Designing Beautiful
    Ruby APIs V.2
          ihower@gmail.com
           2010/4/25 & 6/26
    RubyConf Taiwan & RubyCon...
About Me
•              a.k.a. ihower
    •   http://ihower.tw

    •   http://twitter.com/ihower

• Rails Developer since...
Ruby Taiwan
RubyConf Taiwan 2010
Agenda
                    10 techniques and a sub-talk
•   Arguments processing
•   Code blocks
•   Module
•   method_mis...
Define “beautiful”

• Readable: easy to understand
• Efficient: easy to write
• Flexible: easy to extend
1.Argument Processing
Pseudo-Keyword
   Arguments
def blah(options)
  puts options[:foo]
  puts options[:bar]
end

blah(:foo => "test", :bar => ...
Treating Arguments as
       an Array
       def sum(*args)
           puts args[0]
           puts args[1]
           put...
Rails helper usage
             example
# USAGE-1 without block
<% link_to 'Posts list', posts_path, :class => 'posts'
%>
...
# Rails3's source code
def link_to(*args, &block)
  if block_given?
    options       = args.first || {}
    html_options ...
ActiveSupport#extract_options!
            extract hash from *args


      def foobar(*args)
        options = args.extrac...
2.Code Blocks
A trivial example 1
     def call_block
       puts "start"
       yield(" foobar")
       puts "end"
     end

     call_...
A trivial example 2
    def call_block(&block)
      puts "start"
      block.call("foobar")
      puts "end"
    end

   ...
pre- and Post-processing
     usage example
   f = File.open("myfile.txt", 'w')
   f.write("Lorem ipsum dolor sit amet")
 ...
pre- and Post-
       processing
# without code block
def send_message(msg)
  socket = TCPSocket.new(@ip, @port) # Pre-
  ...
# with code block
def send_message(msg)
  connection do |socket|
    socket.puts("foobar")
    socket.gets
  end
end

def ...
Dynamic Callbacks
     Sinatra usage example

   get '/posts' do
     #.. show something ..
   end

   post '/posts' do
  ...
Dynamic Callbacks
server = Server.new

server.handle(/hello/) do
  puts "Hello at #{Time.now}"
end

server.handle(/goodbye...
class Server

  def initialize
    @handlers = {}
  end

  def handle(pattern, &block)
    @handlers[pattern] = block
  en...
Self Yield
      gemspec example

spec = Gem::Specification.new
spec.name = "foobar"
spec.version = "1.1.1"



# using blo...
Self Yield
            gemspec example


class Gem::Specification
    def initialize name = nil, version = nil
        # ....
3.Module
A trivial example
    module Mixin
        def foo
            puts "foo"
        end
    end

    class A
        include...
obj.extend(Mod)
       Implementing class behavior


module Mixin
    def foo
        puts "foo"                   class A...
obj.extend(Mod)
 Implementing per-object behavior

  module Mixin
      def foo
          puts "foo"
      end
  end

  a ...
Dual interface
module Logger
    extend self

      def log(message)
          $stdout.puts "#{message} at #{Time.now}"
  ...
Mixin with class methods
      module Mixin
        def foo
          puts "foo"
        end

        module ClassMethods
...
module Mixin

  # self.included is a hook method
  def self.included(base)
    base.extend(ClassMethods)
  end

  def foo
...
module Mixin

  def self.included(base)
    base.extend(ClassMethods)
    base.send(:include, InstanceMethods)
  end

  mo...
4.method_missing?
ActiveRecord example

 class Person < ActiveRecord::Base
 end

 p1 = Person.find_by_name("ihower")
 p2 = Person.find_by_em...
A trivial example
    car = Car.new

    car.go_to_taipei
    # go to taipei

    car.go_to_shanghai
    # go to shanghai
...
class Car
    def go(place)
        puts "go to #{place}"
    end

      def method_missing(name, *args)
          if name...
Don’t abuse method
        missing
• method_missing? is slow
• only use it when you can not define
  method in advance
 • M...
XML builder example
builder = Builder::XmlMarkup.new(:target=>STDOUT, :indent=>2)
builder.person do |b|
  b.name("Jim")
  ...
We need BlankSlate or
  BasicObject to avoid name conflict
# Jim Weirich's BlankSlate from XML builder
>> BlankSlate.instan...
BlankSlate usage example
class Proxy < BlankSlate

  def initialize(obj)
    @obj = obj
  end

  def method_missing(sym, *...
5.const_missing
ActiveSupport::Dependencies
 example (lazy class loading)

•   Person

•   Ruby calls const_missing

•   const_missing cal...
Global constant
class Module
    original_c_m = instance_method(:const_missing)

      define_method(:const_missing) do |n...
Localized constant
    (you can use super here)
  class Color

    def self.const_missing(name)
      if name.to_s =~ /[a-...
6.Methods chaining
an Array example
[1,1,2,3,3,4,5].uniq!.reject!{ |i| i%2 == 0 }.reverse

# 5,3,1
a trivial example
     class Demo
       def foo
         puts "foo"
         self
       end

       def bar
         put...
Object#tap
                          Ruby 1.8.7 later




puts "dog".reverse
          .tap{ |o| puts "reversed: #{o}" } #...
Object#tap
   Ruby 1.8.7 later



  class Object

    def tap
      yield self
      self
    end

  end
7.Core extension
NilClass#try example
person = Person.find_by_email(params[:email])
# but we don't know @person exists or not

