Adapters: Reasonable Metaprogramming in Ruby

Implementing an adapter using method_missing is an opportunity to learn about Ruby's method lookup and the contracts that metaprogramming should not break. No prior experience required!

Transcript

1. adapters Reasonable metaprogramming in Ruby.
2. Bringing more creativity into the world. gonzalo-bulnes
3. gonzalo-bulnes/dredd-rack
4. 1. Method lookup in Ruby 2. Introducing metaprogramming 3. What’s in an adapter? e.g. Dredd::Rack 4. Dive into method_missing 5. Abstraction
5. Method lookup in Ruby Making objects respond to messages
6. Let’s take an example
7. ~/dev/dredd-rack$ irb -Ilib irb:001> require ‘dredd/rack/runner’ => true irb:002> dredd = Dredd::Rack::Runner.new => #<Dredd::Rack::Runner:0x007f9d9285f7c8 ...> irb:003> dredd.object_id # for example => 70157372423140 1. Create a new instance of Dredd::Rack::Runner 2. Send the object_id message to our runner object 3. Get the expected value!
8. What just happened?
9. #<Dredd::Rack::Runner :0x007f9d9285f7c8> dredd.object_id # is there an object_id method? 1. Message received 2. Ruby’s first question
10. “One step to the right, then all the way up.” Metaprogramming Ruby 2 by Paolo Perrotta
11. #<Dredd::Rack::Runner :0x007f9d9285f7c8> Dredd::Rack::Runner BasicObject Object is_a? instance_of? dredd.object_id # is there an object_id method?
12. #<Dredd::Rack::Runner :0x007f9d9285f7c8> Dredd::Rack::Runner BasicObject dredd.object_id Object is_a? instance_of? object_id() 70157372423140 <=
13. What if no method exists with that name? Introducing: method_missing
14. irb:006> dredd.make_coffee(:long, :black) NoMethodError: undefined method `make_coffee' for #<Dredd::Rack::Runner:0x007f9d9285f7c8> from lib/dredd/rack/runner.rb:144:in `method_missing'
15. #<Dredd::Rack::Runner :0x007f9d9285f7c8> Dredd::Rack::Runner BasicObject is_a? instance_of? dredd.make_coffee(:long, :black) Object nope :(
16. #<Dredd::Rack::Runner :0x007f9d9285f7c8> dredd.method_missing(:make_coffee, :long, :black) 1. Method lookup unsuccessful 2. Ruby notifies the object sending a new message 3. Message received Which means... 4. Method lookup round 2!
17. #<Dredd::Rack::Runner :0x007f9d9285f7c8> Dredd::Rack::Runner BasicObject Object is_a? instance_of? dredd.method_missing(:make_coffee, :long, :black) method_missing(symbol [, *args]) NoMethodError: undefined method `make_coffee'
18. method_missing is called every time an object receives a message it’s not prepared to respond to
19. A very short introduction to Metaprogramming
20. Metaprogamming: code that writes code
21. One example: define_method
22. - Right from the standard Ruby library - usually we define methods using def, with names chosen at write time - we can also use define_method, and choose the names at load time - or even at runtime!
23. class Recipe def initialize @ingredients = [] end def list_ingredients @ingredients.map{ |ingredient| ingredient.name } end # Metaprogramming! Yay! define_method(:add_ingredient) do |ingredient| @ingredients << ingredient end end
24. Dredd::Rack Let’s get practical!
25. What is Dredd?
26. $ dredd api-blueprint.apib http://localhost:3000 \ --level=info --color --sorted # FORMAT: 1A7 # GET /message + Response 200 (text/plain) Hello World! $ dredd api-blueprint.apib http://lo PASS /hello-world PASS /question/1/answers FAIL /wip PASS /status
27. We can test-drive Ruby apps with Dredd!
28. # Wouldn’t it be great to create and configure a runner ONCE... dredd = Dredd::Rack::Runner.new.level(:info).color!.sorted! # And run Dredd without worrying about configuring it or # restarting the application server? dredd.run $ dredd api-blueprint.apib http://localhost:3000 \ --level=info --color --sorted
29. Dredd::Rack is an adapter It adapts Dredd for use with Rack apps* * Ruby web apps, e.g. Rails, Sinatra, Grape apps...
30. What is an adapter?
31. Adapters wrap an API into a more convenient one
32. # Part of the application code, alongside the specs dredd = Dredd::Rack::Runner.new.level(:info).color!.sorted! dredd.run $ # Written manually to the command line $ dredd api-blueprint.apib http://localhost:3000 \ --level=info --color --sorted
33. Dredd::Rack::Runner in a nutshell
34. - a “runner” instance allows to run Dredd - it provides methods to set Dredd options - the configuration is readable code - that is stored close to the app code
35. class Dredd::Rack::Runner def initialize @command_options = [] end def command ([@dredd_command, @paths_to_blueprints, @api_endpoint] + \ @command_options).join(' ') end def run raise InvalidCommandError.new(command) unless command_valid? start_server! unless api_remote? Kernel.system(command) end # ... end
