for knowledge and defense, never for attack."

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Re: Hook::LexWrap doesn't capture return value of subroutine in post() wrapper by Roger (Parson) on Dec 23, 2003 at 04:10 UTC
Closer examination revealed that the value of @_ varies depending on how the function is called. `#!/usr/bin/perl -w use strict; use Hook::LexWrap; use Data::Dumper; sub get { return "Foo!" } wrap get, post => sub { print Dumper(\@_) }; my @n = get(); my $n = get(); get();` [download] And the output - `$VAR1 = [ [ 'Foo!' ] ]; $VAR1 = [ 'Foo!' ]; $VAR1 = [ bless( sub { "DUMMY" }, 'Hook::LexWrap::Cleanup' ) ];` [download] Looking at the documentation, the author states that: `In a post-wrapper, $_[-1] contains the return value produced by the wrapped subroutine. In a scalar return context, this value is the scalar return value. In an list return context, this value is a reference to the array of return values. $_[-1] may be assigned to in a post-wrapper, and this changes the return value accordingly.` [download] What the author didn't mention is that in a void context, `$_[-1]` is the object. Thus giving you the error in 'warn'.	[reply] [d/l] [select]

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Re: Hook::LexWrap doesn't capture return value of subroutine in post() wrapper
by Roger (Parson) on Dec 23, 2003 at 04:10 UTC

#!/usr/bin/perl -w
use strict;
use Hook::LexWrap;
use Data::Dumper;

sub get { return "Foo!" }
wrap get, post => sub { print Dumper(\@_) };
my @n = get();
my $n = get();
get();
[download]

$VAR1 = [
          [
            'Foo!'
          ]
        ];
$VAR1 = [
          'Foo!'
        ];
$VAR1 = [
          bless( sub { "DUMMY" }, 'Hook::LexWrap::Cleanup' )
        ];
[download]

In a post-wrapper, $_[-1] contains the return value produced by the wrapped subroutine. In a scalar return context, this value is the scalar return value. In an list return context, this value is a reference to the array of return values. $_[-1] may be assigned to in a post-wrapper, and this changes the return value accordingly.
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$_[-1]

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