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Intro to MOP (presented at YAPC::NA)
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Upasana
June 22, 2016
Programming
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Intro to MOP (presented at YAPC::NA)
Upasana
June 22, 2016
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Transcript
Meta Object Protocol (MOP) Upasana
[email protected]
About me • Software developer at booking.com
We are hiring https://workingatbooking.com/
Backstory • GNOME Outreach Program for Women internship in 2013
• Structured exceptions in Moose • Want to share whatever I learnt during my internship
Topics 1) Little bit about object oriented programming (OOP) 2)
Little bit about OOP in perl (the old style) 3) Meta object protocol (MOP) 4) History of MOP 5) Applications of MOP
Topics 6) Implementing MOP in Perl (the easy way) 7)
Why #6 might not be a good idea 8) Metaclass incompatibility 9) Mixins 10) MOP in Moose 11) Drawbacks of MOP 12) Where to go next?
How a class looks like? • Class name • Superclasses
How a class looks like? • Attributes – is read
only or read-write – type (int, float etc.) – default value if any – getter method (accessor) – setter method (mutator)
How a class looks like? • Methods – method name
– body
Classes in Perl • Perl doesn't provide any special syntax
for classes • Perl packages are classes
Attributes in Perl classes • No special syntax or support
for declaring and manipulating attributes • Attributes are stored in the object itself • As a hash of key-value pairs
Object? • A hash reference • blessed into a class
OOP in Perl package Rectangle; sub new { my $self
= shift; my $attributes = { @_ }; bless $attributes, $self; } Rectangle->new( height => 10, width => 20, );
OOP in Perl package Rectangle; sub new { my $self
= shift; my $attributes = { @_ }; bless $attributes, $self; } Rectangle->new( height => 10, width => 20, );
OOP in Perl package Rectangle; sub new { my $self
= shift; my $attributes = { @_ }; bless $attributes, $self; } Rectangle->new( height => 10, width => 20, );
OOP in Perl package Rectangle; sub new { my $self
= shift; my $attributes = { @_ }; bless $attributes, $self; } Rectangle->new( height => 10, width => 20, );
OOP in Perl package Rectangle; sub new { my $self
= shift; my $attributes = { @_ }; bless $attributes, $self; } Rectangle->new( height => 10, width => 20, );
OOP in Perl package Rectangle; sub new { my $self
= shift; my $attributes = { @_ }; bless $attributes, $self; } Rectangle->new( height => 10, width => 20, );
OOP in Perl package Rectangle; sub new { my $self
= shift; my $attributes = { @_ }; bless $attributes, $self; } Rectangle->new( height => 10, width => 20, );
What is MOP? • provides the vocabulary to access and
manipulate the structure and behavior of objects.
Functions of MOP • Creating new classes • Deleting existing
classes • Changing the class structure • Changing methods of the class • At runtime
History of MOP • First introduced in the Smalltalk •
Common LISP Object System (CLOS) was influenced by Smalltalk • CLOS allowed multiple inheritance unlike Smalltalk
MOP in modern languages • Javascript has Joose • OpenC++
• Java has Reflection API • Perl has Moose
Why do we need a MOP?
Testing
Testing • I work at booking.com • Our website is
moving very fast • Many rollouts in a day
Testing • We don't test everything • At one point,
rollouts became hard • Some things need to be tested manually
Testing package Web::Handler { has 'search' => ( url =>
'/search', #... ); has 'hotel' => ( url => '/hotel', #... ); # ... }
Testing package Web::Handler { has 'search' => ( url =>
'/search', #... ); has 'hotel' => ( url => '/hotel', #... ); # ... }
Testing package Web::Handler { has 'search' => ( url =>
'/search', #... ); has 'hotel' => ( url => '/hotel', #... ); # ... }
Testing • Give me all the attributes of Web::Handler. •
Run tests for all the attributes.
