The Ruby Module Builder Pattern

Ruby The Module Builder Pattern The Module Builder Pattern Chris
Salzberg ^

About Me • Writer, programmer, translator • @shioyama on github,
twitter, SO • I program in Ruby • I live in Tokyo • I am the author of Mobility • I blog at dejimata.com

We’re Hiring!

Introduction Introduction

MODULE

MODULE What the heck is a anyway?

“A self-contained component of a system, often interchangeable, which has
a well- defined interface to the other components.”

Modular design is an attempt to combine the advantages of
standardization with those of customization. - Wikipedia Modular Design

Elements of Module Design

Elements of Module Design • Abstraction Extract common pattern into
functional unit

functional unit • Specification Define contractual interface for unit

functional unit • Specification Define contractual interface for unit • Encapsulation Hide implementation behind public interface

Scaling Rule The broader the scope of your abstraction the
more expressive your specification must be the tighter your encapsulation must be

Modularity in Ruby • Module (modularity through inclusion) – collection
of methods and constants – callback method (included) • Class (modularity through inheritance) – initializer, methods, constants – callback method (inherited)

Gems are also Modules! • Self-contained component – collection of
Classes, Modules, Objects and code • Well-defined interface to other components – gem/ruby dependencies (gemspec), class/object interfaces • As flexible as its components allow it to be gem install degica

A Modular Gem: Mobility • Pluggable “Translation Framework” • Abstracts
storage strategies into “backends” • Supports a flexible collection of plugins Plugins Backend

The Ruby Module module MyModule def foo “foo” end def
self.included(base) puts “included in #{base.name}” end end module MyModule def foo “foo” end def self.included(base) puts “included in #{base.name}” end end class A include MyModule def foo super + “bar” end end # prints “included in A” a = A.new a.foo #=> “foobar” class A include MyModule def foo super + “bar” end end # prints “included in A” a = A.new a.foo #=> “foobar”

Flexible? Nope. module MyModule(config) def foo config end end module
MyModule(config) def foo config end end

Flexible? Nope. module MyModule(config) #=> SyntaxError: unexpected '\n'... def foo
config end end module MyModule(config) #=> SyntaxError: unexpected '\n'... def foo config end end Class A include MyModule(config) end #=> NoMethodError: undefined method `MyModule' for A:Class Class A include MyModule(config) end #=> NoMethodError: undefined method `MyModule' for A:Class

Encapsulated? Nope. module MyModule def foo “foo” + bar end
private def bar “bar” end end module MyModule def foo “foo” + bar end private def bar “bar” end end

Encapsulated? Nope. module MyModule def foo “foo” + bar end
private def bar “bar” end end module MyModule def foo “foo” + bar end private def bar “bar” end end class A include MyModule def bar “baz” end end a = A.new a.foo #=> “foobaz” class A include MyModule def bar “baz” end end a = A.new a.foo #=> “foobaz”

Solution?

Metaprogramming!!!

Building with Modules Building with Modules

module Addable def +(point) self.class.new(point.x + x, point.y + y)
end end module Addable def +(point) self.class.new(point.x + x, point.y + y) end end A Simple Module Point = Struct.new(:x, :y) Point.include Addable p1 = Point.new(1, 2) p2 = Point.new(2, 3) p1 + p2 #=> #<Point:.. @x=3, @y=5> Point = Struct.new(:x, :y) Point.include Addable p1 = Point.new(1, 2) p2 = Point.new(2, 3) p1 + p2 #=> #<Point:.. @x=3, @y=5>

class Point < Struct.new(:x, :y) include Addable def +(point) puts
“Enter Adder...” super.tap { “Exit Adder...” } end end p1 = Point.new(1, 2) p2 = Point.new(2, 3) p1 + p2 Enter Adder... Exit Adder... #=> #<Point:.. @x=3, @y=5> class Point < Struct.new(:x, :y) include Addable def +(point) puts “Enter Adder...” super.tap { “Exit Adder...” } end end p1 = Point.new(1, 2) p2 = Point.new(2, 3) p1 + p2 Enter Adder... Exit Adder... #=> #<Point:.. @x=3, @y=5> Module Composition

Problem: Abstraction • Module only defines addition for fixed pair
of accessors x and y • Can we abstract our module so that defines addition on any set of accessors? • Can we do it in a way that supports method composition?