# Without tr...
class NilClass
  def try(*args)
    nil
  end
end
Numeric#bytes example

    123.kilobytes # 125952
    456.megabytes # 478150656
    789.gigabytes # 847182299136
class Numeric
  KILOBYTE = 1024
  MEGABYTE = KILOBYTE * 1024
  GIGABYTE = MEGABYTE * 1024

  def bytes
    self
  end
  al...
Object#blank? example
    [1,2,3].blank? # false
    "blah".blank? # false
    "".blank? # true

    class Demo
      def ...
class Object

  def blank?
    respond_to?(:empty?) ? empty? : !self
  end

  def present?
    !blank?
  end

end

class N...
sub-talk



Ruby Object Model
       and
Meta-programming
self
(current object)
 class Demo
     puts self # Demo

       def blah
           puts self # <Demo:0x10180f398> object
...
Everything in Ruby is
 object, even class.
Ruby Object Model
class A                          Object
end
                                    super
class B < A
end
  ...
class object is an object
      of the class Class
class A                          Object
end
                           ...
module
class A
end
                              A
module B
end                            super

module C                ...
what’s metaclass?
class A
  def foo                         Object
  end
end                                  super

obj1 ...
metaclass
            also known as singleton, eigenclass, ghost class, virtual class.
class A                every object...
class object has his metaclass too.
class A     so the singleton method is class
  # way1
  def self.foo          method!!...
If we define method
   inside class Class
class Class
  def blah
    puts "all class has blah class method"
  end
end

clas...
method definition(current class)
    "def" defines instance method for current class



           class Demo
              ...
class << changes the method
   definition(current class)
   class Demo
     class << self
       # the current class is Dem...
class << also
changes the self (current object)

       class Demo

         puts self # Demo

         class << self
    ...
We can get metaclass by
    using class<<
    class Object
      def metaclass
        class << self; self; end end
      ...
method definition
  mechanism    self (current object)                      new scope?
                                    ...
Meta-programming
 How to write code to write code?
Two types of meta-
     programming
• Code Generation (Not talk about it today)
 • eg. Rails scaffold
• Reflection
 • eg. C...
Specifically, How to write a
method to define method?
class Demo
  # the current class is Demo

  def define_blah1
    # the current class is Demo
    def blah1
      puts "bla...
And how to write a method
to define singleton method?
class Demo

  def define_blah1
    # self is Demo's instance
    def self.blah1
      puts "blah1" # define singleton meth...
Not useful really, we
need more dynamic
      power
The key of power is
  variable scope!!
Variable scope




 {
      module MyDemo
        var = 1




    {
        class Demo
          var = 2



       {
     ...
block variable scope
var1 = "foo"

[1,2,3].each do |i|
    puts var
    var1 = "bar"
    var2 = "baz"
end

puts var1 # foo...
define_method
unlike “def”, you can access outside variable!!
      class Demo
          # define instance methods
        ...
define_method will
defines instance method
   for current object
   (it’s must be class object or module)
class Class
                    def define_more_methods
                      # define instance methods
                  ...
module Mixin
                  module ClassMethods
                    def define_more_methods
                      # def...
So unlike “def”,
define_method will not
  create new scope
So we maybe need those methods
     inside define_method:
                (because the scope is not changed)



  •   Objec...
Not dynamic enough?
  Because class <<
will create new scope
         still!!
we need even more
 dynamic power
var = 1
String.class_eval do

  puts var # 1
  puts self # the current object is String

  # the current class is String
 ...
class_eval + define_method
      name = "say"
      var = "it’s awesome"

      String.class_eval do

        define_method...
But how to define
singleton method using
     class_eval and
    define_method?
Wrong!
name = "foo"
var = "bar"

String.class_eval do

  class << self
    define_method(name) do
      puts var
    end
 ...
Fixed!
   you need find out metaclass and class_eval it!

name = "foo"
var = "bar"

metaclass = (class << String; self; end...
How about apply to
   any object?
 (because class_eval only works on class object or module)
instance_eval for any
       object
 obj = "blah"
 obj.instance_eval do
   puts self # obj

   # the current class is obj'...
how about class object?
  String.instance_eval do
    puts self # String

    # the current class is String's metaclass
  ...
method definition
   mechanism        self (current object)                      new scope?
                               ...
8. Class Macro
 (Ruby’s declarative style)
ActiveRecord example
class User < ActiveRecord::Base

  validates_presence_of     :login
  validates_length_of       :logi...
Class Bodies Aren’t
         Special
class Demo
  a = 1
  puts a

  def self.say
    puts "blah"
  end

  say # you can ex...
Memorize example
class Account

  def calculate
    @calculate ||= begin
      sleep 10 # expensive calculation
      5
  ...
memoize method
class Account

  def calculate
      sleep 2 # expensive calculation
      5
    end
  end

  memoize :calc...
class Class
  def memoize(name)
    original_method = "_original_#{name}"
    alias_method :"#{original_method}", name

  ...
It’s general for any class
    class Car

      def run
          sleep 100 # expensive calculation
          “done”
     ...
BTW, how to keep original
 method and call it later?
• alias_method
 • most common way
 • example above
• method binding
 ...
9.instance_eval
DSL calls it create implicit context
Rack example

