Moo the universe and everything

Moo, the universe and everything…
Cincinnati.pm
Henry Van Styn
<vanstyn@cpan.org>
Wednesday, 22 October, 2014

Agenda
• Overview of Moo
• Differences between Moo and Moose
• Moosey concepts
• Within the larger context of Perl OO
• Some useful patterns (and anti-patterns)

Moo is…
• Light version of Moose
• Object system for Perl written in Perl
• Sugar
• Result of lessons learned from Moose, Mouse
MooseX, etc
• Current state-of-the-art*

Differences from Moose
• Smaller set of features (~2/3 of Moose)
• “Less is more”
• Faster startup, more straightforward APIs
• Types - simple CodeRef interface*
• also use CodeRefs for default, coerce, etc…*

Moosey concepts
• Declarative OO
• MOP
• Provides additional, higher-level object concepts:
attributes, roles, method modifiers
• Perl is still perl: classes (packages), methods,
inheritance, blessed references

Peeling back the onion -- Perl OO…
• “Bag of functions”
• classes == packages + inheritance
• blessed refs: attach data to a class == objects
• runtime introspection and modification…

class-based OO
• Push package names on @ISA (inheritance)
• Early sugar: use base, use parent
Some::Class::method();
# ^^ calls method() in Some::Class
Some::Class->method();
# - or the next parent of Some::Class *
# - with ‘Some::Class’ passed as the first argument

object-based OO
my $obj = bless($ref, ’Some::Class’);
$obj->method();
# - or the next parent of Some::Class *
# - with $obj passed as the first argument
# - $obj is like a merger of $ref and ‘Some::Class’

Multiple Inheritance
• Push multiple package names on @ISA
• Which is the “next parent” ?
• Flattened (linearized) according to ‘MRO’
use mro ‘c3’;
# ^^ Don’t be stupid
# next::method() is now a thing
# MRO::Compat before Perl 5.10
# perldoc mro

constructors
• Perl doesn’t have or need the concept
• Do it however you want
sub new {
my $class = shift;
bless({},$class);
}

object attributes, etc
• Perl doesn’t have or need the concept
• Do it however you want
sub thing {
my ($self,$new_val) = @_;
$self->{thing} = $new_val if ($new_val);
$self->{thing}
}

What Moo does
• Sugar to write classes following a specific pattern
• Designed to be instantiated into objects (via new)
• Subclass using ‘extends’
• Declare attributes with ‘has’

How it does it
• Uses introspection to modify the package
• ‘extends’ modifies @ISA like base and parent
• ‘has’ adds/changes methods

Roles (‘with’)
• Also uses introspection to add/modify methods
• Accomplish MI like designs w/o actually using Perl’s
“MI”
• No need to worry about MRO
• You can still do MI if you really want to:
pass multiple class names to ‘extends’
• But don’t - just use Roles

Method-Modifiers
• Access MI concepts but also w/o worry about MRO
around ‘foo’ => sub {
my ($orig,$self,@args) = @_;
$self->$orig(@args)
};
sub foo {
my $self = shift;
$self->next::method(@_)
}

Useful patterns
• Chaining (like DBIC ResultSets)
• Methods written to work on class or object
• Using class names as params
• Type::Tiny for types
• Coercions

When to use Moo
• When writing object classes
• When extending anything
• Extend Moose classes

Anti-Patterns
• Getting too carried away
• Doing things for dogma rather than benefit
• Forgetting you are still writing Perl (and that it is
wonderful)
• There really is more than one way to do it

Code Examples
https://github.com/vanstyn/LJ-moose-article-examples
https://github.com/vanstyn/DBIx-Class-Schema-Diff
https://github.com/vanstyn/Hash-Layout

Moo the universe and everything

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