Refactoring Ruby with Monads

Monads are in danger of becoming a bit of a joke: for every person who raves about them, there’s another person asking what in the world they are, and a third person writing a confusing tutorial about them. With their technical-sounding name and forbidding reputation, monads can seem like a complex, abstract idea that’s only relevant to mathematicians and Haskell programmers.

Forget all that! In this pragmatic talk we roll up our sleeves and get stuck into refactoring some awkward Ruby code, using the good parts of monads to tackle the problems we encounter along the way. We see how the straightforward design pattern underlying monads can help us to make our code simpler, clearer and more reusable by uncovering its hidden structure, and we all leave with a shared understanding of what monads actually are and why people won’t shut up about them.

Given at eurucamp 2014 (http://2014.eurucamp.org/), BaRuCo 2014 (http://lanyrd.com/2014/baruco/) and GoGaRuCo 2014 (http://lanyrd.com/2014/gogaruco/). The accompanying code is at https://github.com/tomstuart/monads, and an article version is available at https://tomstu.art/refactoring-ruby-with-monads.

Transcript

1. REFACTORING RUBY
   WITH MONADS
   @tomstuart • GoGaRuCo • 2014-09-19
   

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2. WARM UP
   

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3. ANALOGIES
   

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4. WHAT’S A
   STACK?
   

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5. A KIND OF VALUE…
   

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6. WITH CERTAIN
   OPERATIONS
   …
   …
   

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7. #push(value)
   #pop
   #top
   #empty?
   .empty
   

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8. THAT FOLLOW
   CERTAIN RULES
   …
   

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9. stack.push(value).top == value
   stack.push(value).pop == stack
   empty.empty? == truee
   stack.push(value).empty? == false
   

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10. WE CAN
    IMPLEMENT THEM
    HOWEVER WE LIKE
    

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11. ArrayStack = Struct.new(:values) do
    def push(value)
    ArrayStack.new([value] + values)
    end
    !
    def top
    values.first
    end
    !
    def pop
    ArrayStack.new(values.drop(1))
    end
    !
    def empty?
    values.empty?
    end
    !
    def self.empty
    new([])
    end
    end
    

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12. LinkedListStack = Struct.new(:top, :pop) do
    def push(value)
    LinkedListStack.new(value, self)
    end
    !
    # def top
    # # we get this for free!
    # end
    !
    # def pop
    # # and this!
    # end
    !
    def empty?
    pop.nil?
    end
    !
    def self.empty
    new(nil, nil)
    end
    end
    

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13. WE CAN USE THEM
    WITHOUT KNOWING
    THEIR IMPLEMENTATION
    

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14. >> stack = ArrayStack.empty
    => #
    >> stack.push('hello').push('world').pop.top
    => "hello"
    >> stack = LinkedListStack.empty
    => #
    >> stack.push('hello').push('world').pop.top
    => "hello"
    

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15. WE CAN
    DEFINE MORE
    OPERATIONS
    

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16. module Stack
    def size
    if empty?
    0
    else
    pop.size + 1
    end
    end
    end
    !
    ArrayStack.include(Stack)
    LinkedListStack.include(Stack)
    

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17. >> ArrayStack.empty.size
    => 0
    >> ArrayStack.empty.push('hello').push('world').size
    => 2
    >> LinkedListStack.empty.size
    => 0
    >> LinkedListStack.empty.push('hello').push('world').size
    => 2
    

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18. OPERATIONS
    &
    RULES
    

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19. COMMON INTERFACE
    &
    SHARED FUNCTIONALITY
    

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20. WHAT’S A
    COLLECTION?
    

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21. A KIND OF VALUE…
    

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22. WITH CERTAIN
    OPERATIONS
    …
    …
    

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23. #each
    

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24. THAT FOLLOW
    CERTAIN RULES
    …
    

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25. #each
    calls a block with a value
    zero or more times
    in immediate sequence
    

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26. WE CAN
    IMPLEMENT IT
    HOWEVER WE LIKE
    

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27. class HardcodedCollection
    def each(&block)
    block.call(1)
    block.call(2)
    block.call(3)
    block.call(4)
    block.call(5)
    end
    end
    

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28. class GeneratedCollection
    def each(&block)
    number = 1
    while number <= 5
    block.call(number)
    number = number + 1
    end
    end
    end
    

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29. WE CAN USE IT
    WITHOUT KNOWING
    ITS IMPLEMENTATION
    

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30. >> collection = HardcodedCollection.new
    => #
    >> collection.each { |n| puts n }
    1
    2
    3
    4
    5
    => nil
    >> collection = GeneratedCollection.new
    => #
    >> collection.each { |n| puts n }
    1
    2
    3
    4
    5
    => nil
    