 Rack::Builder.new do
   use Some::Middleware, param
   use Some::Other::Middleware
   run Application
 end
How is instance_eval
       doing?
• It changes the “self” (current object) to
  caller
• Any object can call instance_eva...
a trivial example
    class Demo
      def initialize
        @a = 99
      end
    end

    foo = Demo.new

    foo.insta...
instance_eval with block
      class Foo
        attr_accessor :a,:b

        def initialize(&block)
          instance_ev...
Strings of Code
     eval*() family can accept string of code




• No editor’s syntax highlight
• Not report syntax error...
10.Class.new
anonymous class
  klass = Class.new
    def move_left
      puts "left"
    end

    def move_right
      puts "right"
   ...
variable scope matters
           you can access outside variable

var = "it’s awesome"
klass = Class.new

      puts var
...
var = "it’s awesome"
klass = Class.new do

      puts var

      define_method :my_method do
          puts var
      end
...
Subclassing with a
generator using Class.new
   def disable_string_class(method_name)
     Class.new(String) do
       und...
Parameterized subclassing
    Camping example
      module Camping::Controllers
        class Edit < R '/edit/(d+)'
      ...
Parameterized
subclassing example
     def Person(name)
       if name == "ihower"
         Class.new do
            def m...
class Foo < Person 'ihower'
  def name
    "foo"
  end
end

class Bar < Person 'not_ihower'
  def name
    "bar"
  end
end...
Conclusion
Story 1:
      DSL or NoDSL by José Valim
            at Euruko 2010
http://blog.plataformatec.com.br/2010/06/dsl-or-nodsl...
a DSL
class ContactForm < MailForm::Base
  to "jose.valim@plataformatec.com.br"
  from "contact_form@app_name.com"
  subje...
DSL fails
class ContactForm < MailForm::Base
  to :author_email
  from { |c| "#{c.name} <#{c.email}>" }
  subject "Contact...
NoDSL
class ContactForm < MailForm::Base
  attributes :name, :email, :message

  def headers
    {
      :to => author_ema...
Other examples

• Rake v.s. Thor
• RSpec v.s. Unit::test
Rake example
task :process do
  # do some processing
end

namespace :app do
  task :setup do
    # do some setup
    Rake:...
Thor example
 class Default < Thor
   def process
     # do some processing
   end
 end

 class App < Thor
   def setup
  ...
José’s Conclusion

• DSL or NoDSL - don't actually have
  answer
• Replace "It provides a nice DSL" with
  "it relies on a...
Story 2:

Rails 2 to 3
 API changes
Routes
                            nice DSL
# Rails 2
map.resources :people, :member => { :dashboard => :get,
            ...
AR queries (1)
                     method chaining


# Rails 2
users = User.find(:all, :conditions => { :name =>
'ihower'...
AR queries (2)
      Unify finders, named_scope, with_scope to Relation
# Rails 2
users = User
users = users.some_named_sco...
AR queries (2)
Unify finders, named_scope, with_scope to Relation


# Rails   3
users =   User
users =   users.some_scope i...
AR queries (3)
Using class methods instead of scopes when you need lambda
    # Rails 3
    class Product < ActiveRecord::...
AR validation (1)
# Rails 2
class User < ActiveRecord::Base
  validates_presence_of :email
  validates_uniqueness_of :emai...
AR validation (2)
                           custom validator

# Rails 3
class User < ActiveRecord::Base
  validates :emai...
ActionMailer
# Rails 2
class UserMailer < ActionMailer::Base

  def signup(user)
    recipients user.email
    from 'ihowe...
ActionMailer
# Rails 3
class UserMailer < ActionMailer::Base

  default :from => "ihower@gmail.com"

  def signup(user)
  ...
I think it’s a Ruby APIs
     paradigm shift
 Apparently, Rails is the most successful Ruby open source codebase
         ...
References
•   Ruby Best Practices, O’Reilly

•   The Ruby Object Model and Metaprogrammin, Pragmatic

•   Programming Rub...
The End
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Designing Ruby APIs