Testing # This is pseudocode, don't expect this # to
compile my $attr = Web::Handler->meta->get_attributes_list; foreach my $a ( @$attr ) { next unless $a->attribute_exists('url'); my $url = $a->get_attribute('url'); die “test fails...\n” if( !LWP::Simple::get($url) ); }
Testing # This is pseudocode, don't expect this # to
compile my $attr = Web::Handler->meta->get_attributes_list; foreach my $a ( @$attr ) { next unless $a->attribute_exists('url'); my $url = $a->get_attribute('url'); die “test fails...\n” if( !LWP::Simple::get($url) ); }
Testing # don't expect this to compile my $attr =
Web::Handler->meta->get_attributes_list; foreach my $a ( @$attr ) { next unless $a->attribute_exists('url'); my $url = $a->get_attribute('url'); die “test fails...\n” if( !LWP::Simple::get($url) ); }
Testing # don't expect this to compile my $attr =
Web::Handler->meta->get_attributes_list; foreach my $a ( @$attr ) { next unless $a->attribute_exists('url'); my $url = $a->get_attribute('url'); die “test fails...\n” if( !LWP::Simple::get($url) ); }
Testing # don't expect this to compile my $attr =
Web::Handler->meta->get_attributes_list; foreach my $a ( @$attr ) { next unless $a->attribute_exists('url'); my $url = $a->get_attribute('url'); die “test fails...\n” if( !LWP::Simple::get($url) ); }
Testing # don't expect this to compile my $attr =
Web::Handler->meta->get_attributes_list; foreach my $a ( @$attr ) { next unless $a->attribute_exists('url'); my $url = $a->get_attribute('url'); die “test fails...\n” if( !LWP::Simple::get($url) ); }
Object Relational Mapping (ORM)
ORM my $create_table_statement =<<END; CREATE TABLE Hotel ( id INT
PRIMARY KEY, name VARCHAR(255), address VARCHAR(255) ); END
ORM my $sql_parser = SQL::Parser->new( $create_table_statement ); my $class_name =
$sql_parser->table_name; my $c = Moose::Meta::Class->create( $class_name );
ORM my $sql_parser = SQL::Parser->new( $create_table_statement ); my $class_name =
$sql_parser->table_name; my $c = Moose::Meta::Class->create( $class_name );
ORM my $sql_parser = SQL::Parser->new( $create_table_statement ); my $class_name =
$sql_parser->table_name; my $c = Moose::Meta::Class->create( $class_name );
ORM my $sql_parser = SQL::Parser->new( $create_table_statement ); my $class_name =
$sql_parser->table_name; my $c = Moose::Meta::Class->create( $class_name );
ORM foreach my $f ( $sql_parser->fields ) { my $tc
= find_type_constraint( $f->type ); $c->add_attribute( Moose::Meta::Attribute->new( $f->name, isa => $tc, reader => 'get_' . $f->name, writer => 'set_' . $f->name, ); ); }
ORM foreach my $f ( $sql_parser->fields ) { my $tc
= find_type_constraint( $f->type ); $c->add_attribute( Moose::Meta::Attribute->new( $f->name, isa => $tc, reader => 'get_' . $f->name, writer => 'set_' . $f->name, ); ); }
ORM foreach my $f ( $sql_parser->fields ) { my $tc
= find_type_constraint( $f->type ); $c->add_attribute( Moose::Meta::Attribute->new( $f->name, isa => $tc, reader => 'get_' . $f->name, writer => 'set_' . $f->name, ); ); }
ORM foreach my $f ( $sql_parser->fields ) { my $tc
= find_type_constraint( $f->type ); $c->add_attribute( Moose::Meta::Attribute->new( $f->name, isa => $tc, reader => 'get_' . $f->name, writer => 'set_' . $f->name, ); ); }
ORM foreach my $f ( $sql_parser->fields ) { my $tc
= find_type_constraint( $f->type ); $c->add_attribute( Moose::Meta::Attribute->new( $f->name, isa => $tc, reader => 'get_' . $f->name, writer => 'set_' . $f->name, ); ); }
ORM foreach my $f ( $sql_parser->fields ) { my $tc
= find_type_constraint( $f->type ); $c->add_attribute( Moose::Meta::Attribute->new( $f->name, isa => $tc, reader => 'get_' . $f->name, writer => 'set_' . $f->name, ); ); }
ORM foreach my $f ( $sql_parser->fields ) { my $tc
= find_type_constraint( $f->type ); $c->add_attribute( Moose::Meta::Attribute->new( $f->name, isa => $tc, reader => 'get_' . $f->name, writer => 'set_' . $f->name, ); ); }
ORM package SomeDB::Class::Thing; ..... sub retrieve { .... } sub
search_where {....} 1; $c->set_superclass( 'SomeDB::Class::Thing' );
ORM # return me the hotel with id 123 my
$h = Hotel->retrieve( 123 ); my $hotel_name = $h->name; $h->set_name( 'asdfasdf' );
ORM # return me the hotel with id 123 my