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31. WE CAN
    DEFINE MORE
    OPERATIONS
    

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32. module Collection
    def select(&block)
    results = []
    each do |value|
    results << value if block.call(value)
    end
    results
    end
    end
    !
    HardcodedCollection.include(Collection)
    GeneratedCollection.include(Collection)
    

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33. >> HardcodedCollection.new.select { |n| n.odd? }
    => [1, 3, 5]
    !
    >> GeneratedCollection.new.select { |n| n.odd? }
    => [1, 3, 5]
    

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34. OPERATIONS
    &
    RULES
    

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35. COMMON INTERFACE
    &
    SHARED FUNCTIONALITY
    

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36. WHAT ARE
    STACKS &
    COLLECTIONS?
    

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37. ABSTRACT DATA TYPE
    

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38. (ABSTRACT,
    BUT GOOD!)
    

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39. LET’S DO SOME
    REFACTORING!
    

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40. HANDLING NIL
    

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41. Project = Struct.new(:creator)
    Person = Struct.new(:address)
    Address = Struct.new(:country)
    Country = Struct.new(:capital)
    City = Struct.new(:weather)
    

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42. def weather_for(project)
    project.creator.address.
    country.capital.weather
    end
    

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43. >> city = City.new('sunny')
    => #
    !
    >> country = Country.new(city)
    => #
    !
    >> address = Address.new(country)
    => #
    !
    >> person = Person.new(address)
    => #
    !
    >> project = Project.new(person)
    => #
    

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44. >> weather_for(project)
    => "sunny"
    

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45. >> bad_project = Project.new(Person.new(Address.new(nil)))
    => #
    !
    >> weather_for(bad_project)
    NoMethodError: undefined method `capital' for nil:NilClass
    

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46. def weather_for(project)
    project.creator.address.
    country.capital.weather
    end
    

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47. def weather_for(project)
    creator = project.creator
    address = creator.address
    country = address.country
    capital = country.capital
    weather = capital.weather
    end
    

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48. def weather_for(project)
    unless project.nil?
    creator = project.creator
    unless creator.nil?
    address = creator.address
    unless address.nil?
    country = address.country
    unless country.nil?
    capital = country.capital
    unless capital.nil?
    weather = capital.weather
    end
    end
    end
    end
    end
    end
    

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49. def weather_for(project)
    unless project.nil?
    creator = project.creator
    end
    !
    unless creator.nil?
    address = creator.address
    end
    !
    unless address.nil?
    country = address.country
    end
    !
    unless country.nil?
    capital = country.capital
    end
    !
    unless capital.nil?
    weather = capital.weather
    end
    end
    

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50. # activesupport/lib/active_support/core_ext/object/try.rb
    !
    class Object
    def try(*a, &b)
    if a.empty? && block_given?
    yield self
    else
    public_send(*a, &b) if respond_to?(a.first)
    end
    end
    end
    !
    class NilClass
    def try(*args)
    nil
    end
    end
    

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51. def weather_for(project)
    unless project.nil?
    creator = project.creator
    end
    !
    unless creator.nil?
    address = creator.address
    end
    !
    unless address.nil?
    country = address.country
    end
    !
    unless country.nil?
    capital = country.capital
    end
    !
    unless capital.nil?
    weather = capital.weather
    end
    end
    

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52. def weather_for(project)
    creator = project.try(:creator)
    address = creator.try(:address)
    country = address.try(:country)
    capital = country.try(:capital)
    weather = capital.try(:weather)
    end
    

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53. def weather_for(project)
    project.
    try(:creator).
    try(:address).
    try(:country).
    try(:capital).
    try(:weather)
    end
    

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54. # activesupport/lib/active_support/core_ext/object/try.rb
    !
    class Object
    def try(*a, &b)
    if a.empty? && block_given?
    yield self
    else
    public_send(*a, &b) if respond_to?(a.first)
    end
    end
    end
    !
    class NilClass
    def try(*args)
    nil
    end
    end
    

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55. class Object
    remove_method :try
    end
    !
    class NilClass
    remove_method :try
    end
    

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56. Optional = Struct.new(:value)
    

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57. >> optional_string = Optional.new('hello')
    => #
    >> optional_string.value
    => "hello"
    >> optional_string = Optional.new(nil)
    => #
    >> optional_string.value
    => nil
    

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58. class Optional
    def try(*args, &block)
    if value.nil?
    nil
    else
    value.public_send(*args, &block)
    end
    end
    end
    

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59. >> optional_string = Optional.new('hello')
    => #
    >> length = optional_string.try(:length)
    => 5
    >> optional_string = Optional.new(nil)
    => #
    >> length = optional_string.try(:length)
    => nil
    