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Designing Ruby APIs

  1. 1. Designing Beautiful Ruby APIs V.2 ihower@gmail.com 2010/4/25 & 6/26 RubyConf Taiwan & RubyConf China
  2. 2. About Me • a.k.a. ihower • http://ihower.tw • http://twitter.com/ihower • Rails Developer since 2006 • The Organizer of Ruby Taiwan Community • http://ruby.tw • http://rubyconf.tw
  3. 3. Ruby Taiwan
  4. 4. RubyConf Taiwan 2010
  5. 5. Agenda 10 techniques and a sub-talk • Arguments processing • Code blocks • Module • method_missing? • const_missing • Methods chaining • Core extension • (sub-talk) Ruby Object Model and Meta-programming • Class macro • instance_eval • Class.new
  6. 6. Define “beautiful” • Readable: easy to understand • Efficient: easy to write • Flexible: easy to extend
  7. 7. 1.Argument Processing
  8. 8. Pseudo-Keyword Arguments def blah(options) puts options[:foo] puts options[:bar] end blah(:foo => "test", :bar => "test")
  9. 9. Treating Arguments as an Array def sum(*args) puts args[0] puts args[1] puts args[2] puts args[3] end sum(1,2,3) # 1 # 2 # 3 # nil
  10. 10. Rails helper usage example # USAGE-1 without block <% link_to 'Posts list', posts_path, :class => 'posts' %> # USAGE-2 with block <% link_to posts_path, :class => 'posts' do %> <p>Posts list</p> <% end %>
  11. 11. # Rails3's source code def link_to(*args, &block) if block_given? options = args.first || {} html_options = args.second link_to(capture(&block), options, html_options) else name = args[0] options = args[1] || {} html_options = args[2] html_options = convert_options_to_data_attributes(options, html_options) url = url_for(options) if html_options html_options = html_options.stringify_keys href = html_options['href'] tag_options = tag_options(html_options) else tag_options = nil end href_attr = "href="#{url}"" unless href "<a #{href_attr}#{tag_options}>#{ERB::Util.h(name || url)}</a>".html_safe end end
  12. 12. ActiveSupport#extract_options! extract hash from *args def foobar(*args) options = args.extract_options! end foobar(1, 2) # options is {} foobar(1, 2, :a => :b) # options is { :a => :b }
  13. 13. 2.Code Blocks
  14. 14. A trivial example 1 def call_block puts "start" yield(" foobar") puts "end" end call_block do |str| puts " here" puts str puts " here" end # start # here # foobar # here # end
  15. 15. A trivial example 2 def call_block(&block) puts "start" block.call("foobar") puts "end" end call_block do |str| puts "here" puts str puts "here" end # start # here # foobar # here # end
  16. 16. pre- and Post-processing usage example f = File.open("myfile.txt", 'w') f.write("Lorem ipsum dolor sit amet") f.write("Lorem ipsum dolor sit amet") f.close # using block File.open("myfile.txt", 'w') do |f| f.write("Lorem ipsum dolor sit amet") f.write("Lorem ipsum dolor sit amet") end
  17. 17. pre- and Post- processing # without code block def send_message(msg) socket = TCPSocket.new(@ip, @port) # Pre- socket.puts(msg) response = socket.gets ensure socket.close # Post- end end
  18. 18. # with code block def send_message(msg) connection do |socket| socket.puts("foobar") socket.gets end end def connection socket = TCPSocket.new(@ip, @port) # Pre- yield(socket) ensure socket.close # Post- end end
  19. 19. Dynamic Callbacks Sinatra usage example get '/posts' do #.. show something .. end post '/posts' do #.. create something .. end put '/posts/:id' do #.. update something .. end delete '/posts/:id' do #.. annihilate something .. end
  20. 20. Dynamic Callbacks server = Server.new server.handle(/hello/) do puts "Hello at #{Time.now}" end server.handle(/goodbye/) do puts "goodbye at #{Time.now}" end server.execute("/hello") # Hello at Wed Apr 21 17:33:31 +0800 2010 server.execute("/goodbye") # goodbye at Wed Apr 21 17:33:42 +0800 2010
  21. 21. class Server def initialize @handlers = {} end def handle(pattern, &block) @handlers[pattern] = block end def execute(url) @handlers.each do |pattern, block| if match = url.match(pattern) block.call break end end end end
  22. 22. Self Yield gemspec example spec = Gem::Specification.new spec.name = "foobar" spec.version = "1.1.1" # using block Gem::Specification.new do |s| s.name = "foobar" s.version = "1.1.1" #... end
  23. 23. Self Yield gemspec example class Gem::Specification def initialize name = nil, version = nil # ... yield self if block_given? # ... end end
  24. 24. 3.Module
  25. 25. A trivial example module Mixin def foo puts "foo" end end class A include Mixin end A.new.foo # foo
  26. 26. obj.extend(Mod) Implementing class behavior module Mixin def foo puts "foo" class A end extend Mixin end end the same as A.extend(Mixin) class << A include Mixin A.foo # class method end
  27. 27. obj.extend(Mod) Implementing per-object behavior module Mixin def foo puts "foo" end end a = "test" class << a the same as a.extend(Mixin) include Mixin a.foo # foo end b = "demo" b.foo # NoMethodError
  28. 28. Dual interface module Logger extend self def log(message) $stdout.puts "#{message} at #{Time.now}" end end Logger.log("test") # as Logger’s class method class MyClass include Logger end MyClass.new.log("test") # as MyClass’s instance method
  29. 29. Mixin with class methods module Mixin def foo puts "foo" end module ClassMethods def bar puts "bar" end end end class MyClass include Mixin extend Mixin::ClassMethods end
