This document provides examples of metaprogramming in Ruby. Metaprogramming allows writing programs that manipulate other programs as data. The examples demonstrate dynamically defining methods and attributes, generating code using templates, and building a domain-specific language for processing items. A key example shows how to build a rules engine that loads rules from a file and applies matching rules to items by evaluating code blocks in their context.

1. metaprogramming
+
domain
specific
languages

2. % cat ddfreyne.txt
DENIS DEFREYNE
==============
web: http://stoneship.org
twitter: ddfreyne
nanoc: a static ruby web site publishing system
http://nanoc.stoneship.org/
% _

3. what is
metaprogramming?

4. Metaprogramming is the
writing of computer
programs that write or
manipulate other
programs (or themselves)
as their data.

5. class Person
attr_reader :friends
end

6. class Document < ActiveRecord::Base
has_many :pages
end

7. task :test do
puts "Hello, I am a rake task!"
end

8. example (i)

9. class Bob
my_attr_reader :foo, :bar
end

10. class Module
def my_attr_reader(*fields)
fields.each do |field|
class_eval "
def #{field}
@#{field}
end
"
end
end
end

11. class Module
def my_attr_reader(*fields)
fields.each do |field|
class_eval "
def #{field}
@#{field}
end
"
end
end
end

12. def #{field}
@#{field}
end

13. def foo
@foo
end

14. def bar
@bar
end

15. class Module
def my_attr_reader(*fields)
fields.each do |field|
class_eval "
def #{field}
@#{field}
end
"
end
end
end

16. DEMO

17. example (ii)

18. class Module
def my_attr_reader(*fields)
fields.each do |field|
class_eval "
def #{field}
@#{field}
end
"
end
end
end

19. class Module
def battr_reader(*fields)
fields.each do |field|
class_eval "
def #{field}?
!!@#{field}
end
"
end
end
end

20. class Bob
battr_reader :foo, :bar
end
p bob.foo?
p bob.bar?

21. DEMO

22. example (iii)

23. <p>Hello. My name is <%= @first_name %>.</p>

24. template = "<p>Hello. My name is <%=
@first_name %>.</p>"
@first_name = "Bob"
p ERB.new(template).result

25. DEMO

26. example (iv)

27. class Product
def initialize
@title = "My First Ruby Book"
@id = "39t8zfeg"
end
end
template = "<p>Product <%=
@id %> has title <%= @title %>.</p>"
product = Product.new
# TODO use the product details
p ERB.new(template).result

28. class Product
def initialize
@title = "My First Ruby Book"
@id = "39t8zfeg"
end
def print
template = "<p>Product <%=
@id %> has title <%= @title %>.</p>"
puts ERB.new(template).result(binding)
end
end

29. class Product
def initialize
@title = "My First Ruby Book"
@id = "39t8zfeg"
end
def print
template = "<p>Product <%=
@id %> has title <%= @title %>.</p>"
puts ERB.new(template).result(binding)
end
end

30. class Product
def initialize
@title = "My First Ruby Book"
@id = "39t8zfeg"
end
end

31. class Product
def initialize
@title = "My First Ruby Book"
@id = "39t8zfeg"
end
def get_binding
binding
end
end

32. def get_binding
binding
end

33. def get_binding
binding
end

34. def binding
binding
end

35. def binding
binding
end
SystemStackError: stack level too deep

36. template = "<p>Product <%=
@id %> has title <%= @title %>.</p>"
product = Product.new
p ERB.new(template).result

37. template = "<p>Product <%=
@id %> has title <%= @title %>.</p>"
product = Product.new
p ERB.new(template).result(product.get_binding)

38. DEMO

39. example (v)

40. compile '/articles/*/' do
filter :erb
filter :bluecloth
layout 'article'
filter :rubypants
end

41. my-­‐site/
config.yaml
Rules
content/
layouts/
lib/

42. … simpler
process /oo/ do |item|
puts "I am rule /oo/!"
puts "I am processing #{item.inspect}!"
end

