Practical Metaprogramming in Application

RubyKaigi 2014 talk. Points for *practical* use of metaprogramming in Ruby.

Transcript

1. Practical Metaprogramming
   in Application
   Kyosuke MOROHASHI (@moro)
   @2014-09-20 RubyKaigi 2014
   

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2. ॾڮګհ(@moro)
   Kyosuke MOROHASHI
   

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5. https://idobata.io/
   

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6. Practical Metaprogramming
   in Application
   MOROHASHI Kyosuke (@moro)
   @2014-09-20 RubyKaigi 2014
   

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7. GOAL of the talk
   Feel close to Ruby's
   metaprogramming.
   Understand practical use of
   metaprogramming.
   

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8. What is
   metaprogramming ?
   

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9. “Metaprogramming is
   writing code that
   manipulates language
   constructs at
   runtime
   Metaprogramming Ruby 2
   

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10. Ruby constructs
    

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11. class / module
    object (& its state, @ivar)
    method
    a series of procedures
    

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12. class Book
    def title
    @title
    end
    def title=(title)
    @title = title
    end
    end
    

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13. class Book
    def title
    @title
    end
    def title=(title)
    @title = title
    end
    end
    

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14. You will think...
    Is there more efficient way
    using repeated 'title' by
    manipulating language
    constructs ?
    

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15. class Book
    attr_accessor :title
    end
    

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16. Just example, real code is written in C.
    def attr_accessor(name)
    define_method(name) do
    instance_variable_get("@#{name}")
    end
    define_method("#{name}=") do |val|
    instance_variable_set("@#{name}", val)
    end
    end
    

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17. define_method(:title) do
    instance_variable_get("@title")
    end
    define_method("title=") do |val|
    instance_variable_set("@title", val)
    end
    

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18. Define methods dynamically.
    define_method(:title) do
    instance_variable_get("@title")
    end
    define_method("title=") do |val|
    instance_variable_set("@title", val)
    end
    

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19. Manipulate @ivar based on its name
    define_method(:title) do
    instance_variable_get("@title")
    end
    define_method("title=") do |val|
    instance_variable_set("@title", val)
    end
    

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20. “
    the ability to examine and modify
    the structure and behavior of the
    program at runtime
    Extracted from http://en.wikipedia.org/wiki/Reflection_(computer_programming)
    Reflection APIs
    

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21. Metaprogramming is
    so familiar in Ruby
    

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22. Sum
    m
    ary
    What is metaprogramming ?
    Writing code that manipulates
    language constructs
    So familiar in Ruby
    Rubyists are using them
    everyday
    

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23. Practical
    Metaprogramming
    

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24. Pitfall
    of
    Metaprogramming
    

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25. DON'T READ: Overuse of metaprogramming
    Book = Class.new do
    define_method(:initialize) do |attrs|
    attrs.each do |key, value|
    if respond_to?("#{key}=")
    send("#{key}=", value)
    else
    instance_variable_set("@#{key}", value)
    end
    end
    end
    define_method(:price=) do |price_str|
    instance_variable_set(
    '@price',
    price_str.delete(',').to_i
    )
    end
    end
    

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26. Standard Ruby code
    class Book
    def initialize(attrs)
    @author = attrs[:author]
    self.price = attrs[:price]
    end
    def price=(price_str)
    @price = Integer(price_str.delete(','))
    end
    end
    Book.new(author: 'moro', price: '1,980')
    #=> #@price=1980>
    

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27. Metaprogramming with
    overuse of reflection API
    tends to be complicated.
    

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28. You will think...
    Is there any way to write
    simple code with
    metaprogramming?
    

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29. Extract code from
    'meta' aspect
    &
    write code simply
    

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30. ActiveRecord::Base.has_many assoc_name
    Specifies a one-to-many association named
    assoc_name.
    

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31. Many methods are added
    class Books < ActiveRecord::Base
    has_many :reviews
    end
    Book#reviews
    #reviews=
    #review_ids
    #review_ids=
    ...
    Book#reviews.build
    reviews.create
    reviews.each {|review| ... }
    reviews.where(...)
    