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60. def weather_for(project)
    creator = project.try(:creator)
    address = creator.try(:address)
    country = address.try(:country)
    capital = country.try(:capital)
    weather = capital.try(:weather)
    end
    

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61. def weather_for(project)
    optional_project = Optional.new(project)
    optional_creator = Optional.new(optional_project.try(:creator))
    optional_address = Optional.new(optional_creator.try(:address))
    optional_country = Optional.new(optional_address.try(:country))
    optional_capital = Optional.new(optional_country.try(:capital))
    optional_weather = Optional.new(optional_capital.try(:weather))
    weather = optional_weather.value
    end
    

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62. class Optional
    def try(*args, &block)
    if value.nil?
    nil
    else
    value.public_send(*args, &block)
    end
    end
    end
    

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63. class Optional
    def try(&block)
    if value.nil?
    nil
    else
    block.call(value)
    end
    end
    end
    

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64. def weather_for(project)
    optional_project = Optional.new(project)
    optional_creator = Optional.new(optional_project.try(:creator))
    optional_address = Optional.new(optional_creator.try(:address))
    optional_country = Optional.new(optional_address.try(:country))
    optional_capital = Optional.new(optional_country.try(:capital))
    optional_weather = Optional.new(optional_capital.try(:weather))
    weather = optional_weather.value
    end
    

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65. def weather_for(project)
    optional_project = Optional.new(project)
    optional_creator = optional_project.try { |project| Optional.new(project.creator) }
    optional_address = optional_creator.try { |creator| Optional.new(creator.address) }
    optional_country = optional_address.try { |address| Optional.new(address.country) }
    optional_capital = optional_country.try { |country| Optional.new(country.capital) }
    optional_weather = optional_capital.try { |capital| Optional.new(capital.weather) }
    weather = optional_weather.value
    end
    

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66. >> weather_for(project)
    => "sunny"
    !
    >> weather_for(bad_project)
    NoMethodError: undefined method `capital' for nil:NilClass
    

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67. class Optional
    def try(&block)
    if value.nil?
    nil
    else
    block.call(value)
    end
    end
    end
    

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68. class Optional
    def try(&block)
    if value.nil?
    Optional.new(nil)
    else
    block.call(value)
    end
    end
    end
    

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69. >> weather_for(project)
    => "sunny"
    !
    >> weather_for(bad_project)
    => nil
    

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70. def weather_for(project)
    optional_project = Optional.new(project)
    optional_creator = optional_project.try { |project| Optional.new(project.creator) }
    optional_address = optional_creator.try { |creator| Optional.new(creator.address) }
    optional_country = optional_address.try { |address| Optional.new(address.country) }
    optional_capital = optional_country.try { |country| Optional.new(country.capital) }
    optional_weather = optional_capital.try { |capital| Optional.new(capital.weather) }
    weather = optional_weather.value
    end
    

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71. class Optional
    def try(&block)
    if value.nil?
    Optional.new(nil)
    else
    block.call(value)
    end
    end
    end
    

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72. class Optional
    def and_then(&block)
    if value.nil?
    Optional.new(nil)
    else
    block.call(value)
    end
    end
    end
    

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73. def weather_for(project)
    optional_project = Optional.new(project)
    optional_creator = optional_project.and_then { |project| Optional.new(project.creator) }
    optional_address = optional_creator.and_then { |creator| Optional.new(creator.address) }
    optional_country = optional_address.and_then { |address| Optional.new(address.country) }
    optional_capital = optional_country.and_then { |country| Optional.new(country.capital) }
    optional_weather = optional_capital.and_then { |capital| Optional.new(capital.weather) }
    weather = optional_weather.value
    end
    

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74. def weather_for(project)
    Optional.new(project).
    and_then { |project| Optional.new(project.creator) }.
    and_then { |creator| Optional.new(creator.address) }.
    and_then { |address| Optional.new(address.country) }.
    and_then { |country| Optional.new(country.capital) }.
    and_then { |capital| Optional.new(capital.weather) }.
    value
    end
    

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75. class Optional
    def method_missing(*args, &block)
    and_then do |value|
    Optional.new(value.public_send(*args, &block))
    end
    end
    end
    

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76. def weather_for(project)
    Optional.new(project).
    and_then { |project| Optional.new(project.creator) }.
    and_then { |creator| Optional.new(creator.address) }.
    and_then { |address| Optional.new(address.country) }.
    and_then { |country| Optional.new(country.capital) }.
    and_then { |capital| Optional.new(capital.weather) }.
    value
    end
    

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77. def weather_for(project)
    Optional.new(project).
    creator.
    address.
    country.
    capital.
    weather.
    value
    end
    

