Metaprogramming

Object Model and Metaprogramming in Ruby Monday, November 1, 2010

Santiago Pastorino @spastorino Monday, November 1, 2010

Monday, November 1, 2010

Ruby Rails Monday, November 1, 2010

Java get/set version 1 public class User { private String name; public String getName() { return name; } public void setName(String name) { this.name = name; } } Monday, November 1, 2010

.NET get/set version 1 class User { private string name; public string Name { get { return name; } set { name = value; } } } Monday, November 1, 2010

Ruby get/set version 1 class User def name @name end def name=(value) @name = value end end Monday, November 1, 2010

Java get/set version 2 ? public property String name; Monday, November 1, 2010

.NET get/set version 2 class User { public string Name { get; set; } } Monday, November 1, 2010

Ruby get/set version 2 The right way class User attr_accessor :name end Monday, November 1, 2010

Metaprogramming Monday, November 1, 2010

Metaprogramming “Metaprogramming is writing code that writes code.” Monday, November 1, 2010

Metaprogramming “Metaprogramming is writing code that writes code.” “Metaprogramming is writing code that manipulates language constructs at runtime.” Monday, November 1, 2010

Metaprogramming Monday, November 1, 2010

Metaprogramming • Classes are always open • Everything is an Object even classes and modules • All methods calls have a receiver Monday, November 1, 2010

Classes are Open class Hash def except!(*keys) keys.each { |key| delete(key) } self end end hsh = {:a => 1, :b => 2} hsh.except!(:a) # => {:b => 2} Monday, November 1, 2010

Everything is an Object class User puts self # => User puts self.class # => Class end User.methods.grep /get/ => [:const_get, :class_variable_get, :instance_variable_get] Monday, November 1, 2010

All method calls have a receiver class User attr_accessor :name end Monday, November 1, 2010

Metaprogramming • class_eval • deﬁne_method • method_missing Monday, November 1, 2010

attr_reader (class_eval) class User def initialize(name) @name = name end attr_reader :name end u = User.new(‘Santiago’) u.name # => ‘Santiago’ Monday, November 1, 2010

attr_reader (class_eval) class Module def attr_reader(sym) class_eval “def #{sym}; @#{sym}; end” end end class User attr_reader :name # def name; @name; end end Monday, November 1, 2010

attr_reader (class_eval) class Module private def attr_reader(sym) class_eval <<-READER, __FILE__, __LINE__ + 1 def #{sym} @#{sym} end READER end end Monday, November 1, 2010

attr_writer (class_eval) class Module private def attr_writer(sym) class_eval <<-WRITER, __FILE__, __LINE__ + 1 def #{sym}=(value) @#{sym} = value end WRITER end end Monday, November 1, 2010

attr_accessor class Module private def attr_accessor(*syms) attr_reader(syms) attr_writer(syms) end end Monday, November 1, 2010

deﬁne_method class User attr_accessor_with_default :age, 22 end user = User.new user.age # => 22 user.age = 26 user.age # => 26 Monday, November 1, 2010

deﬁne_method class Module def attr_accessor_with_default(sym, default) class_eval <<-READER, __FILE__, __LINE__ + 1 def #{sym} #{default} end # + something else READER end end Monday, November 1, 2010

deﬁne_method class User attr_accessor_with_default :age, 22 end user = User.new user.age # => 22 Monday, November 1, 2010

deﬁne_method class USer attr_accessor_with_default :complex_obj, Struct.new(:x, :y) end user = User.new user.complex_obj # => nil Monday, November 1, 2010

deﬁne_method 22.to_s # => “22” Struct.new(:x, :y).to_s # => "#<Class:0x00000100966210>" Monday, November 1, 2010

deﬁne_method class Module def attr_accessor_with_default(sym, default) class_eval <<-READER, __FILE__, __LINE__ + 1 def #{sym} default end # + something else READER end end Monday, November 1, 2010

deﬁne_method class User attr_accessor_with_default :complex_obj, Struct.new(:x, :y) end user = User.new user.complex_obj NameError: undefined local variable or method `default' for #<User:0x00000100978050> Monday, November 1, 2010

deﬁne_method class Module def attr_accessor_with_default(sym, default) define_method(sym, Proc.new { default }) end end Monday, November 1, 2010