$h = Hotel->retrieve( 123 ); # return me the hotel id 123 my $hotel_name = $h->name; $h->set_name( 'asdfasdf' );
ORM # return me the hotel with id 123 my
$h = Hotel->retrieve( 123 ); # return me the hotel id 123 my $hotel_name = $h->name; $h->set_name( 'asdfasdf' );
Implementing MOP in Perl
Creating a class at runtime • Perl class is a
package • Every package has a symbol table
Symbol table • Hash of subroutines/variables defined in a package
• package name with two colons appended $Rectangle::
Symbol table • Hash of subroutines/variables defined in a package
• package name with two colons appended $Rectangle::
package Metaclass; sub create_class { my ($self, %options) = @_;
my $class = $options{ package }; my $methods = $options{ methods }; while( my ($method, $body) = each( %$methods ) ) { no strict 'refs'; *{ "${class}::$method" } = $body; } # end while loop } 1;
package Metaclass; sub create_class { my ($self, %options) = @_;
my $class = $options{ package }; my $methods = $options{ methods }; while( my ($method, $body) = each( %$methods ) ) { no strict 'refs'; *{ "${class}::$method" } = $body; } # end while loop } 1;
package Metaclass; sub create_class { my ($self, %options) = @_;
my $class = $options{ package }; my $methods = $options{ methods }; while( my ($method, $body) = each( %$methods ) ) { no strict 'refs'; *{ "${class}::$method" } = $body; } # end while loop } 1;
package Metaclass; sub create_class { my ($self, %options) = @_;
my $class = $options{ package }; my $methods = $options{ methods }; while( my ($method, $body) = each( %$methods ) ) { no strict 'refs'; *{ "${class}::$method" } = $body; } # end while loop } 1;
package Metaclass; sub create_class { my ($self, %options) = @_;
my $class = $options{ package }; my $methods = $options{ methods }; while( my ($method, $body) = each( %$methods ) ) { no strict 'refs'; *{ "${class}::$method" } = $body; } # end while loop } 1;
package Metaclass; sub create_class { my ($self, %options) = @_;
my $class = $options{ package }; my $methods = $options{ methods }; while( my ($method, $body) = each( %$methods ) ) { no strict 'refs'; *{ "${class}::$method" } = $body; } # end while loop } 1;
package Metaclass; sub create_class { my ($self, %options) = @_;
my $class = $options{ package }; my $methods = $options{ methods }; while( my ($method, $body) = each( %$methods ) ) { no strict 'refs'; *{ "${class}::$method" } = $body; } # end while loop } 1;
package Metaclass; sub create_class { my ($self, %options) = @_;
my $class = $options{ package }; my $methods = $options{ methods }; while( my ($method, $body) = each( %$methods ) ) { no strict 'refs'; *{ "${class}::$method" } = $body; } # end while loop } 1;
Metaclass->create_class( package => 'Rectangle', methods => { new => sub
{ my ($self) = shift; my $attributes = { @_ }; return bless $attributes, $self; }, } ); Rectangle->new( height => 10, Width => 20 );
Metaclass->create_class( package => 'Rectangle', methods => { new => sub
{ my ($self) = shift; my $attributes = { @_ }; return bless $attributes, $self; }, } ); Rectangle->new( height => 10, Width => 20 );
Metaclass->create_class( package => 'Rectangle', methods => { new => sub
{ my ($self) = shift; my $attributes = { @_ }; return bless $attributes, $self; }, } ); Rectangle->new( height => 10, Width => 20 );
Metaclass->create_class( package => 'Rectangle', methods => { new => sub
{ my ($self) = shift; my $attributes = { @_ }; return bless $attributes, $self; }, } ); Rectangle->new( height => 10, Width => 20 );
Metaclass->create_class( package => 'Rectangle', methods => { new => sub
{ my ($self) = shift; my $attributes = { @_ }; return bless $attributes, $self; }, } ); Rectangle->new( height => 10, Width => 20 );
sub create_class { my ($self, %options) = @_; my $class
= $options{ package }; $options{ methods }->{ meta } = \&get_meta; my $methods = $options{ methods }; while( my ($method, $body) = each( %$methods ) ) { no strict 'refs'; *{ "${class}::$method" } = $body; } # end while loop }
my %meta_to_class; sub get_meta { my $class = shift; Metaclass->get_metaclass(
$class ); }; sub get_metaclass { my $class = shift; return bless $meta_to_class{ $_[ 0 ] }, $class; }
my %meta_to_class; sub get_meta { my $class = shift; Metaclass->get_metaclass(
