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Re: Binary data structure to moose class.

by tobyink (Abbot)
on Jan 02, 2013 at 16:07 UTC ( #1011298=note: print w/ replies, xml ) Need Help??


in reply to Binary data structure to moose class.

One of the core ideas behind Moose is that of metaprogramming. That is; don't write programs - write programs which write programs.

For example, rather than defining our attributes the old-fashioned way, like:

sub some_attribute { my $self = shift; $self->{some_attribute} = shift if @_; return $self->{some_attribute}; }

We just write:

has some_attribute => (is => 'rw');

The has function is a "program which writes programs". It makes our sub some_attribute for us!

So the solution is to write something that does the same sort of job as has, but has some domain-specific knowledge about grabbing raw data, unpacking it, etc. Here's an example (untested):

{ package Binary::Humax::HmtData; use DateTime; use Moose; has raw_data_block => ( is => 'rw', isa => 'Str', required => 1, ); # Shortcut function for defining attributes # sub _my_has { my ($name, %spec) = @_; my $meta = __PACKAGE__->meta; # Default attribute to 'rw' $spec{is} //= 'rw'; # Set up lazy builder if (my $unpack = delete $spec{unpack}) { $spec{lazy} //= 1; $spec{builder} //= "_build_$name"; if (my $postprocess = delete $spec{postprocess}) { $meta->add_method($spec{builder}, sub { my $self = shift; local $_ = unpack($unpack, $self->raw_data_block); $postprocess->(); }); } else { $meta->add_method($spec{builder}, sub { my $self = shift; return unpack($unpack, $self->raw_data_block); }); } } $meta->add_attribute($name, \%spec); } # Now use that shortcut to define each attribute. # _my_has last_play => (isa => 'Int', unpack => '@5 S'); _my_has chan_num => (isa => 'Int', unpack => '@17 S'); _my_has start_time => ( isa => 'DateTime', unpack => '@5 S', postprocess => sub { DateTime->from_epoch(epoch => $_, time_zone => 'GMT'); }, ); _my_has file_name => (isa => 'Str', unpack => '@33 A512'); # Create an alternative constructor which wraps "new". # sub new_from_file { my ($class, $filename) = @_; open my $fh, '>', $filename; my $slurp = do { local $/ = <$fh> }; return $class->new(r); } } # # USAGE # my $hmt_data = Binary::Humax::HmtData->new_from_file($path_name); my $field = $hmt_data->start_time;

"Secondly, when complete, I plan to upload the module to CPAN. Have I chosen a good name for it, or should it live in a different name space?"

No, it seems like a bad name. You're putting it in "Binary" because it's a binary file format. But presumably end users of your module won't care whether it's a binary file format, a text-based one, or XML-based; they don't care about the file format at all, because they've downloaded your module to abstract those sort of details away, haven't they?

I would have thought something in the "TV" namespace more fitting.

UPDATE:; we can go even "more meta" by replacing our has workalike with an attribute trait. This has the advantage of allowing introspection of each attribute to read back its "unpack" code.

{ package Binary::Humax::HmtData::Trait::Attribute; use Moose::Role; has unpack => (is => 'ro', isa => 'Str'); has postprocess => (is => 'ro', isa => 'CodeRef'); before _process_options => sub { my ($meta, $name, $spec) = @_; if ($spec->{unpack}) { $spec->{lazy} //= 1; $spec->{builder} //= "_build_$name"; $spec->{is} //= 'rw'; } }; after attach_to_class => sub { my $attr = shift; my $class = $attr->associated_class; my $unpack = $attr->unpack or return; if (my $postprocess = $attr->postprocess) { $class->add_method($attr->builder, sub { my $self = shift; local $_ = unpack($unpack, $self->raw_data_block); $postprocess->(); }); } else { $class->add_method($attr->builder, sub { my $self = shift; return unpack($unpack, $self->raw_data_block); }); } }; } { package Binary::Humax::HmtData; use DateTime; use Moose; use constant MAGIC => 'Binary::Humax::HmtData::Trait::Attribute'; has raw_data_block => ( is => 'rw', isa => 'Str', required => 1, ); has last_play => ( traits => [ MAGIC ], isa => 'Int', unpack => '@5 S', ); has chan_num => ( traits => [ MAGIC ], isa => 'Int', unpack => '@17 S', ); has start_time => ( traits => [ MAGIC ], isa => 'DateTime', unpack => '@5 S', postprocess => sub { DateTime->from_epoch(epoch => $_, time_zone => 'GMT'); }, ); has file_name => ( traits => [ MAGIC ], isa => 'Str', unpack => '@33 A512', ); } # Attribute introspection print Binary::Humax::HmtData->meta->get_attribute('start_time')->unpac +k, "\n";