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78. def weather_for(project)
    Optional.new(project).
    creator.address.country.capital.weather.
    value
    end
    

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79. Optional = Struct.new(:value) do
    def and_then(&block)
    if value.nil?
    Optional.new(nil)
    else
    block.call(value)
    end
    end
    !
    def method_missing(*args, &block)
    and_then do |value|
    Optional.new(value.public_send(*args, &block))
    end
    end
    end
    

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80. def weather_for(project)
    Optional.new(project).
    creator.address.country.capital.weather.
    value
    end
    

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81. def weather_for(project)
    project.creator.address.
    country.capital.weather
    end
    

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82. MULTIPLE RESULTS
    

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83. Blog = Struct.new(:categories)
    Category = Struct.new(:posts)
    Post = Struct.new(:comments)
    

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84. def words_in(blogs)
    blogs.flat_map { |blog|
    blog.categories.flat_map { |category|
    category.posts.flat_map { |post|
    post.comments.flat_map { |comment|
    comment.split(/\s+/)
    }
    }
    }
    }
    end
    

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85. blogs = [
    Blog.new([
    Category.new([
    Post.new(['I love cats', 'I love dogs']),
    Post.new(['I love mice', 'I love pigs'])
    ]),
    Category.new([
    Post.new(['I hate cats', 'I hate dogs']),
    Post.new(['I hate mice', 'I hate pigs'])
    ])
    ]),
    Blog.new([
    Category.new([
    Post.new(['Red is better than blue'])
    ]),
    Category.new([
    Post.new(['Blue is better than red'])
    ])
    ])
    ]
    

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86. >> words_in(blogs)
    => ["I", "love", "cats", "I", "love", "dogs", "I",
    "love", "mice", "I", "love", "pigs", "I",
    "hate", "cats", "I", "hate", "dogs", "I",
    "hate", "mice", "I", "hate", "pigs", "Red",
    "is", "better", "than", "blue", "Blue", "is",
    "better", "than", "red"]
    

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87. def words_in(blogs)
    blogs.flat_map { |blog|
    blog.categories.flat_map { |category|
    category.posts.flat_map { |post|
    post.comments.flat_map { |comment|
    comment.split(/\s+/)
    }
    }
    }
    }
    end
    

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88. Many = Struct.new(:values) do
    def and_then(&block)
    Many.new(values.map(&block).flat_map(&:values))
    end
    end
    

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89. def words_in(blogs)
    blogs.flat_map { |blog|
    blog.categories.flat_map { |category|
    category.posts.flat_map { |post|
    post.comments.flat_map { |comment|
    comment.split(/\s+/)
    }
    }
    }
    }
    end
    

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90. def words_in(blogs)
    Many.new(blogs).and_then do |blog|
    Many.new(blog.categories).and_then do |category|
    Many.new(category.posts).and_then do |post|
    Many.new(post.comments).and_then do |comment|
    Many.new(comment.split(/\s+/))
    end
    end
    end
    end.values
    end
    

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91. def words_in(blogs)
    Many.new(blogs).and_then do |blog|
    Many.new(blog.categories)
    end.and_then do |category|
    Many.new(category.posts)
    end.and_then do |post|
    Many.new(post.comments)
    end.and_then do |comment|
    Many.new(comment.split(/\s+/))
    end.values
    end
    

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92. def words_in(blogs)
    Many.new(blogs).
    and_then { |blog | Many.new(blog.categories) }.
    and_then { |category| Many.new(category.posts) }.
    and_then { |post | Many.new(post.comments) }.
    and_then { |comment | Many.new(comment.split(/\s+/)) }.
    values
    end
    

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93. class Many
    def method_missing(*args, &block)
    and_then do |value|
    Many.new(value.public_send(*args, &block))
    end
    end
    end
    

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94. def words_in(blogs)
    Many.new(blogs).
    and_then { |blog | Many.new(blog.categories) }.
    and_then { |category| Many.new(category.posts) }.
    and_then { |post | Many.new(post.comments) }.
    and_then { |comment | Many.new(comment.split(/\s+/)) }.
    values
    end
    

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95. def words_in(blogs)
    Many.new(blogs).
    categories.
    posts.
    comments.
    split(/\s+/).
    values
    end
    

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96. def words_in(blogs)
    Many.new(blogs).
    categories.posts.comments.split(/\s+/).
    values
    end
    

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97. def words_in(blogs)
    blogs.categories.posts.comments.split(/\s+/)
    end
    

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98. Many = Struct.new(:values) do
    def and_then(&block)
    Many.new(values.map(&block).flat_map(&:values))
    end
    !
    def method_missing(*args, &block)
    and_then do |value|
    Many.new(value.public_send(*args, &block))
    end
    end
    end
    