deﬁne_method class Module def attr_accessor_with_default(sym, default) define_method(sym, Proc.new { default }) class_eval(<<-EVAL, __FILE__, __LINE__ + 1) def #{sym}=(value) class << self; attr_accessor :#{sym} end @#{sym} = value end EVAL end end Monday, November 1, 2010

deﬁne_method class Person attr_accessor_with_default :complex_obj, Struct.new(:x, :y) end person = Person.new person.complex_obj # => #<Class:0x00000101045c70> person.complex_obj = Struct.new(:a, :b) person.complex_obj # => #<Class:0x000001009be9d8> Monday, November 1, 2010

Method Lookup BasicObject Kernel Kernel Object x XML Monday, November 1, 2010

method_missing (An internal DSL) XML.generate(STDOUT) do html do head do title { ‘pagetitle’ } comment ‘This is a test’ end body do h1 style: ‘font-family: sans-serif‘ do ‘pagetitle’ end ul id: ‘info’ do li { Time.now } li { RUBY_VERSION } end end end Monday, November 1, 2010

method_missing (An internal DSL) <html> <head> <title>pagetitle</title>  </head> <body> <h1 style=”font-family: sans-serif”> pagetitle </h1> <ul id=”info”> <li>2010-10-30 17:39:45 -0200</li> <li>1.9.2</li> </ul> </body> </html> Monday, November 1, 2010

method_missing (An internal DSL) class XML def initialize(out) @out = out end def content(text) @out << text.to_s end def comment(text) @out << “” end end Monday, November 1, 2010

method_missing (An internal DSL) def method_missing(tagname, attributes={}) @out << “<#{tagname}” attributes.each { |attr, value| @out << “#{attr}=’#{value}’” } if block_given? @out << ‘>’ content = yield if content @out << content.to_s end @out << “</#{tagname}>” else @out << ‘/>’ end end Monday, November 1, 2010

method_missing (An internal DSL) class XML def self.generate(out, &block) XML.new(out).instance_eval(&block) end end XML.generate(STDOUT) do # code end Monday, November 1, 2010

method_missing (An internal DSL) XML.generate(STDOUT) do html do head do title { ‘pagetitle’ } comment ‘This is a test’ end body do h1 style: ‘font-family: sans-serif‘ do ‘pagetitle’ end ul id: ‘info’ do li { Time.now } li { RUBY_VERSION } end end end Monday, November 1, 2010

It’s all about the self Monday, November 1, 2010

Singleton Method d = “9/5/1982” def d.to_date # code here end # or class << d def to_date # code here end end Monday, November 1, 2010

Object Model BasicObject Kernel Kernel Object String d (d) Monday, November 1, 2010

Class Methods? class String def self.to_date # code here end # or class << self def to_date # code here end end end Monday, November 1, 2010

The same d = “9/5/1982” def d.to_date # code here end # or class << d def to_date # code here end end Monday, November 1, 2010

Object Model BasicObject (BasicObject) Object (Object) Module String (String) Class Monday, November 1, 2010

Everything about self class User p self # => User class << self p self # => <Class:User> end end Monday, November 1, 2010

Real life use cases for ﬁbers? require 'fiber' module Eigenclass def eigenclass class << self; self end end module_function :eigenclass public :eigenclass end class Object include Eigenclass end Monday, November 1, 2010

Real life use cases for ﬁbers? class EigenclassesGenerator def initialize(obj) @eigenclasses = [obj.eigenclass] @fiber = Fiber.new do loop do Fiber.yield @eigenclasses.last @eigenclasses[@eigenclasses.length] = @eigenclasses.last.eigenclass end end end def [](index) if index >= @eigenclasses.length (index - @eigenclasses.length + 2).times { @fiber.resume } end @eigenclasses[index] end end Monday, November 1, 2010

Rails Magic? class User < ActiveRecord::Base belongs_to :bill_address has_one :role has_many :orders validates_presence_of :name, :country validates_acceptance_of :terms validates_uniqueness_of :name end Monday, November 1, 2010

Questions? Monday, November 1, 2010

Thank you! Monday, November 1, 2010

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Metaprogramming

by Santiago Pastorino on Nov 01, 2010

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