The slight ugliness with this method is that the attribute trait has some knowledge of the class it's being applied to - it knows that the class has a raw_data_block attribute. With a little more work that problem could be eliminated.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'


Comment on Re: Binary data structure to moose class.
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Re^2: Binary data structure to moose class.
by tobyink (Abbot) on Jan 02, 2013 at 21:05 UTC

    OK, here's a massively generalized version of the above:

    use 5.010; use strict; use warnings; BEGIN { package Trait::Attribute::Derived; no thanks; use MooseX::Role::Parameterized; use List::MoreUtils 'any'; use namespace::autoclean; use base do { package Trait::Attribute::Derived::__CLASS_METHODS__; use Sub::Install 'install_sub'; use namespace::autoclean; my @saved; sub make_trait { my ($pkg, %args) = @_; push @saved, $pkg->meta->generate_role(parameters => \%arg +s); return $saved[-1]->name; } sub import { my $pkg = shift; my $caller = caller; while (@_) { my $name = shift; my $trait = $pkg->make_trait(%{+shift}); install_sub { into => $caller, as => $name, code => sub () { $trait }, } } } __PACKAGE__; }; parameter processor => (is => 'ro', required => 1, isa => 'Code +Ref'); parameter fields => (is => 'ro', required => 1, isa => 'Hash +Ref'); parameter is => (is => 'ro', default => 'ro', isa => 'Str' +); parameter source => (is => 'ro', required => 1, isa => 'Str' +); role { my $p = shift; my @fields = keys %{ $p->fields }; has postprocessor => (is => 'ro', isa => 'CodeRef'); for my $attr (@fields) { has $attr => (is => 'ro', isa => $p->fields->{$attr}); } before _process_options => sub { my ($meta, $name, $spec) = @_; $spec->{is} //= $p->is; $spec->{lazy} //= 1; $spec->{builder} //= "_build_$name"; }; after attach_to_class => sub { my $attr = shift; my $class = $attr->associated_class; return if $class->has_method($attr->builder); my $source = $p->source; my $processor = $p->processor; my $postprocess = $attr->postprocessor; my %data = map { ; $_ => $attr->$_ } @fields; $class->add_method($attr->builder, sub { my $self = shift; local %_ = %data; local $_ = $self->$source; $_ = $self->$processor($_, +{%data}); return $_ unless $postprocess; return $self->$postprocess($_, +{%data}); }); }; }; }; { package Person; use Moose; use Trait::Attribute::Derived Split => { source => 'full_name', fields => { segment => 'Num' }, processor => sub { (split)[$_{segment}] }, }; has full_name => (is => 'ro', isa => 'Str'); has first_name => (traits => [Split], segment => +0); has initial => (traits => [Split], segment => +0, postprocessor + => sub { substr $_, 0, 1 }); has last_name => (traits => [Split], segment => -1); } my $bob = Person->new(full_name => 'Robert Redford'); say $bob->first_name; say $bob->initial; say $bob->last_name;

    If you can understand that and how to apply it to your problem, then you're a true metahacker! :-)

    Update: this is now on CPAN as Trait::Attribute::Derived.

    perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'
Re^2: Binary data structure to moose class.
by chrestomanci (Priest) on Jan 04, 2013 at 18:14 UTC

    Thank you tobyink, that looks like the solution I was looking for, especialy the second code example you posted. though it will take a bit of experementaion to understand how it works.

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