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32. def has_many(name, opts = {})
    class_eval <<-RUBY
    def #{name}
    klass = #{name.to_s.classify}
    klass.where(#{fk}: id)
    end
    def #{name}=(value)
    values.each do |value|
    #{name.to_s.classify}.create!(...)
    end
    end
    RUBY
    en
    Seems difficult internal (not actual code)
    

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33. ActiveRecord::Base.has_many assoc_name
    Define wrapper methods to access
    `association object` that represents
    a one-to-many association.
    

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34. Book#reviews
    Returns an association object
    that represents a book
    has_many reviews.
    NOT returns reviews directly
    

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35. def define_readers
    mixin.class_eval <<-CODE, __FILE__, __LINE__ +
    def #{name}(*args)
    association(:#{name}).reader(*args)
    end
    CODE
    end
    Returns association object.
    

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36. Has owner and target
    Define join relations on
    owner.pk = target.fk
    Load target & cache them
    Manage create/delete and
    callbacks.
    Association Object
    

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37. = Active Record Associations
    This is the root class of all associations
    ('+ Foo' signifies an included module Foo):
    Association
    SingularAssociation
    HasOneAssociation
    HasOneThroughAssociation
    + ThroughAssociation
    BelongsToAssociation
    BelongsToPolymorphicAssociation
    CollectionAssociation
    HasAndBelongsToManyAssociation
    HasManyAssociation
    HasManyThroughAssociation
    + ThroughAssociation
    

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38. Returns "association
    object" instead of children
    directly.
    Then, what is useful API to
    access association?
    

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39. Define thin wrapper with reflection APIs.
    def define_readers
    mixin.class_eval <<-CODE, __FILE__, __LINE__ +
    def #{name}(*args)
    association(:#{name}).reader(*args)
    end
    CODE
    end
    

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40. Sum
    m
    ary
    Points for
    Practical metaprogramming
    Extract code from 'meta' aspect
    & write code simply
    Define thin wrapper with
    reflection APIs.
    

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41. 1 more example for
    Extraction from
    meta aspect
    

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42. Post/Comment/Photo
    Censoring system
    

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43. Censors post, comment and photo.
    Submit each content to external
    censoring service API.
    Post:
    title + body
    Comment:
    body
    Photo:
    public URL
    

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44. class Post < AR::Base
    after_save :submit_to_censoring_service
    private
    def submit_to_censoring_service
    content = [title, body].join("\n\n")
    url = Rails.config.censoring_service_url
    req = Net::HTTP::Post.new(url.path)
    req.set_form_data(id: to_global_id, text: conte
    Net::HTTP.start(url.hostname, url.port) do |htt
    http.request(req)
    end
    end
    

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45. class Comment < AR::Base
    ...
    def submit_to_censoring_service
    content = body
    ...
    end
    end
    class Photo < AR::Base
    ...
    def submit_to_censoring_service
    content = "http://img.example.com/#{id}"
    ...
    end
    end
    

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46. You will think...
    First of all,
    extract obvious
    duplications.
    

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47. class Post < AR::Base
    after_save :submit_to_censoring_service
    private
    def submit_to_censoring_service
    content = [title, body].join("\n\n")
    url = Rails.config.censoring_endpoint
    req = Net::HTTP::Post.new(url.path)
    req.set_form_data(id: to_global_id, text: conte
    Net::HTTP.start(url.hostname, url.port) do |htt
    http.request(req)
    end
    end
    

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48. class Post < AR::Base
    after_save :submit_to_censoring_service
    private
    def submit_to_censoring_service
    CensorRequest.new(
    id: to_global_id,
    text: [title, body].join("\n\n")
    ).submit
    end
    end
    

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49. class CensorRequest
    @@url = URI(Rails.config.censoring_endpoint)
    def initialize(content)
    @content = content
    end
    def submit
    req = Net::HTTP::Post.new(@@url.path)
    req.set_form_data(content)
    Net::HTTP.start(@@url.hostname, @@url.port) do
    http.request(req)
    end
    end
    end
    

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50. class Post < AR::Base
    after_save :submit_to_censoring_service
    private
    def submit_to_censoring_service
    CensorRequest.new(
    id: to_global_id,
    text: [title, body].join("\n\n")
    ).submit
    end
    end
    

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51. You will think...
    Extract code from 'meta' aspect
    There should be a class
    which builds content per
    each censored class &
    submits it to the service ?
    

