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