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

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WARM UP

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ANALOGIES

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WHAT’S A STACK?

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A KIND OF VALUE…

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WITH CERTAIN OPERATIONS … …

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

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THAT FOLLOW CERTAIN RULES …

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

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WE CAN IMPLEMENT THEM HOWEVER WE LIKE

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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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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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WE CAN USE THEM WITHOUT KNOWING THEIR IMPLEMENTATION

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>> 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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WE CAN DEFINE MORE OPERATIONS

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

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>> 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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OPERATIONS & RULES

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COMMON INTERFACE & SHARED FUNCTIONALITY

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WHAT’S A COLLECTION?

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A KIND OF VALUE…

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WITH CERTAIN OPERATIONS … …

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#each

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THAT FOLLOW CERTAIN RULES …

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

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WE CAN IMPLEMENT IT HOWEVER WE LIKE

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

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WE CAN USE IT WITHOUT KNOWING ITS IMPLEMENTATION

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>> 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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WE CAN DEFINE MORE OPERATIONS

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

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OPERATIONS & RULES

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COMMON INTERFACE & SHARED FUNCTIONALITY

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WHAT ARE STACKS & COLLECTIONS?

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ABSTRACT DATA TYPE

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(ABSTRACT, BUT GOOD!)

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LET’S DO SOME REFACTORING!

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HANDLING NIL

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

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

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

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>> weather_for(project) => "sunny"

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

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

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

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

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# 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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class Object remove_method :try end ! class NilClass remove_method :try end

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Optional = Struct.new(:value)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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MULTIPLE RESULTS

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

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

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

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

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

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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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ASYNCHRONOUS CODE

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GET https://api.github.com/

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

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

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

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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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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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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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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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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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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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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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SO… WHAT?

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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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Optional, Many & Eventually ARE MONADS

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ABSTRACT DATA TYPE

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SOME OPERATIONS

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#and_then .from_value

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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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SOME RULES

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

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#and_then returns an instance of the same monad

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

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COMMON INTERFACE

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#and_then lets us connect together a sequence of operations

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

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

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

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SHARED FUNCTIONALITY

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#within

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

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

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

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>> 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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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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(ABSTRACT, BUT GOOD!)

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

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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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thanks! @tomstuart • [email protected] • computationbook.com https://github.com/tomstuart/monads gem install monads