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52. class ContentCensor
    def initialize(type, &builder)
    @type = type
    @builder = builder
    end
    def submit(record)
    CensorRequest.new(
    id: record.global_id,
    @type => @builder.call(record)
    ).submit
    end
    end
    # ----------------
    censor = ContentCensor.new(:text) do |post|
    [post.title, post.body].join("\n\n")
    end
    censor.submit(post)
    

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53. class Post < AR::Base
    @@censor = ContentCensor.new(:text) do |post|
    [post.title, post.body].join("\n\n")
    end
    after_save :submit_to_censoring_service
    private
    def submit_to_censoring_service
    @@censor.submit(self)
    end
    end
    

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54. class Comment < AR::Base
    @@censor = ContentCensor.new(:text) do |comment|
    comment.body
    end
    after_save :submit_to_censoring_service
    private
    def submit_to_censoring_service
    @@censor.submit(self)
    end
    end
    

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55. class Photo < AR::Base
    @@censor = ContentCensor.new(:image) do |photo|
    "http://img.example.com/#{photo.id}"
    end
    after_save :submit_to_censoring_service
    private
    def submit_to_censoring_service
    @@censor.submit(self)
    end
    end
    

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56. You will think...
    Define thin wrapper with reflection APIs.
    Does extraction of
    ContentCensor call make
    the code cleaner ?
    

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57. module CensorDsl
    def censor_content(type, &block)
    censor = CensorContent.new(type, block)
    after_save {|record| censor.submit(record) }
    end
    end
    

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58. class Post < AR::Base
    extend CensorDsl
    censor_content(:text) do |post|
    [post.title, post.body].join("\n\n")
    end
    end
    

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59. You will think...
    protip:
    Try to clean up codes like
    Rails framework with
    ActiveSupport::Concern
    

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60. module Censorable
    extend ActiveSupport::Concern
    module ClassMethods
    def censor_content(type, &block)
    censor = Censorable::Censor.new(type, block)
    after_save {|record| censor.submit(record) }
    end
    end
    class Censor
    ...
    class Request
    ...
    end
    

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61. class Post < ActiveRecord::Base
    include Censorable
    censor_content(:text) do |post|
    [post.title, post.body].join("\n\n")
    end
    end
    

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62. Before
    class Post < ActiveRecord::Base
    after_save :submit_to_censoring_service
    private
    def submit_to_censoring_service
    content = [title, body].join("\n\n")
    url = Rails.config.censoring_service_url
    req = Net::HTTP::Post.new(url.path)
    req.set_form_data(id: to_global_id, text: conte
    Net::HTTP.start(url.hostname, url.port) do |htt
    http.request(req)
    end
    end
    

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63. After
    class Post < ActiveRecord::Base
    include Censorable
    censor_content(:text) do |post|
    [post.title, post.body].join("\n\n")
    end
    end
    

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64. Sum
    m
    ary
    Points for
    Practical metaprogramming
    Extract code from 'meta' aspect
    & write code simply
    Define thin wrapper with
    reflection APIs.
    REVISED
    

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65. Conclusion
    

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66. GOAL of the talk
    Feel closer to Ruby's
    metaprogramming.
    Understand practical use of
    metaprogramming.
    

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67. There isn't so much difference
    between standard programming
    and metaprogramming while we
    code in Ruby and/or Rails.
    

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68. It's helpful to extract code from
    'meta' aspect & code simply to
    avoid complicated code by over
    use of metaprogramming.
    

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69. But, I know it's so fun to use reflection APIs
    that I recommend to practice extreme
    metaprogramming :-)
    

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70. You should extract code from
    'meta' aspects step by step.
    Ruby's flexibility helps you.
    

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71. “Ruby͸܅Λ৴པ͢ΔɻRuby͸܅Λ
    ෼ผͷ͋ΔϓϩάϥϚͱͯ͠ѻ͏ɻ
    Ruby͸ϝλϓϩάϥϛϯάͷΑ͏ͳ
    ڧྗͳྗΛ༩͑Δɻͨͩ͠ɺେ͍ͳ
    Δྗʹ͸ɺେ͍ͳΔ੹೚͕൐͏͜ͱ
    Λ๨Εͯ͸͍͚ͳ͍ɻͦΕͰ͸ɺ
    RubyͰͨͷ͍͠ϓϩάϥϛϯάΛɻ
    ʮϝλϓϩάϥϛϯάRubyʯ ংจΑΓ
    Matz says:
    

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