36. $ dredd --help [...] Options: --dry-run, -y Do not run any real HTTP transaction... --hookfiles, -f Specifies a pattern to match files... --language, -a Language of hookfiles. Possible options... --sandbox, -b Load and run non trusted hooks code... --server, -g Run API backend server command and kill... --server-wait Set delay time in seconds between running... --init, -i Run interactive configuration. ... --custom, -j Pass custom key-value configuration data... --names, -n Only list names of requests... --only, -x Run only specified transaction name. ... --reporter, -r Output additional report format. ... --output, -o Specifies output file when using... [...] --version Show version number. class Dredd::Rack::Runner def dry_run! @command_options << ‘--dry-run’ end # ... def names! @command_options << ‘--names’ end def only(arg) @command_options += [‘--only’, arg] end # ... # many many more... end
37. Adapters are often tightly coupled to the API they wrap
38. Adapters are usually not DRY ”we enjoy typing”
39. Keeping up with new releases
40. There will be time pressure That’s where easy & repetitive becomes dangerous
41. git commit -m “Add support for Dredd v1.5.0”
42. For the sake of sustainability Automate critical procedures “People don’t fail, processes do.”
43. An introduction to Metaprogramming
44. method_missing is called every time an object receives a message it’s not prepared to respond to
45. How could we take advantage of that?
46. What if we could do whatever needs to be done using method_missing?
47. What if every time we got there...
48. 1. method name matches an option? 2. yes: do what the method would do 3. no: raise the usual NoMethodError
49. class Dredd::Rack::Runner def initialize @command_options = [] end # ... def method_missing(name, *args) ... end # and that replaces ALL the “option” methods! end
50. Metaprogramming can go wrong (like everything else) “The question is not if... but why?”
51. Good craft requires a bit of care Do not forget to respond_to_missing!
52. class Dredd::Rack::Runner # ... def method_missing # ... end def respond_to_missing?(method, include_private=false) OPTIONS_NAMES.include?(method.to_sym) || super end end
53. Abstraction Thinking simpler
54. Metaprogramming increases complexity What about readability?
55. As humans, abstracting is what we do Abstractions are readable.
56. Abstractions are readable complexity
57. The option abstraction in Dredd::Rack
58. $ dredd --help [...] Options: --dry-run, -y Do not run any real HTTP transaction... --hookfiles, -f Specifies a pattern to match files... --language, -a Language of hookfiles. Possible options... --sandbox, -b Load and run non trusted hooks code... --server, -g Run API backend server command and kill... --server-wait Set delay time in seconds between running... --init, -i Run interactive configuration. ... --custom, -j Pass custom key-value configuration data... --names, -n Only list names of requests... --only, -x Run only specified transaction name. ... --reporter, -r Output additional report format. ... --output, -o Specifies output file when using... [...] --version Show version number.
59. class Dredd::Rack::Runner NEGATABLE_BOOLEAN_OPTIONS = [:details!, :color!, ...] META_OPTIONS = [:help, :version] BOOLEAN_OPTIONS = NEGATABLE_BOOLEAN_OPTIONS + META_OPTIONS SINGLE_ARGUMENT_OPTIONS = [:language, :only, :output, ...] OPTIONS = BOOLEAN_OPTIONS + SINGLE_ARGUMENT_OPTIONS end
60. Would I understand the abstraction when Dredd releases a new version? Is it reasonable?
61. Takeaways - Method lookup in Ruby (it’s a thing) - Metaprogramming is a collection of techniques. Some of them are simple. - Adapters’ are good candidates for automation - Metaprogramming should be reasonable - Abstraction is reasonable, can be a criterion
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63. Come talk to me, I’m a nice person ; ) You can read this presentation again on SpeakerDeck. Copyright (C) 2017 Gonzalo Bulnes Guilpain Except where otherwise stated, this presentation is published under the term of the Creative Commons Attribution-ShareAlike 4.0 International License (CC BY-SA). All logos and trademarks belong to their respective authors. Artworks displayed in this presentation belong to their authors, see the corresponding credits page for details. The code shown here is part of Dredd::Rack. Dredd::Rack is free software, see details at https:/ /github.com/gonzalo-bulnes/dredd-rack. Want to become a better programmer? Join the Recurse Center! More at https:/ /www.recurse.com/faq