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99. ASYNCHRONOUS CODE
    

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100. GET https://api.github.com/
     

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101. {
     "current_user_url": "https://api.github.com/user",
     "authorizations_url": "https://api.github.com/authorizations",
     "emails_url": "https://api.github.com/user/emails",
     "emojis_url": "https://api.github.com/emojis",
     "events_url": "https://api.github.com/events",
     "feeds_url": "https://api.github.com/feeds",
     "following_url": "https://api.github.com/user/following{/target}",
     "gists_url": "https://api.github.com/gists{/gist_id}",
     "hub_url": "https://api.github.com/hub",
     "issues_url": "https://api.github.com/issues",
     "keys_url": "https://api.github.com/user/keys",
     "notifications_url": "https://api.github.com/notifications",
     "organization_url": "https://api.github.com/orgs/{org}",
     "public_gists_url": "https://api.github.com/gists/public",
     "rate_limit_url": "https://api.github.com/rate_limit",
     "repository_url": "https://api.github.com/repos/{owner}/{repo}",
     "starred_url": "https://api.github.com/user/starred{/owner}{/repo}",
     "starred_gists_url": "https://api.github.com/gists/starred",
     "team_url": "https://api.github.com/teams",
     "user_url": "https://api.github.com/users/{user}"
     }
     

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102. GET https://api.github.com/
     GET https://api.github.com/orgs/ruby
     GET https://api.github.com/orgs/ruby/repos
     GET https://api.github.com/repos/ruby/ruby
     GET https://api.github.com/repos/ruby/ruby/contributors
     GET https://api.github.com/users/nobu
     

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103. {
     "login": "nobu",
     …
     "name": "Nobuyoshi Nakada",
     …
     }
     

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104. def get_json(url, &success)
     Thread.new do
     uri = URI.parse(url)
     json = Net::HTTP.get(uri)
     value = JSON.parse(json)
     success.call(value)
     end
     end
     

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105. get_json('https://api.github.com/') do |urls|
     org_url_template = URITemplate.new(urls['organization_url'])
     org_url = org_url_template.expand(org: 'ruby')
     !
     get_json(org_url) do |org|
     repos_url = org['repos_url']
     !
     get_json(repos_url) do |repos|
     most_popular_repo = repos.max_by { |repo| repo['watchers_count'] }
     repo_url = most_popular_repo['url']
     !
     get_json(repo_url) do |repo|
     contributors_url = repo['contributors_url']
     !
     get_json(contributors_url) do |users|
     most_prolific_user = users.max_by { |user| user['contributions'] }
     user_url = most_prolific_user['url']
     !
     get_json(user_url) do |user|
     puts "The most influential Rubyist is #{user['name']} (#{user['login']})"
     end
     end
     end
     end
     end
     end
     

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106. Eventually = Struct.new(:block) do
     def initialize(&block)
     super(block)
     end
     !
     def run(&success)
     block.call(success)
     end
     end
     

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107. class Eventually
     def and_then(&block)
     Eventually.new do |success|
     run do |value|
     block.call(value).run(&success)
     end
     end
     end
     end
     

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108. get_json('https://api.github.com/') do |urls|
     org_url_template = URITemplate.new(urls['organization_url'])
     org_url = org_url_template.expand(org: 'ruby')
     !
     get_json(org_url) do |org|
     repos_url = org['repos_url']
     !
     get_json(repos_url) do |repos|
     most_popular_repo = repos.max_by { |repo| repo['watchers_count'] }
     repo_url = most_popular_repo['url']
     !
     get_json(repo_url) do |repo|
     contributors_url = repo['contributors_url']
     !
     get_json(contributors_url) do |users|
     most_prolific_user = users.max_by { |user| user['contributions'] }
     user_url = most_prolific_user['url']
     !
     get_json(user_url) do |user|
     puts "The most influential Rubyist is #{user['name']} (#{user['login']})"
     end
     end
     end
     end
     end
     end
     

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109. Eventually.new { |s| get_json('https://api.github.com/', &s) }.and_then do |urls|
     org_url_template = URITemplate.new(urls['organization_url'])
     org_url = org_url_template.expand(org: 'ruby')
     !
     Eventually.new { |s| get_json(org_url, &s) }.and_then do |org|
     repos_url = org['repos_url']
     !
     Eventually.new { |s| get_json(repos_url, &s) }.and_then do |repos|
     most_popular_repo = repos.max_by { |repo| repo['watchers_count'] }
     repo_url = most_popular_repo['url']
     !
     Eventually.new { |s| get_json(repo_url, &s) }.and_then do |repo|
     contributors_url = repo['contributors_url']
     !
     Eventually.new { |s| get_json(contributors_url, &s) }.and_then do |users|
     most_prolific_user = users.max_by { |user| user['contributions'] }
     user_url = most_prolific_user['url']
     !
     Eventually.new { |s| get_json(user_url, &s) }
     end
     end
     end
     end
     end.run do |user|
     puts "The most influential Rubyist is #{user['name']} (#{user['login']})"
     end
     