$class ); }; sub get_metaclass { my $class = shift; return bless $meta_to_class{ $_[ 0 ] }, $class; }
my %meta_to_class; sub get_meta { my $class = shift; Metaclass->get_metaclass(
$class ); }; sub get_metaclass { my $class = shift; return bless $meta_to_class{ $_[ 0 ] }, $class; }
my %meta_to_class; sub get_meta { my $class = shift; Metaclass->get_metaclass(
$class ); }; sub get_metaclass { my $class = shift; return bless $meta_to_class{ $_[ 0 ] }, $class; }
my %meta_to_class; sub get_meta { my $class = shift; Metaclass->get_metaclass(
$class ); }; sub get_metaclass { my $class = shift; return bless $meta_to_class{ $_[ 0 ] }, $class; }
my %meta_to_class; sub get_meta { my $class = shift; Metaclass->get_metaclass(
$class ); }; sub get_metaclass { my $class = shift; return bless $meta_to_class{ $_[ 0 ] }, $class; }
my %meta_to_class; sub get_meta { my $class = shift; Metaclass->get_metaclass(
$class ); }; sub get_metaclass { my $class = shift; return bless $meta_to_class{ $_[ 0 ] }, $class; }
sub create_class { my ($self, %options) = @_; my $class
= $options{ package }; $options{ methods }->{ meta } = \&get_meta; my $methods = $options{ methods }; no strict 'refs'; while( my ($method, $body) = each( %$methods ) ) { *{ "${class}::$method" } = $body; } # end while loop use strict; set_metaclass( $class, \%options ); }
my %meta_to_class; sub get_meta { my $class = shift; Metaclass->get_metaclass(
$class ); }; sub get_metaclass { my $class = shift; return bless $meta_to_class{ $_[ 0 ] }, $class; } sub set_metaclass { $meta_to_class{ $_[ 0 ] } = $_[ 1 ]; }
my %meta_to_class; sub get_meta { my $class = shift; Metaclass->get_metaclass(
$class ); }; sub get_metaclass { my $class = shift; return bless $meta_to_class{ $_[ 0 ] }, $class; } sub set_metaclass { $meta_to_class{ $_[ 0 ] } = $_[ 1 ]; }
Introspection Metaclass->create_class( package => 'Rectangle', methods => { new =>
sub { my ($self) = shift; my $attributes = { @_ }; return bless $attributes, $self; }, }, ); print Dumper( Rectangle->meta );
bless({ 'package' => 'Rectangle', 'methods' => { 'meta' => sub
{ "DUMMY" }, 'new' => sub { "DUMMY" } } }, 'Metaclass' );
bless({ 'package' => 'Rectangle', 'methods' => { 'meta' => sub
{ "DUMMY" }, 'new' => sub { "DUMMY" } } }, 'Metaclass' );
Inheritance • Every package's symbol table has an array named
ISA • @PackageName::ISA
Inheritance if( $options{ superclasses } && @{$options{ superclasses }} )
{ @{"${class}::ISA"} = @{$options{ superclasses }} }
Inheritance if( $options{ superclasses } && @{$options{ superclasses }} )
{ @{"${class}::ISA"} = @{$options{ superclasses }}; }
Metaclass->create_class( package => 'ColoredRectangle', superclasses => [ 'Rectangle' ], );
Metaclass->create_class( package => 'ColoredRectangle', superclasses => [ 'Rectangle' ], );
And it works, I can do ColoredRectangle->new();
But please don't try aforementioned things
It's incomplete & may be fragile
But why?
“Manipulating stashes (Perl's symbol tables) is occasionally necessary, but incredibly
messy, and easy to get wrong. This module hides all of that behind a simple API.” `man Package::Stash`
But why? • use Package::Stash; • use Symbol::Table;
But why? • Metaclass.pm is very basic • But actually
Metaclasses are not so simple • Look at Moose
Moose • Metaclasses for attributes • Metaclasses for methods
Inheritance & metaclass compatibility • A has a method i-foo
– Calls c-bar of MetaA • B inherits from A – B has i-foo • MetaB may not have c-bar
Inheritance & metaclass compatibility package MetaA; .... sub c_bar {
print "in c_bar\n"; } 1;
Inheritance & metaclass compatibility MetaA->create_class( package => 'A', methods =>
{ new => sub { my ($self) = shift; my $attributes = { @_ }; return bless $attributes, $self; }, i_foo => sub { my ($self) = shift; my $meta = $self->meta; $meta->c_bar; }, }, ); A->i_foo();
Inheritance & metaclass incompatibility MetaB->create_class( package => 'B', methods =>
{ new => sub { my ($self) = shift; my $attributes = { @_ }; return bless $attributes, $self; }, }, superclasses => [ 'A' ], ); B->i_foo;
Inheritance & metaclass incompatibility Can't locate object method "c_bar" via
package "MetaB" at test.pl line 24.