43. class Item
attr_reader :identifier
def initialize(identifier)
@identifier = identifier
end
end

44. items = [
Item.new('foo'),
Item.new('foobar'),
Item.new('quxbar'),
Item.new('moo')
]
magically_load_rules
items.each do |item|
magically_process(item)
end

45. class Rule
def initialize(pattern, block)
@pattern = pattern
@block = block
end
def applicable_to?(item)
item.identifier =~ @pattern
end
def apply_to(item)
@block.call(item)
end
end

46. class Application
def initialize
@rules = []
end
def load_rules
rules_content = File.read('Rules')
dsl = DSL.new(@rules)
dsl.instance_eval(rules_content)
end
⋮

47. class Application
def initialize
@rules = []
end
def load_rules
rules_content = File.read('Rules')
dsl = DSL.new(@rules)
dsl.instance_eval(rules_content)
end
⋮

48. class Bob
def initialize
@secret = "abc"
end
end

49. bob = Bob.new
p bob.secret
NoMethodError: undefined method `secret'
for #<Bob:0x574324>

50. bob = Bob.new
p bob.instance_eval { @secret }
abc

51. bob = Bob.new
p bob.instance_eval "@secret"
abc

52. class Application
def initialize
@rules = []
end
def load_rules
rules_content = File.read('Rules')
dsl = DSL.new(@rules)
dsl.instance_eval(rules_content)
end
⋮

53. ⋮
def process(item)
rule = rules.find do |r|
r.applicable_to?(item)
end
rule.apply_to(item)
end
end

54. class DSL
def initialize(rules)
@rules = rules
end
def process(pattern, &block)
@rules << Rule.new(pattern, block)
end
end

55. class DSL
def initialize(rules)
@rules = rules
end
def process(pattern, &block)
@rules << Rule.new(pattern, block)
end
end

56. process /oo/ do |item|
puts "I am rule /oo/!"
puts "I am processing #{item.inspect}!"
end

57. process /oo/ do |item|
puts "I am rule /oo/!"
puts "I am processing #{item.inspect}!"
end

58. items = [
Item.new('foo'),
Item.new('foobar'),
Item.new('quxbar'),
Item.new('moo')
]
app = App.new
app.load_rules
items.each do |item|
app.process(item)
end

59. DEMO

60. example (vi)

61. process /oo/ do |item|
puts "I am rule /oo/!"
puts "I am processing #{item.inspect}!"
end

62. process /oo/ do
puts "I am rule /oo/!"
puts "I am processing #{item.inspect}!"
end

63. class RuleContext
def initialize(item)
@item = item
end
end

64. class Rule
def initialize(pattern, block)
@pattern = pattern
@block = block
end
def applicable_to?(item)
item.identifier =~ @pattern
end
def apply_to(item)
# original way:
@block.call(item)
end
end

65. class Rule
def initialize(pattern, block)
@pattern = pattern
@block = block
end
def applicable_to?(item)
item.identifier =~ @pattern
end
def apply_to(item)
rule_context = RuleContext.new(item)
rule_context.instance_eval(&@block)
end
end

66. process /oo/ do
puts "I am rule /oo/!"
puts "I am processing #{@item.inspect}!"
end

67. process /oo/ do
puts "I am rule /oo/!"
puts "I am processing #{item.inspect}!"
end

68. class RuleContext
def initialize(item)
@item = item
end
end

69. class RuleContext
def initialize(item)
@item = item
end
def item
@item
end
end

70. process /oo/ do
puts "I am rule /oo/!"
puts "I am processing #{item.inspect}!"
end

71. DEMO

72. ‣ The Ruby Object Model
and Metaprogramming
http://www.pragprog.com/screencasts/v-
dtrubyom/the-ruby-object-model-and-
metaprogramming
‣ How nanocʼs Rules DSL Works
http://stoneship.org/journal/2009/how-
nanocs-rules-dsl-works/

73. you can haz
questions?

74. k thx bai