module AdderDefiner def define_adder(*keys) define_method :+ do |other| self.class.new( *(keys.map
{ |key| send(key) + other.send(key) }) ) end end end module AdderDefiner def define_adder(*keys) define_method :+ do |other| self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end end Bootstrap Methods • Define a bootstrap method which builds instance methods dynamically from arguments

class LineItem < Struct.new(:amount, :tax) extend AdderDefiner define_adder(:amount, :tax) end
class LineItem < Struct.new(:amount, :tax) extend AdderDefiner define_adder(:amount, :tax) end Bootstrapping a Method

class LineItem < Struct.new(:amount, :tax) extend AdderDefiner define_adder(:amount, :tax) end Bootstrapping a Method def +(other) self.class.new(amount + other.amount, tax + other.tax) end

class LineItem < Struct.new(:amount, :tax) extend AdderDefiner define_adder(:amount, :tax) end Bootstrapping a Method l1 = LineItem.new(9.99, 1.50) l2 = LineItem.new(15.99, 2.40) l1 + l2 #=> #<LineItem:... @amount=25.98, @tax=3.9> l1 = LineItem.new(9.99, 1.50) l2 = LineItem.new(15.99, 2.40) l1 + l2 #=> #<LineItem:... @amount=25.98, @tax=3.9>

class LineItem < Struct.new(:amount, :tax) extend AdderDefiner define_adder(:amount, :tax) def
+(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end class LineItem < Struct.new(:amount, :tax) extend AdderDefiner define_adder(:amount, :tax) def +(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end Bootstrapping + Composition

+(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end class LineItem < Struct.new(:amount, :tax) extend AdderDefiner define_adder(:amount, :tax) def +(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end Bootstrapping + Composition l1 = LineItem.new(9.99, 1.50) l2 = LineItem.new(15.99, 2.40) l1 + l2 #=> ??? l1 = LineItem.new(9.99, 1.50) l2 = LineItem.new(15.99, 2.40) l1 + l2 #=> ???

NoMethodError super: no superclass method `+' for #<struct LineItem amount=9.99,
tax=1.5>

+(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end class LineItem < Struct.new(:amount, :tax) extend AdderDefiner define_adder(:amount, :tax) def +(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end Bootstrapping + Composition

class LineItem < Struct.new(:amount, :tax) extend AdderDefiner def +(other) self.class.new(amount
+ other.amount, tax + other.tax) end def +(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end class LineItem < Struct.new(:amount, :tax) extend AdderDefiner def +(other) self.class.new(amount + other.amount, tax + other.tax) end def +(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end Bootstrapping + Composition

Configurable & Composable

Configurable & Composable Define methods directly on extending class

Configurable & Composable Define methods directly on extending class Define
methods on module included into extending class

module AdderIncluder def define_adder(*keys) adder = Module.new do define_method :+
do |other| self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end include adder end end module AdderIncluder def define_adder(*keys) adder = Module.new do define_method :+ do |other| self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end include adder end end Module Boostrapper

do |other| self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end include adder end end module AdderIncluder def define_adder(*keys) adder = Module.new do define_method :+ do |other| self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end include adder end end Module Boostrapper AdderDefiner#define_adder

do |other| self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end include adder end end module AdderIncluder def define_adder(*keys) adder = Module.new do define_method :+ do |other| self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end include adder end end Module Boostrapper Anonymous module

class LineItem < Struct.new(:amount, :tax) extend AdderIncluder define_adder(:amount, :tax) def
+(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end class LineItem < Struct.new(:amount, :tax) extend AdderIncluder define_adder(:amount, :tax) def +(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end Module Composition

class LineItem < Struct.new(:amount, :tax) extend AdderIncluder adder = Module.new
do define_method :+ do |other| self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end include adder def +(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end class LineItem < Struct.new(:amount, :tax) extend AdderIncluder adder = Module.new do define_method :+ do |other| self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end include adder def +(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end Module Composition

class LineItem < Struct.new(:amount, :tax) extend AdderIncluder adder = Module.new
do define_method :+ do |other| self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end include adder def +(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end class LineItem < Struct.new(:amount, :tax) extend AdderIncluder adder = Module.new do define_method :+ do |other| self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end include adder def +(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end Module Composition LineItem.ancestors #=> [LineItem, #<Module:0x...>, ...

