What’s the proper way to create a BUILDARGS method using MooseX::Declare?

Question

I'm having difficulty using MooseX::Declare properly when calling BUILDARGS.

I'm trying to create an object as an interface for a file. (Specifically, I want an interface to a binary file that lets me peek at the next few bytes in the file then chomp them off for further processing.)

I want to be able to create one of these objects like this

my $f = binary_file_buffer->new( $file_name );

and then use it like this

while( my $block_id = $f->peek( $id_offset, $id_length ) ) {
    $block_id = unpack_block_id( $block_id );
    $munge_block{ $block_id }->(
        $f->pop( $block_size[ $block_id ] )
    );
}

My of binary_file_buffer class definition/declaration looks like this

use MooseX::Declare;
class binary_file_buffer {
    use FileHandle;
    use Carp;

    has _file      => ( is => 'ro', isa => 'FileHandle' );
    has _file_name => ( is => 'ro', isa => 'Str' );
    has _buff      => ( is => 'rw', isa => 'Str',  default => '' );

    method BUILDARGS ( Str $file_name ) {
      my $file = FileHandle->new( $file_name );
      carp "unable to open $file_name : $!" unless defined $file;
      $file->binmode;
      return (
        _file_name => $file_name,
        _file      => $file,
      );
    }

    # get the next n bytes from the buffer.
    method pop ( Int $len ) {
        # ... Make sure there is data in _buff
        return substr( $self->{_buff}, 0, $len, '' );
    }

    # Look around inside the buffer without changing the location for pop
    method peek ( Int $offset, Int $len ) {
        # ... Make sure there is data in _buff
        return substr( $self->{_buff}, $offset, $len );
    }
}

(There is buffer loading and managing code that I didn't include here. It is fairly straight forward.)

The problem is, I use the keyword method in the BUILDARGS declaration. So, MooseX::Declare expects a binary_file_buffer object as the first argument to BUILDARGS. But BUILDARGS gets the arguments passed to new, so the first argument is the string a 'binary_file_buffer', the name of the package. As a result it fails the type checking and dies when creating an object using new, like I did in the first code snippet. (At least that's my understanding of what is happening.)

The error message I get is:

Validation failed for 'MooseX::Types::Structured::Tuple[MooseX::Types::Structured::Tuple[Object,Str,Bool],MooseX::Types::Structured::Dict[]]' failed with value [ [ "binary_file_buffer", "drap_iono_t1.log", 0 ], {  } ], Internal Validation Error is: Validation failed for 'MooseX::Types::Structured::Tuple[Object,Str,Bool]' failed with value [ "binary_file_buffer", "drap_iono_t1.log", 0 ] at C:/bin/perl/site/lib/MooseX/Method/Signatures/Meta/Method.pm line 445
 MooseX::Method::Signatures::Meta::Method::validate('MooseX::Method::Signatures::Meta::Method=HASH(0x2a623b4)', 'ARRAY(0x2a62764)') called at C:/bin/perl/site/lib/MooseX/Method/Signatures/Meta/Method.pm line 145
 binary_file_buffer::BUILDARGS('binary_file_buffer', 'drap_iono_t1.log') called at generated method (unknown origin) line 5
 binary_file_buffer::new('binary_file_buffer', 'drap_iono_t1.log') called at logshred.pl line 13

I like the type checking sugar the method keyword supplies for $file_name, but I don't know how to get it since BUILDARGS isn't technically a method.

Does MooseX::Declare have a way to skip the $self creation, or something like that?

Am I doing this the proper MooseX::Declare way? Or am I missing something?

Answer 1

I think you want something like method BUILDARGS (ClassName $class: Str $filename) { ... } in which you explicitly define the invocant as ClassName $class.

Answer 2

I think you want:

#!/use/bin/perl

use strict;
use warnings;

use MooseX::Declare;
class BinaryFile::Buffer {
    use FileHandle;
    use Carp;

    has file      => ( is => 'ro', isa => 'FileHandle');
    has file_name => ( is => 'ro', isa => 'Str');
    has _buff     => (
        is       => 'rw',
        isa      => 'Str',
        default  => '',
        init_arg => undef
    );

    sub BUILDARGS {
        my ($class, $file_name) = @_;
        my $file = FileHandle->new( $file_name ) or do {
            carp "unable to open ", $file_name, " : $!";
            return;
        };
        $file->binmode;
        return $class->SUPER::BUILDARGS(
                file_name => $file_name,
                file      => $file
        );
    }

    # get the next n bytes from the buffer.
    method pop(Int $len) {
        # ... Make sure there is data in _buff
        return substr( $self->buff, 0, $len, '' );
    }

    # Look around inside the buffer without changing the location for pop
    method peek(Int $offset, Int $len) {
        # ... Make sure there is data in _buff
        return substr( $self->buff, $offset, $len );
    }
}

my $f = BinaryFile::Buffer->new($0);

print $f->file_name, "\n";

Answer 3

also a really neat way of doing it (just an expansion of the answer before me):

use MooseX::MultiMethods;

multi method BUILDARGS (ClassName $class: Str $filename) {

#do whatever you want to do if only a strg is passed

}

that way, MooseX::MultiMethods will take care that if you do NOT call FileHandle->new( $file_name ),

but

FileHandle->new(
_filename => $file_name
);

(which is the normal syntax),

it would still work!

Also, you could ( which is not so useful for filenames but in other cases )

add a

multi method ( ClassName $class, Int $some_number ){}

that way, new could now handle hashrefs, integers and strings...

oh the possibilities... ;)