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110. def get_github_api_urls
     github_root_url = 'https://api.github.com/'
     !
     Eventually.new { |success| get_json(github_root_url, &success) }
     end
     

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111. Eventually.new { |s| get_json('https://api.github.com/', &s) }.and_then do |urls|
     org_url_template = URITemplate.new(urls['organization_url'])
     org_url = org_url_template.expand(org: 'ruby')
     !
     Eventually.new { |s| get_json(org_url, &s) }.and_then do |org|
     repos_url = org['repos_url']
     !
     Eventually.new { |s| get_json(repos_url, &s) }.and_then do |repos|
     most_popular_repo = repos.max_by { |repo| repo['watchers_count'] }
     repo_url = most_popular_repo['url']
     !
     Eventually.new { |s| get_json(repo_url, &s) }.and_then do |repo|
     contributors_url = repo['contributors_url']
     !
     Eventually.new { |s| get_json(contributors_url, &s) }.and_then do |users|
     most_prolific_user = users.max_by { |user| user['contributions'] }
     user_url = most_prolific_user['url']
     !
     Eventually.new { |s| get_json(user_url, &s) }
     end
     end
     end
     end
     end.run do |user|
     puts "The most influential Rubyist is #{user['name']} (#{user['login']})"
     end
     

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112. get_github_api_urls.and_then do |urls|
     org_url_template = URITemplate.new(urls['organization_url'])
     org_url = org_url_template.expand(org: 'ruby')
     !
     Eventually.new { |s| get_json(org_url, &s) }.and_then do |org|
     repos_url = org['repos_url']
     !
     Eventually.new { |s| get_json(repos_url, &s) }.and_then do |repos|
     most_popular_repo = repos.max_by { |repo| repo['watchers_count'] }
     repo_url = most_popular_repo['url']
     !
     Eventually.new { |s| get_json(repo_url, &s) }.and_then do |repo|
     contributors_url = repo['contributors_url']
     !
     Eventually.new { |s| get_json(contributors_url, &s) }.and_then do |users|
     most_prolific_user = users.max_by { |user| user['contributions'] }
     user_url = most_prolific_user['url']
     !
     Eventually.new { |s| get_json(user_url, &s) }
     end
     end
     end
     end
     end.run do |user|
     puts "The most influential Rubyist is #{user['name']} (#{user['login']})"
     end
     

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113. def get_org(urls, name)
     org_url_template = URITemplate.new(urls['organization_url'])
     org_url = org_url_template.expand(org: name)
     !
     Eventually.new { |success| get_json(org_url, &success) }
     end
     

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114. get_github_api_urls.and_then do |urls|
     org_url_template = URITemplate.new(urls['organization_url'])
     org_url = org_url_template.expand(org: 'ruby')
     !
     Eventually.new { |s| get_json(org_url, &s) }.and_then do |org|
     repos_url = org['repos_url']
     !
     Eventually.new { |s| get_json(repos_url, &s) }.and_then do |repos|
     most_popular_repo = repos.max_by { |repo| repo['watchers_count'] }
     repo_url = most_popular_repo['url']
     !
     Eventually.new { |s| get_json(repo_url, &s) }.and_then do |repo|
     contributors_url = repo['contributors_url']
     !
     Eventually.new { |s| get_json(contributors_url, &s) }.and_then do |users|
     most_prolific_user = users.max_by { |user| user['contributions'] }
     user_url = most_prolific_user['url']
     !
     Eventually.new { |s| get_json(user_url, &s) }
     end
     end
     end
     end
     end.run do |user|
     puts "The most influential Rubyist is #{user['name']} (#{user['login']})"
     end
     

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115. get_github_api_urls.and_then do |urls|
     !
     !
     !
     get_org(urls, 'ruby').and_then do |org|
     repos_url = org['repos_url']
     !
     Eventually.new { |s| get_json(repos_url, &s) }.and_then do |repos|
     most_popular_repo = repos.max_by { |repo| repo['watchers_count'] }
     repo_url = most_popular_repo['url']
     !
     Eventually.new { |s| get_json(repo_url, &s) }.and_then do |repo|
     contributors_url = repo['contributors_url']
     !
     Eventually.new { |s| get_json(contributors_url, &s) }.and_then do |users|
     most_prolific_user = users.max_by { |user| user['contributions'] }
     user_url = most_prolific_user['url']
     !
     Eventually.new { |s| get_json(user_url, &s) }
     end
     end
     end
     end
     end.run do |user|
     puts "The most influential Rubyist is #{user['name']} (#{user['login']})"
     end
     