class LineItem < Struct.new(:amount, :tax) extend AdderIncluder define_adder(:amount, :tax) def
+(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end class LineItem < Struct.new(:amount, :tax) extend AdderIncluder define_adder(:amount, :tax) def +(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end Now it Works l1 = LineItem.new(9.99, 1.50) l2 = LineItem.new(15.99, 2.40) l1 + l2 Enter Adder... Exit Adder... #=> #<LineItem:... @amount=25.98, @tax=3.9> l1 = LineItem.new(9.99, 1.50) l2 = LineItem.new(15.99, 2.40) l1 + l2 Enter Adder... Exit Adder... #=> #<LineItem:... @amount=25.98, @tax=3.9>

In the Wild • Technique of building anonymous modules to
include in classes is widely used (e.g. Rails)

Recap Addable Defines + method on fixed variables x and
y. AdderDefiner Adds class method define_adder which defines + method on configurable set of variables. AdderIncluder Also adds class method define_adder which defines + method, but in a composable way using anonymous module.

Is this Modular? • Boostrap method(s) extended as class methods
• Instance methods defined on anonymous module • Configuration state stored on including class, referenced by methods on anonymous module define_foo define_foo extend def foo(*args) self.class.foo end def foo(*args) self.class.foo end include MyClass @foo Module.new

The Module Builder The Module Builder

Classes, Modules & Objects

class Module Object Model Revisited • Module is a Class
is a

class ModuleBuilder < Module Object Model Revisited • Module is
a Class • A class can have subclasses

class ModuleBuilder < Module def initialize(method_name) Object Model Revisited •
Module is a Class • A class can have subclasses • (Sub)classes can define methods

class ModuleBuilder < Module def initialize(method_name) define_method method_name do Object
Model Revisited • Module is a Class • A class can have subclasses • (Sub)classes can define methods • Modules can also define methods

Elements of a Module Builder class Attributes < Module def
initialize(*attribute_names) @names = attribute_names # ... # ... end def included(klass) @names.each do |name| define_method name do # ... end # ... end end end class Post include Attributes.new(:title, :content) end class Attributes < Module def initialize(*attribute_names) @names = attribute_names # ... # ... end def included(klass) @names.each do |name| define_method name do # ... end # ... end end end class Post include Attributes.new(:title, :content) end Subclass Module Define module methods in initialize or included hook Include instance of Module subclass in other classes Configure in initializer

Configure in an Initializer class Attributes < Module def initialize(*attribute_names)
@names = attribute_names # ... end # ... end class Post include Attributes.new(:title, :content) end class Attributes < Module def initialize(*attribute_names) @names = attribute_names # ... end # ... end class Post include Attributes.new(:title, :content) end

Encapsulate in a Closure class Attributes < Module # ...
def included(klass) @names.each do |name| define_method name do locale = ... mobility_backend_for(name).read(locale) end end end end class Attributes < Module # ... def included(klass) @names.each do |name| define_method name do locale = ... mobility_backend_for(name).read(locale) end end end end

class AdderBuilder < Module def initialize(*keys) define_method :+ do |other|
self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end end class AdderBuilder < Module def initialize(*keys) define_method :+ do |other| self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end end Addable as Module Builder

class AdderBuilder < Module def initialize(*keys) define_method :+ do |other|
self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end end class AdderBuilder < Module def initialize(*keys) define_method :+ do |other| self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end end Addable as Module Builder AdderDefiner#define_adder

Encapsulated Module Building class LineItem < Struct.new(:amount, :tax) include AdderBuilder.new(:amount,
:tax) end l1 = LineItem.new(9.99, 1.50) l2 = LineItem.new(15.99, 2.40) l1 + l2 #=> #<LineItem:... @amount=25.98, @tax=3.9> class LineItem < Struct.new(:amount, :tax) include AdderBuilder.new(:amount, :tax) end l1 = LineItem.new(9.99, 1.50) l2 = LineItem.new(15.99, 2.40) l1 + l2 #=> #<LineItem:... @amount=25.98, @tax=3.9> • No bootstrap method added to class • No private methods mixed into class

Composition Too class LineItem < Struct.new(:amount, :tax) include (Module.new do
define_method :+ do |other| self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end def +(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end class LineItem < Struct.new(:amount, :tax) include (Module.new do define_method :+ do |other| self.class.new( *(keys.map { |key| send(key) + other.send(key) }) ) end end def +(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end

Composition Too class LineItem < Struct.new(:amount, :tax) include AdderBuilder.new(:amount, :tax)
def +(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end class LineItem < Struct.new(:amount, :tax) include AdderBuilder.new(:amount, :tax) def +(other) puts "Enter Adder..." super.tap { puts "Exit Adder..." } end end LineItem.ancestors [LineItem, #<AdderBuilder:0x...>, ... ] It has a name!