  30. 30. module Mixin # self.included is a hook method def self.included(base) base.extend(ClassMethods) end def foo puts "foo" end module ClassMethods def bar puts "bar" end end end class MyClass include Mixin end
  31. 31. module Mixin def self.included(base) base.extend(ClassMethods) base.send(:include, InstanceMethods) end module InstanceMethods def foo puts "foo" end end module ClassMethods def bar puts "bar" end end end class MyClass include Mixin end
  32. 32. 4.method_missing?
  33. 33. ActiveRecord example class Person < ActiveRecord::Base end p1 = Person.find_by_name("ihower") p2 = Person.find_by_email("ihower@gmail.com")
  34. 34. A trivial example car = Car.new car.go_to_taipei # go to taipei car.go_to_shanghai # go to shanghai car.go_to_japan # go to japan
  35. 35. class Car def go(place) puts "go to #{place}" end def method_missing(name, *args) if name.to_s =~ /^go_to_(.*)/ go($1) else super end end end car = Car.new car.go_to_taipei # go to taipei car.blah # NoMethodError: undefined method `blah`
  36. 36. Don’t abuse method missing • method_missing? is slow • only use it when you can not define method in advance • Meta-programming can define methods dynamically
  37. 37. XML builder example builder = Builder::XmlMarkup.new(:target=>STDOUT, :indent=>2) builder.person do |b| b.name("Jim") b.phone("555-1234") b.address("Taipei, Taiwan") end # <person> # <name>Jim</name> # <phone>555-1234</phone> # <address>Taipei, Taiwan</address> # </person>
  38. 38. We need BlankSlate or BasicObject to avoid name conflict # Jim Weirich's BlankSlate from XML builder >> BlankSlate.instance_methods => ["__send__", "instance_eval", "__id__"] # Ruby 1.9 >> BasicObject.instance_methods => [:==, :equal?, :!, :! =, :instance_eval, :instance_exec, :__send__] # an easy BlankSlate class BlankSlate instance_methods.each { |m| undef_method m unless m =~ /^__/ } end >> BlankSlate.instance_methods => ["__send__", "__id__"]
  39. 39. BlankSlate usage example class Proxy < BlankSlate def initialize(obj) @obj = obj end def method_missing(sym, *args, &block) puts "Sending #{sym}(#{args.join(',')}) to obj" @obj.__send__(sym, *args, &block) end end s = Proxy.new("foo") puts s.reverse # Sending reverse() to obj # "oof"
  40. 40. 5.const_missing
  41. 41. ActiveSupport::Dependencies example (lazy class loading) • Person • Ruby calls const_missing • const_missing calls Dependencies.load_missing_constant(Object, :Person) • require or load person.rb in the list of load path
  42. 42. Global constant class Module original_c_m = instance_method(:const_missing) define_method(:const_missing) do |name| if name.to_s =~ /^U([0-9a-fA-F]{4})$/ [$1.to_i(16)].pack("U*") else original_c_m.bind(self).call(name) end end end puts U0123 # ģ puts U9999 #
  43. 43. Localized constant (you can use super here) class Color def self.const_missing(name) if name.to_s =~ /[a-zA-Z]/ const_set(name, new) else super end end end Color::RED #<Color:0x1018078a0> Color::GREEN #<Color:0x1018059d8>
  44. 44. 6.Methods chaining
  45. 45. an Array example [1,1,2,3,3,4,5].uniq!.reject!{ |i| i%2 == 0 }.reverse # 5,3,1
  46. 46. a trivial example class Demo def foo puts "foo" self end def bar puts "bar" self end def baz puts "baz" self end end Demo.new.foo.bar.baz # foo # bar # baz
  47. 47. Object#tap Ruby 1.8.7 later puts "dog".reverse .tap{ |o| puts "reversed: #{o}" } # .upcase # reversed: god GOD
  48. 48. Object#tap Ruby 1.8.7 later class Object def tap yield self self end end
  49. 49. 7.Core extension
  50. 50. NilClass#try example person = Person.find_by_email(params[:email]) # but we don't know @person exists or not # Without try @person ? @person.name : nil # With try @person.try(:name)
  51. 51. class NilClass def try(*args) nil end end
  52. 52. Numeric#bytes example 123.kilobytes # 125952 456.megabytes # 478150656 789.gigabytes # 847182299136
  53. 53. class Numeric KILOBYTE = 1024 MEGABYTE = KILOBYTE * 1024 GIGABYTE = MEGABYTE * 1024 def bytes self end alias :byte :bytes def kilobytes self * KILOBYTE end alias :kilobyte :kilobytes def megabytes self * MEGABYTE end alias :megabyte :megabytes def gigabytes self * GIGABYTE end alias :gigabyte :gigabytes end
  54. 54. Object#blank? example [1,2,3].blank? # false "blah".blank? # false "".blank? # true class Demo def return_nil end end Demo.new.blank? # false Demo.new.return_nil.blank? # true
  55. 55. class Object def blank? respond_to?(:empty?) ? empty? : !self end def present? !blank? end end class NilClass def blank? true end end class FalseClass def blank? true end end class TrueClass def blank? false end end
  56. 56. sub-talk Ruby Object Model and Meta-programming
  57. 57. self (current object) class Demo puts self # Demo def blah puts self # <Demo:0x10180f398> object [1,2,3].each do |i| puts self # <Demo:0x10180f398> object end end class AnotherDemo puts self # Demo::AnotherDemo end end
  58. 58. Everything in Ruby is object, even class.
  59. 59. Ruby Object Model class A Object end super class B < A end A obj = B.new super obj.class # B B.superclass # A class class B.class # Class obj B Class
  60. 60. class object is an object of the class Class class A Object end super class B < A class end A obj = B.new class super obj.class # B B.superclass # A class class B.class # Class obj B Class class
  61. 61. module class A end A module B end super module C Mixin end B,C class D < A super include B class include C obj D end