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116. def get_repos(org)
     repos_url = org['repos_url']
     !
     Eventually.new { |success| get_json(repos_url, &success) }
     end
     

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117. get_github_api_urls.and_then do |urls|
     !
     !
     !
     get_org(urls, 'ruby').and_then do |org|
     repos_url = org['repos_url']
     !
     Eventually.new { |s| get_json(repos_url, &s) }.and_then do |repos|
     most_popular_repo = repos.max_by { |repo| repo['watchers_count'] }
     repo_url = most_popular_repo['url']
     !
     Eventually.new { |s| get_json(repo_url, &s) }.and_then do |repo|
     contributors_url = repo['contributors_url']
     !
     Eventually.new { |s| get_json(contributors_url, &s) }.and_then do |users|
     most_prolific_user = users.max_by { |user| user['contributions'] }
     user_url = most_prolific_user['url']
     !
     Eventually.new { |s| get_json(user_url, &s) }
     end
     end
     end
     end
     end.run do |user|
     puts "The most influential Rubyist is #{user['name']} (#{user['login']})"
     end
     

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118. get_github_api_urls.and_then do |urls|
     !
     !
     !
     get_org(urls, 'ruby').and_then do |org|
     !
     !
     get_repos(org).and_then do |repos|
     most_popular_repo = repos.max_by { |repo| repo['watchers_count'] }
     repo_url = most_popular_repo['url']
     !
     Eventually.new { |s| get_json(repo_url, &s) }.and_then do |repo|
     contributors_url = repo['contributors_url']
     !
     Eventually.new { |s| get_json(contributors_url, &s) }.and_then do |users|
     most_prolific_user = users.max_by { |user| user['contributions'] }
     user_url = most_prolific_user['url']
     !
     Eventually.new { |s| get_json(user_url, &s) }
     end
     end
     end
     end
     end.run do |user|
     puts "The most influential Rubyist is #{user['name']} (#{user['login']})"
     end
     

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119. get_github_api_urls.and_then do |urls|
     !
     !
     !
     get_org(urls, 'ruby').and_then do |org|
     !
     !
     get_repos(org).and_then do |repos|
     !
     !
     !
     get_most_popular_repo(repos).and_then do |repo|
     !
     !
     get_contributors(repo).and_then do |users|
     !
     !
     !
     get_most_prolific_user(users)
     end
     end
     end
     end
     end.run do |user|
     puts "The most influential Rubyist is #{user['name']} (#{user['login']})"
     end
     

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120. get_github_api_urls.and_then do |urls|
     get_org(urls, 'ruby').and_then do |org|
     get_repos(org).and_then do |repos|
     get_most_popular_repo(repos).and_then do |repo|
     get_contributors(repo).and_then do |users|
     get_most_prolific_user(users)
     end
     end
     end
     end
     end.run do |user|
     puts 'The most influential Rubyist is' +
     " #{user['name']} (#{user['login']})"
     end
     

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121. get_github_api_urls.and_then do |urls|
     get_org(urls, 'ruby')
     end.and_then do |org|
     get_repos(org)
     end.and_then do |repos|
     get_most_popular_repo(repos)
     end.and_then do |repo|
     get_contributors(repo)
     end.and_then do |users|
     get_most_prolific_user(users)
     end.run do |user|
     puts 'The most influential Rubyist is' +
     " #{user['name']} (#{user['login']})"
     end
     

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122. get_github_api_urls.
     and_then { |urls | get_org(urls, 'ruby') }.
     and_then { |org | get_repos(org) }.
     and_then { |repos| get_most_popular_repo(repos) }.
     and_then { |repo | get_contributors(repo) }.
     and_then { |users| get_most_prolific_user(users) }.
     run do |user|
     puts 'The most influential Rubyist is' +
     " #{user['name']} (#{user['login']})"
     end
     

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123. Eventually = Struct.new(:block) do
     def initialize(&block)
     super(block)
     end
     !
     def run(&success)
     block.call(success)
     end
     !
     def and_then(&block)
     Eventually.new do |success|
     run do |value|
     block.call(value).run(&success)
     end
     end
     end
     end
     

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124. SO… WHAT?
     