Applying the Pattern Applying the Pattern

The Module Builder Pattern broader abstraction more expressive specification tighter
encapsulation

Applying the Pattern module Alchemist class ModuleBuilder def initialize category
@category = category end def build build_module do |category_module| define_inspect_method(category_module) define_unit_methods(category_module) end end private attr_reader :category def build_module(&block) Module.new do def self.define_unit_method(names) names.each do |name| define_method(name.to_sym) { Alchemist.measure self, name.to_sym } end end end.tap &block end def define_inspect_method(category_module) category_module.class_eval %(def self.inspect() "#<Module(#{category})>" end) end def define_unit_methods(category_module) category_module.class_eval category_methods end def library Alchemist.library end def category_methods unit_names.map do |name| %(define_method("#{name}") { Alchemist.measure self, :#{name} }) + "\n" + prefixed_methods(name) end.join("\n") end def unit_names library.unit_names(category) end def prefixes_for(name) if library.si_units.include?(name.to_s) library.unit_prefixes.keys else [] end end def prefixed_methods(name) prefixes_for(name).map do |prefix| %(define_method("#{prefix}#{name}") { Alchemist.measure_prefixed self, :#{prefix}, :#{name} }) end.join("\n") end end end module Alchemist class ModuleBuilder def initialize category @category = category end def build build_module do |category_module| define_inspect_method(category_module) define_unit_methods(category_module) end end private attr_reader :category def build_module(&block) Module.new do def self.define_unit_method(names) names.each do |name| define_method(name.to_sym) { Alchemist.measure self, name.to_sym } end end end.tap &block end def define_inspect_method(category_module) category_module.class_eval %(def self.inspect() "#<Module(#{category})>" end) end def define_unit_methods(category_module) category_module.class_eval category_methods end def library Alchemist.library end def category_methods unit_names.map do |name| %(define_method("#{name}") { Alchemist.measure self, :#{name} }) + "\n" + prefixed_methods(name) end.join("\n") end def unit_names library.unit_names(category) end def prefixes_for(name) if library.si_units.include?(name.to_s) library.unit_prefixes.keys else [] end end def prefixed_methods(name) prefixes_for(name).map do |prefix| %(define_method("#{prefix}#{name}") { Alchemist.measure_prefixed self, :#{prefix}, :#{name} }) end.join("\n") end end end BEFORE

Applying the Pattern module Alchemist class ModuleBuilder def initialize category
@category = category end def build build_module do |category_module| define_inspect_method(category_module) define_unit_methods(category_module) end end private attr_reader :category def build_module(&block) Module.new do def self.define_unit_method(names) names.each do |name| define_method(name.to_sym) { Alchemist.measure self, name.to_sym } end end end.tap &block end def define_inspect_method(category_module) category_module.class_eval %(def self.inspect() "#<Module(#{category})>" end) end def define_unit_methods(category_module) category_module.class_eval category_methods end def library Alchemist.library end def category_methods unit_names.map do |name| %(define_method("#{name}") { Alchemist.measure self, :#{name} }) + "\n" + prefixed_methods(name) end.join("\n") end def unit_names library.unit_names(category) end def prefixes_for(name) if library.si_units.include?(name.to_s) library.unit_prefixes.keys else [] end end def prefixed_methods(name) prefixes_for(name).map do |prefix| %(define_method("#{prefix}#{name}") { Alchemist.measure_prefixed self, :#{prefix}, :#{name} }) end.join("\n") end end end module Alchemist class ModuleBuilder def initialize category @category = category end def build build_module do |category_module| define_inspect_method(category_module) define_unit_methods(category_module) end end private attr_reader :category def build_module(&block) Module.new do def self.define_unit_method(names) names.each do |name| define_method(name.to_sym) { Alchemist.measure self, name.to_sym } end end end.tap &block end def define_inspect_method(category_module) category_module.class_eval %(def self.inspect() "#<Module(#{category})>" end) end def define_unit_methods(category_module) category_module.class_eval category_methods end def library Alchemist.library end def category_methods unit_names.map do |name| %(define_method("#{name}") { Alchemist.measure self, :#{name} }) + "\n" + prefixed_methods(name) end.join("\n") end def unit_names library.unit_names(category) end def prefixes_for(name) if library.si_units.include?(name.to_s) library.unit_prefixes.keys else [] end end def prefixed_methods(name) prefixes_for(name).map do |prefix| %(define_method("#{prefix}#{name}") { Alchemist.measure_prefixed self, :#{prefix}, :#{name} }) end.join("\n") end end end module Alchemist class ModuleBuilder < Module def initialize category define_inspect_method(category) define_unit_methods(category) end def define_unit_method(names) names.each do |name| define_method(name.to_sym) { Alchemist.measure self, name.to_sym } end end private def define_inspect_method(category) define_method :inspect do "#<Module(#{category})>" end end def define_unit_methods(category) unit_names(category).map do |name| define_method name do Alchemist.measure self, name.to_sym end prefixes_for(name).map do |prefix| define_method "#{prefix}#{name}" do Alchemist.measure_prefixed self, prefix.to_sym, name.to_sym end end end end def library Alchemist.library end def unit_names(category) library.unit_names(category) end def prefixes_for(name) if library.si_units.include?(name.to_s) library.unit_prefixes.keys else [] end end end end module Alchemist class ModuleBuilder < Module def initialize category define_inspect_method(category) define_unit_methods(category) end def define_unit_method(names) names.each do |name| define_method(name.to_sym) { Alchemist.measure self, name.to_sym } end end private def define_inspect_method(category) define_method :inspect do "#<Module(#{category})>" end end def define_unit_methods(category) unit_names(category).map do |name| define_method name do Alchemist.measure self, name.to_sym end prefixes_for(name).map do |prefix| define_method "#{prefix}#{name}" do Alchemist.measure_prefixed self, prefix.to_sym, name.to_sym end end end end def library Alchemist.library end def unit_names(category) library.unit_names(category) end def prefixes_for(name) if library.si_units.include?(name.to_s) library.unit_prefixes.keys else [] end end end end BEFORE AFTER