  62. 62. what’s metaclass? class A def foo Object end end super obj1 = A.new class obj2 A obj2 = A.new class obj2
  63. 63. metaclass also known as singleton, eigenclass, ghost class, virtual class. class A every object has his own metaclass def foo end end Object obj1 = A.new super obj2 = A.new class def obj2.bar obj2 A # only obj2 has bar method, # called singleton method super end class obj2’s obj2 metaclass # another way class << obj1 def baz #only obj1 has baz method end end P.S. well, number and symbol have no metaclass
  64. 64. class object has his metaclass too. class A so the singleton method is class # way1 def self.foo method!! end # way2 class << self def bar class B end Class end end super class A’s # way3 A metaclass def A.baz end A.foo A.bar A.baz
  65. 65. If we define method inside class Class class Class def blah puts "all class has blah class method" end end class A end A.blah # all class has blah class method String.blah # all class has blah class method
  66. 66. method definition(current class) "def" defines instance method for current class class Demo # the current class is Demo def foo puts "foo" end end Demo.new.foo # foo
  67. 67. class << changes the method definition(current class) class Demo class << self # the current class is Demo's metaclass def foo puts "foo" end end end Demo.foo # foo
  68. 68. class << also changes the self (current object) class Demo puts self # Demo class << self puts self # Demo's metaclass end end "abc".metaclass
  69. 69. We can get metaclass by using class<< class Object def metaclass class << self; self; end end end end "abc".metaclass String.metaclass # Ruby 1.9.2 "abc".singleton_class => #<Class:#<String:0x000001009e26f8>>
  70. 70. method definition mechanism self (current object) new scope? (current_class) class Foo Foo Foo yes class << Foo Foo’s Foo’s yes metaclass metaclass
  71. 71. Meta-programming How to write code to write code?
  72. 72. Two types of meta- programming • Code Generation (Not talk about it today) • eg. Rails scaffold • Reflection • eg. Class Macro (talk later)
  73. 73. Specifically, How to write a method to define method?
  74. 74. class Demo # the current class is Demo def define_blah1 # the current class is Demo def blah1 puts "blah1" end end def self.define_blah2 # the current class is Demo (the same as above) def blah2 puts "blah2" end end end Demo.new.blah1 # NoMethodError: undefined method `blah1' Demo.new.define_blah1 Demo.new.blah1 # blah1 Demo.new.blah2 # NoMethodError: undefined method `blah2' Demo.define_blah2 Demo.new.blah2 #blah2
  75. 75. And how to write a method to define singleton method?
  76. 76. class Demo def define_blah1 # self is Demo's instance def self.blah1 puts "blah1" # define singleton method end end def self.define_blah2 #self is Demo def self.blah2 puts "blah2" # define singleton method (class method) end end end a = Demo.new a.define_blah1 a.blah1 # blah1 Demo.new.blah1 # NoMethodError: undefined method `blah1' Demo.new.blah2 # NoMethodError: undefined method `blah2' Demo.define_blah2 Demo.blah2 #blah2
  77. 77. Not useful really, we need more dynamic power
  78. 78. The key of power is variable scope!!
  79. 79. Variable scope { module MyDemo var = 1 { class Demo var = 2 { def foo var = 3 end end end
  80. 80. block variable scope var1 = "foo" [1,2,3].each do |i| puts var var1 = "bar" var2 = "baz" end puts var1 # foo puts var2 # NameError: undefined local variable or method
  81. 81. define_method unlike “def”, you can access outside variable!! class Demo # define instance methods ["aaa", "bbb", "ccc"].each do |name| define_method(name) do puts name.upcase end end # define class methods class << self ["xxx", "yyy", "zzz"].each do |name| define_method(name) do puts name.upcase end end end end Demo.new.aaa # AAA Demo.new.bbb # BBB Demo.yyy # YYY Demo.zzz # ZZZ
  82. 82. define_method will defines instance method for current object (it’s must be class object or module)
  83. 83. class Class def define_more_methods # define instance methods ["aaa", "bbb", "ccc"].each do |name| Use class define_method(name) do puts name.upcase end end method to # define class methods class << self define methods ["xxx", "yyy", "zzz"].each do |name| define_method(name) do puts name.upcase (global) end end end end end class Demo define_more_methods end
  84. 84. module Mixin module ClassMethods def define_more_methods # define instance methods ["aaa", "bbb", "ccc"].each do |name| define_method(name) do Use class puts name.upcase end end method to # define class methods class << self ["xxx", "yyy", "zzz"].each do |name| define methods define_method(name) do puts name.upcase end (localized) end end end end end class Demo extend Mixin::ClassMethods define_more_methods end
  85. 85. So unlike “def”, define_method will not create new scope
  86. 86. So we maybe need those methods inside define_method: (because the scope is not changed) • Object#instance_variable_get • Object#instance_variable_set • Object#remove_instance_variable • Module#class_variable_get • Module#class_variable_set • Module#remove_class_variable
  87. 87. Not dynamic enough? Because class << will create new scope still!!
  88. 88. we need even more dynamic power
  89. 89. var = 1 String.class_eval do puts var # 1 puts self # the current object is String # the current class is String def foo puts "foo" end def self.bar puts "bar" end class_eval (only for class object or module) class << self def baz puts "baz" end end end "abc".foo # foo String.bar # bar String.baz # baz
