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
2. WARM UP
3. ANALOGIES
4. WHAT’S A STACK?
5. A KIND OF VALUE…
6. WITH CERTAIN OPERATIONS … …
7. #push(value) #pop #top #empty? .empty
8. THAT FOLLOW CERTAIN RULES …
9. stack.push(value).top == value stack.push(value).pop == stack empty.empty? == truee stack.push(value).empty? == false
10. WE CAN IMPLEMENT THEM HOWEVER WE LIKE
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
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
13. WE CAN USE THEM WITHOUT KNOWING THEIR IMPLEMENTATION
14. >> stack = ArrayStack.empty => #<struct ArrayStack values=[]> >> stack.push('hello').push('world').pop.top => "hello" >> stack = LinkedListStack.empty => #<struct LinkedListStack top=nil, rest=nil> >> stack.push('hello').push('world').pop.top => "hello"
15. WE CAN DEFINE MORE OPERATIONS
16. module Stack def size if empty? 0 else pop.size + 1 end end end ! ArrayStack.include(Stack) LinkedListStack.include(Stack)
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
18. OPERATIONS & RULES
19. COMMON INTERFACE & SHARED FUNCTIONALITY
20. WHAT’S A COLLECTION?
21. A KIND OF VALUE…
22. WITH CERTAIN OPERATIONS … …
23. #each
24. THAT FOLLOW CERTAIN RULES …
25. #each calls a block with a value zero or more times in immediate sequence
26. WE CAN IMPLEMENT IT HOWEVER WE LIKE
27. class HardcodedCollection def each(&block) block.call(1) block.call(2) block.call(3) block.call(4) block.call(5) end end
28. class GeneratedCollection def each(&block) number = 1 while number <= 5 block.call(number) number = number + 1 end end end
29. WE CAN USE IT WITHOUT KNOWING ITS IMPLEMENTATION
30. >> collection = HardcodedCollection.new => #<HardcodedCollection> >> collection.each { |n| puts n } 1 2 3 4 5 => nil >> collection = GeneratedCollection.new => #<GeneratedCollection> >> collection.each { |n| puts n } 1 2 3 4 5 => nil
31. WE CAN DEFINE MORE OPERATIONS
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)
33. >> HardcodedCollection.new.select { |n| n.odd? } => [1, 3, 5] ! >> GeneratedCollection.new.select { |n| n.odd? } => [1, 3, 5]
34. OPERATIONS & RULES
35. COMMON INTERFACE & SHARED FUNCTIONALITY
36. WHAT ARE STACKS & COLLECTIONS?
37. ABSTRACT DATA TYPE
38. (ABSTRACT, BUT GOOD!)
39. LET’S DO SOME REFACTORING!
40. HANDLING NIL
41. Project = Struct.new(:creator) Person = Struct.new(:address) Address = Struct.new(:country) Country = Struct.new(:capital) City = Struct.new(:weather)
42. def weather_for(project) project.creator.address. country.capital.weather end
43. >> city = City.new('sunny') => #<struct City …> ! >> country = Country.new(city) => #<struct Country …> ! >> address = Address.new(country) => #<struct Address …> ! >> person = Person.new(address) => #<struct Person …> ! >> project = Project.new(person) => #<struct Project …>
44. >> weather_for(project) => "sunny"
45. >> bad_project = Project.new(Person.new(Address.new(nil))) => #<struct Project …> ! >> weather_for(bad_project) NoMethodError: undefined method `capital' for nil:NilClass
46. def weather_for(project) project.creator.address. country.capital.weather end
47. def weather_for(project) creator = project.creator address = creator.address country = address.country capital = country.capital weather = capital.weather end
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
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
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
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
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
53. def weather_for(project) project. try(:creator). try(:address). try(:country). try(:capital). try(:weather) end
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
55. class Object remove_method :try end ! class NilClass remove_method :try end
56. Optional = Struct.new(:value)
57. >> optional_string = Optional.new('hello') => #<struct Optional value="hello"> >> optional_string.value => "hello" >> optional_string = Optional.new(nil) => #<struct Optional value=nil> >> optional_string.value => nil
58. class Optional def try(*args, &block) if value.nil? nil else value.public_send(*args, &block) end end end
59. >> optional_string = Optional.new('hello') => #<struct Optional value="hello"> >> length = optional_string.try(:length) => 5 >> optional_string = Optional.new(nil) => #<struct Optional value=nil> >> length = optional_string.try(:length) => nil
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
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
62. class Optional def try(*args, &block) if value.nil? nil else value.public_send(*args, &block) end end end
63. class Optional def try(&block) if value.nil? nil else block.call(value) end end end
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
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
66. >> weather_for(project) => "sunny" ! >> weather_for(bad_project) NoMethodError: undefined method `capital' for nil:NilClass
67. class Optional def try(&block) if value.nil? nil else block.call(value) end end end
68. class Optional def try(&block) if value.nil? Optional.new(nil) else block.call(value) end end end
69. >> weather_for(project) => "sunny" ! >> weather_for(bad_project) => nil
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
71. class Optional def try(&block) if value.nil? Optional.new(nil) else block.call(value) end end end
72. class Optional def and_then(&block) if value.nil? Optional.new(nil) else block.call(value) end end end
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
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
75. class Optional def method_missing(*args, &block) and_then do |value| Optional.new(value.public_send(*args, &block)) end end end
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
77. def weather_for(project) Optional.new(project). creator. address. country. capital. weather. value end
78. def weather_for(project) Optional.new(project). creator.address.country.capital.weather. value end