Compact and Versatile class GeoLocation include Dry::Equalizer.new(:latitude, :longitude) attr_reader :latitude,
:longitude def initialize(latitude, longitude) @latitude, @longitude = latitude, longitude end end class GeoLocation include Dry::Equalizer.new(:latitude, :longitude) attr_reader :latitude, :longitude def initialize(latitude, longitude) @latitude, @longitude = latitude, longitude end end def initialize(*keys) @keys = keys define_methods freeze end def initialize(*keys) @keys = keys define_methods freeze end def define_methods define_cmp_method define_hash_method define_inspect_method end def define_methods define_cmp_method define_hash_method define_inspect_method end

Compact and Versatile point_a = GeoLocation.new(1, 2) point_b = GeoLocation.new(1,
2) point_c = GeoLocation.new(2, 2) point_a.inspect #=> "#<GeoLocation latitude=1 longitude=2>" point_a == point_b # => true point_a.hash == point_b.hash # => true point_a.eql?(point_b) # => true point_a.equal?(point_b) # => false point_a == point_c # => false point_a.hash == point_c.hash # => false point_a.eql?(point_c) # => false point_a.equal?(point_c) # => false point_a = GeoLocation.new(1, 2) point_b = GeoLocation.new(1, 2) point_c = GeoLocation.new(2, 2) point_a.inspect #=> "#<GeoLocation latitude=1 longitude=2>" point_a == point_b # => true point_a.hash == point_b.hash # => true point_a.eql?(point_b) # => true point_a.equal?(point_b) # => false point_a == point_c # => false point_a.hash == point_c.hash # => false point_a.eql?(point_c) # => false point_a.equal?(point_c) # => false

Flexible and Extendable Title Backend #<Fallbacks> #<Default> read(:fr) Storage Strategy
... class Post extend Mobility translates :title, fallbacks: { fr: :en }, default: ‘No Translation’ end I18n.locale = :fr post = Post.new post.title #=> post.title_backend.read(:fr) class Post extend Mobility translates :title, fallbacks: { fr: :en }, default: ‘No Translation’ end I18n.locale = :fr post = Post.new post.title #=> post.title_backend.read(:fr) • Mobility uses Module Builders to: – Customize translation accessors per-attribute – Add attribute-specific customization to models

But umm, is this... okay?

Don’t be Afraid! • A Module Builder is: – Just
a subclass – Just configuring an instance – Just defining methods on a module

But most of all, a Module Builder is just...

But most of all, a Module Builder is just... PLAIN
OLD RUBY RUBY

Thanks! Thanks!

The Ruby Module Builder Pattern

The Ruby Module Builder Pattern

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