  90. 90. class_eval + define_method name = "say" var = "it’s awesome" String.class_eval do define_method(name) do puts var end end "ihower".say # it’s awesome
  91. 91. But how to define singleton method using class_eval and define_method?
  92. 92. Wrong! name = "foo" var = "bar" String.class_eval do class << self define_method(name) do puts var end end end # ArgumentError: interning empty string # we can not get name and var variable, because class << create new scope
  93. 93. Fixed! you need find out metaclass and class_eval it! name = "foo" var = "bar" metaclass = (class << String; self; end) metaclass.class_eval do define_method(name) do puts var end end String.foo # bar
  94. 94. How about apply to any object? (because class_eval only works on class object or module)
  95. 95. instance_eval for any object obj = "blah" obj.instance_eval do puts self # obj # the current class is obj's metaclass def foo puts "foo" end end obj.foo # singleton method
  96. 96. how about class object? String.instance_eval do puts self # String # the current class is String's metaclass def foo puts "bar" end end String.foo # singleton method (class method)
  97. 97. method definition mechanism self (current object) new scope? (current_class) class Foo Foo Foo yes class << Foo Foo’s metaclass Foo’s metaclass yes Foo.class_eval Foo Foo no Foo.instance_eval Foo Foo’s metaclass no
  98. 98. 8. Class Macro (Ruby’s declarative style)
  99. 99. ActiveRecord example class User < ActiveRecord::Base validates_presence_of :login validates_length_of :login, :within => 3..40 validates_presence_of :email belongs_to :group has_many :posts end
  100. 100. Class Bodies Aren’t Special class Demo a = 1 puts a def self.say puts "blah" end say # you can execute class method in class body end # 1 # blah
  101. 101. Memorize example class Account def calculate @calculate ||= begin sleep 10 # expensive calculation 5 end end end a = Account.new a.caculate # need waiting 10s to get 5 a.caculate # 5 a.caculate # 5 a.caculate # 5
  102. 102. memoize method class Account def calculate sleep 2 # expensive calculation 5 end end memoize :calculate end a = Account.new a.calculate # need waiting 10s to get 5 a.calculate # 5
  103. 103. class Class def memoize(name) original_method = "_original_#{name}" alias_method :"#{original_method}", name define_method name do cache = instance_variable_get("@#{name}") if cache return cache else result = send(original_method) # Dynamic Dispatches instance_variable_set("@#{name}", result) return result end end end end
  104. 104. It’s general for any class class Car def run sleep 100 # expensive calculation “done” end end memoize :run end c = Car.new c.run # need waiting 100s to get done c.run # done
  105. 105. BTW, how to keep original method and call it later? • alias_method • most common way • example above • method binding • can avoid to add new method • example in const_missing
  106. 106. 9.instance_eval DSL calls it create implicit context
  107. 107. Rack example Rack::Builder.new do use Some::Middleware, param use Some::Other::Middleware run Application end
  108. 108. How is instance_eval doing? • It changes the “self” (current object) to caller • Any object can call instance_eval (unlike class_eval)
  109. 109. a trivial example class Demo def initialize @a = 99 end end foo = Demo.new foo.instance_eval do puts self # foo instance puts @a # 99 end
  110. 110. instance_eval with block class Foo attr_accessor :a,:b def initialize(&block) instance_eval &block end def use(name) # do some setup end end bar = Foo.new do self.a = 1 self.b = 2 use "blah" use "blahblah" end
  111. 111. Strings of Code eval*() family can accept string of code • No editor’s syntax highlight • Not report syntax error until be evaluated (in runtime!) • Security problem
  112. 112. 10.Class.new
  113. 113. anonymous class klass = Class.new def move_left puts "left" end def move_right puts "right" end end object = klass.new object.move_left # left Car = klass # naming it Car car = Car.new car.move_right # right
  114. 114. variable scope matters you can access outside variable var = "it’s awesome" klass = Class.new puts var def my_method puts var # undefined local variable or method `var' end end puts klass.new.my_method
  115. 115. var = "it’s awesome" klass = Class.new do puts var define_method :my_method do puts var end end puts klass.new.my_method # it’s awesome # it’s awesome
  116. 116. Subclassing with a generator using Class.new def disable_string_class(method_name) Class.new(String) do undef_method method_name end end klass1 = disable_string_class(:reverse) a = klass1.new("foobar") a.reverse # NoMethodError klass2 = disable_string_class(:upcase) b = klass2.new("foobar") b.upcase # NoMethodError
  117. 117. Parameterized subclassing Camping example module Camping::Controllers class Edit < R '/edit/(d+)' def get(id) # ... end end end
  118. 118. Parameterized subclassing example def Person(name) if name == "ihower" Class.new do def message puts "good" end end else Class.new do def message puts "bad" end end end end
  119. 119. class Foo < Person 'ihower' def name "foo" end end class Bar < Person 'not_ihower' def name "bar" end end f = Foo.new f.message # good! b = Bar.new b.message # bad!
  120. 120. Conclusion