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
80. def weather_for(project) Optional.new(project). creator.address.country.capital.weather. value end
81. def weather_for(project) project.creator.address. country.capital.weather end
82. MULTIPLE RESULTS
83. Blog = Struct.new(:categories) Category = Struct.new(:posts) Post = Struct.new(:comments)
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
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']) ]) ]) ]
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"]
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
88. Many = Struct.new(:values) do def and_then(&block) Many.new(values.map(&block).flat_map(&:values)) end end
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
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
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
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
93. class Many def method_missing(*args, &block) and_then do |value| Many.new(value.public_send(*args, &block)) end end end
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
95. def words_in(blogs) Many.new(blogs). categories. posts. comments. split(/\s+/). values end
96. def words_in(blogs) Many.new(blogs). categories.posts.comments.split(/\s+/). values end
97. def words_in(blogs) blogs.categories.posts.comments.split(/\s+/) end
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
99. ASYNCHRONOUS CODE
100. GET https://api.github.com/
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}" }
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
103. { "login": "nobu", … "name": "Nobuyoshi Nakada", … }
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
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
106. Eventually = Struct.new(:block) do def initialize(&block) super(block) end ! def run(&success) block.call(success) end end
107. class Eventually def and_then(&block) Eventually.new do |success| run do |value| block.call(value).run(&success) end end end end
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
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
110. def get_github_api_urls github_root_url = 'https://api.github.com/' ! Eventually.new { |success| get_json(github_root_url, &success) } end
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
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
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
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
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
116. def get_repos(org) repos_url = org['repos_url'] ! Eventually.new { |success| get_json(repos_url, &success) } end
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
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
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
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
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
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
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
124. SO… WHAT?
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
126. Optional, Many & Eventually ARE MONADS
127. ABSTRACT DATA TYPE
128. SOME OPERATIONS
129. #and_then .from_value
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
131. SOME RULES
132. #and_then calls a block with a value zero or more times at some point
133. #and_then returns an instance of the same monad
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
135. #and_then and .from_value don’t mess with the value
136. COMMON INTERFACE
137. #and_then lets us connect together a sequence of operations
138. Optional#and_then: do the next thing if the value isn’t nil
139. Many#and_then: do the next thing once for each value
140. Eventually#and_then: do the next thing whenever the value is ready
141. SHARED FUNCTIONALITY
142. #within
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)
144. we can write code that uses #within without knowing which monad it is
145. def description_from(containing_json) containing_json.within do |json| JSON.parse(json) end.within do |hash| "#{hash['name']} (#{hash['login']})" end end
146. >> optional_json = Optional.new('{ "login": "nobu", "name": "Nobuyoshi Nakada" }') => #<struct Optional value="{ \"login\": \"nobu\", \"name\": \"Nobuyoshi Nakada\" }"> >> optional_description = description_from(optional_json) => #<struct Optional value="Nobuyoshi Nakada (nobu)"> >> optional_description.value => "Nobuyoshi Nakada (nobu)"
147. >> optional_json = Optional.new(nil) => #<struct Optional value=nil> >> optional_description = description_from(optional_json) => #<struct Optional value=nil> >> optional_description.value => nil
148. >> many_jsons = Many.new([ '{ "login": "nobu", "name": "Nobuyoshi Nakada" }', '{ "login": "matz", "name": "Yukihiro Matsumoto" }' ]) => #<struct Many values=[ "{ \"login\": \"nobu\", \"name\": \"Nobuyoshi Nakada\" }", "{ \"login\": \"matz\", \"name\": \"Yukihiro Matsumoto\" }" ]> >> many_descriptions = description_from(many_jsons) => #<struct Many values=[ "Nobuyoshi Nakada (nobu)", "Yukihiro Matsumoto (matz)" ]> >> many_descriptions.values => ["Nobuyoshi Nakada (nobu)", "Yukihiro Matsumoto (matz)"]
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 => #<struct Eventually block=#<Proc>> >> eventually_description = description_from(eventually_json) => #<struct Eventually block=#<Proc>> >> eventually_description.run { |description| puts description } => #<Thread run> Nobuyoshi Nakada (nobu)
150. def description_from(containing_json) containing_json.within do |json| JSON.parse(json) end.within do |hash| "#{hash['name']} (#{hash['login']})" end end
151. (ABSTRACT, BUT GOOD!)
152. https://github.com/tomstuart/monads gem install monads
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/
154. thanks! @tomstuart • tom@codon.com • computationbook.com https://github.com/tomstuart/monads gem install monads