Inheritance & metaclass incompatibility i_foo => sub { my ($self)
= shift; my $meta = $self->meta; $meta->c_bar; },
Inheritance & metaclass incompatibility package MetaB; .... # NO c_bar
1;
Inheritance & metaclass incompatibility • MetaA has a method c-foo
• c-foo needs to call i- bar in A • MetaB inherits from MetaA • B has to has i-bar
Inheritance & metaclass incompatibility package MetaA; ..... sub c_foo {
my ( $self, $child ) = @_; $child->i_bar; } 1;
Inheritance & metaclass incompatibility package MetaB; use strict; use warnings;
use parent 'MetaA'; 1;
Inheritance & metaclass incompatibility MetaA->create_class( package => 'A', methods =>
{ new => sub { my ($self) = shift; my $attributes = { @_ }; return bless $attributes, $self; }, i_bar => sub { print "in i_bar\n"; }, }, ); MetaA->c_foo( 'A' );
Inheritance & metaclass incompatibility MetaB->create_class( package => 'B', methods =>
{ new => sub { my ($self) = shift; my $attributes = { @_ }; return bless $attributes, $self; }, # NO i_bar }, ); MetaB->c_foo( 'B' );
Inheritance & metaclass incompatibility Can't locate object method "i_bar" via
package "B" at MetaA.pm line 16.
Inheritance & metaclass compatibility sub c_foo { my ( $self,
$child ) = @_; $child->i_bar; }
Metaclass Incompatibility • Various ways of dealing with this
Metaclass compatibility (Moose) • Does parent & child metaclasses have
any common ancestors? – If yes, then \o/ – else, die • Moose::Exception::CannotFixMetaclassComp atibility
Mixins • A class that contains a combination of methods
from other classes • 'Included' rather than 'inherited' • Moose roles are similar to mixins
Rules of mixins-based inheritance • Order of the mixins matter
• Mixins take precedence over non-mixins
Mixins-based inheritance
Mixins-based inheritance • B => {M1.M2.A}
Rules of mixins-based inheritance • Methods in M2 will take
precedence over A • Methods in M1 will take precedence over M2
Mixins-based inheritance • C => { M3.B.M1.M2.A }
Rules of mixins-based inheritance • Methods in B will take
precedence over M1 • Methods in M3 will take precedence over B
Moose provides a great MOP
Creating a class Moose::Meta::Class->create( 'Rectangle', attributes => { 'height' =>
{ is => 'ro', isa => 'Int', }, ... }, );
Introspection • For getting attributes: Rectangle->meta->get_attributes_list(); • For getting methods:
Rectangle->meta->get_methods_list(); • For getting superclasses: Rectangle->meta->superclasses;
Introspection • For getting attributes: Rectangle->meta->get_attributes_list(); • For getting methods:
Rectangle->meta->get_methods_list(); • For getting superclasses: Rectangle->meta->superclasses;
Introspection • For getting attributes: Rectangle->meta->get_attributes_list(); • For getting methods:
Rectangle->meta->get_methods_list(); • For getting superclasses: Rectangle->meta->superclasses;
Introspection • For getting attributes: Rectangle->meta->get_attributes_list(); • For getting methods:
Rectangle->meta->get_methods_list(); • For getting superclasses: Rectangle->meta->superclasses;
Changing Class definition • For adding a new attribute: Rectangle->meta->add_attribute(...);
• For adding a new method: Rectangle->meta->add_method(...);
Changing Class definition • For adding a new attribute: Rectangle->meta->add_attribute(...);
• For adding a new method: Rectangle->meta->add_method(...);
Changing Class definition • For adding a new attribute: Rectangle->meta->add_attribute(...);
• For adding a new method: Rectangle->meta->add_method(...);
Drawbacks of MOP • Makes things slow • While using
Moose, don't forget to do: __PACKAGE__->meta->make_immutable; – It tells Moose that you are not going to change your class at runtime
Bibliography • The Art of the Metaobject Protocol • Metaclass
Composition Using Mixin-Based Inheritance by Noury Bouraqadi • Wikipedia • Moose documentation • And lots of other random resources on the internet • Stevan Little's awesome brain :)
Thank you for your time
Questions?
• Slides: https://speakerdeck.com/upasana20/intro-to-mop-presented-a t-yapc-na • Code examples: https://github.com/Sweet-kid/Intro-to-MOP-YAPC