  121. 121. Story 1: DSL or NoDSL by José Valim at Euruko 2010 http://blog.plataformatec.com.br/2010/06/dsl-or-nodsl-at-euruko-2010/
  122. 122. a DSL class ContactForm < MailForm::Base to "jose.valim@plataformatec.com.br" from "contact_form@app_name.com" subject "Contact form" attributes :name, :email, :message end ContactForm.new(params[:contact_form]).deliver
  123. 123. DSL fails class ContactForm < MailForm::Base to :author_email from { |c| "#{c.name} <#{c.email}>" } subject "Contact form" headers { |c| { "X-Spam" => "True" } if c.honey_pot } attributes :name, :email, :message def author_email Author.find(self.author_id).email end end
  124. 124. NoDSL class ContactForm < MailForm::Base attributes :name, :email, :message def headers { :to => author_email, :from => "#{name} <#{email}>", :subject => "Contact form" } end def author_email Author.find(self.author_id).email end end
  125. 125. Other examples • Rake v.s. Thor • RSpec v.s. Unit::test
  126. 126. Rake example task :process do # do some processing end namespace :app do task :setup do # do some setup Rake::Task[:process].invoke end end rake app:setup
  127. 127. Thor example class Default < Thor def process # do some processing end end class App < Thor def setup # do some setup Default.new.process end end thor app:setup
  128. 128. José’s Conclusion • DSL or NoDSL - don't actually have answer • Replace "It provides a nice DSL" with "it relies on a simple contract".
  129. 129. Story 2: Rails 2 to 3 API changes
  130. 130. Routes nice DSL # Rails 2 map.resources :people, :member => { :dashboard => :get, :resend => :post, :upload => :put } do |people| people.resource :avatra end # Rails 3 resources :people do resource :avatar member do get :dashboard post :resend put :upload end end
  131. 131. AR queries (1) method chaining # Rails 2 users = User.find(:all, :conditions => { :name => 'ihower' }, :limit => 10, :order => 'age') # Rails 3 users = User.where(:name => 'ihower').limit(20).order('age')
  132. 132. AR queries (2) Unify finders, named_scope, with_scope to Relation # Rails 2 users = User users = users.some_named_scope if params[:some] sort = params[:sort] || "id" conditions = {} if params[:name] conditions = User.merge_conditions( conditions, { :name => params[:name] } ) end if params[:age] conditions = User.merge_conditions( conditions, { :age => params[:age] } ) end find_conditions = { :conditions => conditions, :order => "#{sort} #{dir}" } sort = params[:sort] || "id" users = users.find(:all, :conditions => conditions, :order => sort )
  133. 133. AR queries (2) Unify finders, named_scope, with_scope to Relation # Rails 3 users = User users = users.some_scope if params[:some] users = users.where( :name => params[:name] ) if params[:name] users = users.where( :age => params[:age] ) if params[:age] users = users.order( params[:sort] || "id" )
  134. 134. AR queries (3) Using class methods instead of scopes when you need lambda # Rails 3 class Product < ActiveRecord::Base scope :discontinued, where(:discontinued => true) scope :cheaper_than, lambda { |price| where("price < ?", price) } end # Rails 3, prefer this way more class Product < ActiveRecord::Base scope :discontinued, where(:discontinued => true) def self.cheaper_than(price) where("price < ?", price) end end
  135. 135. AR validation (1) # Rails 2 class User < ActiveRecord::Base validates_presence_of :email validates_uniqueness_of :email validates_format_of :email, :with => /^[wd]+$/ :on => :create, :message => "is invalid" end # Rails 3 class User < ActiveRecord::Base validates :email, :presence => true, :uniqueness => true, :format => { :with => /^([^@s]+)@((?:[-a-z0-9]+.)+[a-z]{2,})$/i } end http://asciicasts.com/episodes/211-validations-in-rails-3
  136. 136. AR validation (2) custom validator # Rails 3 class User < ActiveRecord::Base validates :email, :presence => true, :uniqueness => true, :email_format => true end class EmailFormatValidator < ActiveModel::EachValidator def validate_each(object, attribute, value) unless value =~ /^([^@s]+)@((?:[-a-z0-9]+.)+[a-z]{2,})$/i object.errors[attribute] << (options[:message] || "is not formatted properly") end end end
  137. 137. ActionMailer # Rails 2 class UserMailer < ActionMailer::Base def signup(user) recipients user.email from 'ihower@gmail.com' body :name => user.name subject "Signup" end end
  138. 138. ActionMailer # Rails 3 class UserMailer < ActionMailer::Base default :from => "ihower@gmail.com" def signup(user) @name = user.name mail(:to => user.email, :subject => "Signup" ) end end UserMailer.registration_confirmation(@user).deliver
  139. 139. I think it’s a Ruby APIs paradigm shift Apparently, Rails is the most successful Ruby open source codebase which you can learn from.
  140. 140. References • Ruby Best Practices, O’Reilly • The Ruby Object Model and Metaprogrammin, Pragmatic • Programming Ruby 1.9, Pragmatic • Metaprogramming Ruby, Pragmatic • Advanced Rails, O’Reilly • The Building Blocks of Ruby http://yehudakatz.com/2010/02/07/the-building-blocks-of-ruby/ • The Importance of Executable Class Bodies http://yehudakatz.com/2009/06/04/the-importance-of-executable-class-bodies/ • Metaprogramming in Ruby: It’s All About the Self http://yehudakatz.com/2009/11/15/metaprogramming-in-ruby-its-all-about-the-self/ • Three implicit contexts in Ruby http://yugui.jp/articles/846
  141. 141. The End
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