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125. Optional = Struct.new(:value) do
     def and_then(&block)
     if value.nil?
     Optional.new(nil)
     else
     block.call(value)
     end
     end
     end
     Eventually = Struct.new(:block) do
     def and_then(&block)
     Eventually.new do |success|
     run do |value|
     block.call(value).run(&success)
     end
     end
     end
     end
     Many = Struct.new(:values) do
     def and_then(&block)
     Many.new(
     values.map(&block).flat_map(&:values)
     )
     end
     end
     

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126. Optional, Many &
     Eventually
     ARE MONADS
     

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127. ABSTRACT DATA TYPE
     

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128. SOME OPERATIONS
     

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129. #and_then
     .from_value
     

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130. def Optional.from_value(value)
     Optional.new(value)
     end
     !
     def Many.from_value(value)
     Many.new([value])
     end
     !
     def Eventually.from_value(value)
     Eventually.new { |success| success.call(value) }
     end
     

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131. SOME RULES
     

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132. #and_then
     calls a block with a value
     zero or more times
     at some point
     

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133. #and_then
     returns an instance
     of the same monad
     

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134. Optional = Struct.new(:value) do
     def and_then(&block)
     if value.nil?
     Optional.new(nil)
     else
     block.call(value)
     end
     end
     end
     Eventually = Struct.new(:block) do
     def and_then(&block)
     Eventually.new do |success|
     run do |value|
     block.call(value).run(&success)
     end
     end
     end
     end
     Many = Struct.new(:values) do
     def and_then(&block)
     Many.new(
     values.map(&block).flat_map(&:values)
     )
     end
     end
     

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135. #and_then
     and .from_value
     don’t mess with the value
     

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136. COMMON INTERFACE
     

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137. #and_then
     lets us connect together
     a sequence of operations
     

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138. Optional#and_then:
     do the next thing
     if the value isn’t nil
     

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139. Many#and_then:
     do the next thing
     once for each value
     

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140. Eventually#and_then:
     do the next thing
     whenever the value is ready
     

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141. SHARED
     FUNCTIONALITY
     

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142. #within
     

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143. module Monad
     def within(&block)
     and_then do |value|
     self.class.from_value(block.call(value))
     end
     end
     end
     !
     Optional.include(Monad)
     Many.include(Monad)
     Eventually.include(Monad)
     

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144. we can write code that
     uses #within
     without knowing which
     monad it is
     

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145. def description_from(containing_json)
     containing_json.within do |json|
     JSON.parse(json)
     end.within do |hash|
     "#{hash['name']} (#{hash['login']})"
     end
     end
     

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146. >> optional_json = Optional.new('{ "login": "nobu", "name": "Nobuyoshi Nakada" }')
     => #
     >> optional_description = description_from(optional_json)
     => #
     >> optional_description.value
     => "Nobuyoshi Nakada (nobu)"
     

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147. >> optional_json = Optional.new(nil)
     => #
     >> optional_description = description_from(optional_json)
     => #
     >> optional_description.value
     => nil
     

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148. >> many_jsons = Many.new([
     '{ "login": "nobu", "name": "Nobuyoshi Nakada" }',
     '{ "login": "matz", "name": "Yukihiro Matsumoto" }'
     ])
     => #"{ \"login\": \"nobu\", \"name\": \"Nobuyoshi Nakada\" }",
     "{ \"login\": \"matz\", \"name\": \"Yukihiro Matsumoto\" }"
     ]>
     >> many_descriptions = description_from(many_jsons)
     => #"Nobuyoshi Nakada (nobu)",
     "Yukihiro Matsumoto (matz)"
     ]>
     >> many_descriptions.values
     => ["Nobuyoshi Nakada (nobu)", "Yukihiro Matsumoto (matz)"]
     

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149. >> eventually_json = Eventually.new do |success|
     Thread.new do
     uri = URI.parse('https://api.github.com/users/nobu')
     json = Net::HTTP.get(uri)
     success.call(json)
     end
     end
     => #>
     >> eventually_description = description_from(eventually_json)
     => #>
     >> eventually_description.run { |description| puts description }
     => #
     Nobuyoshi Nakada (nobu)
     

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150. def description_from(containing_json)
     containing_json.within do |json|
     JSON.parse(json)
     end.within do |hash|
     "#{hash['name']} (#{hash['login']})"
     end
     end
     

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151. (ABSTRACT,
     BUT GOOD!)
     

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152. https://github.com/tomstuart/monads
     gem install monads
     

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153. Stuart
     Understanding Computation
     From Simple Machines to Impossible Programs
     Tom Stuart
     Understanding
     Computation
     .com
     @oreillymedia
     k.com/oreilly
     uart, a computer
     t and programmer,
     ounder of Codon, a
     product consultancy
     don. He works as a
     ant, mentor, and
     helping companies
     ove the quality and
     of their approach to
     g software products.
     http://computationbook.com/
     

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154. thanks!
     @tomstuart • tom@codon.com • computationbook.com
     https://github.com/tomstuart/monads
     gem install monads
     

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