Metaprogramming + Ds Ls

metaprogramming
+
domain
speciﬁc
languages

% cat ddfreyne.txt

DENIS DEFREYNE
==============

web: http://stoneship.org
twitter: ddfreyne

nanoc: a static ruby web site publishing system
http://nanoc.stoneship.org/

% _

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

class Person

attr_reader :friends

end

class Document < ActiveRecord::Base

has_many :pages

end

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

class Bob
my_attr_reader :foo, :bar
end

class Module

def my_attr_reader(*fields)
fields.each do |field|

class_eval "
def #{field}
@#{field}
end
"

end
end

end

def #{field}
@#{field}
end

class Module

def battr_reader(*fields)
fields.each do |field|

class_eval "
def #{field}?
!!@#{field}
end
"

end
end

end

class Bob
battr_reader :foo, :bar
end

p bob.foo?
p bob.bar?

Hello. My name is <%= @first_name %>.

template = "Hello. My name is <%=
@first_name %>."

@first_name = "Bob"

p ERB.new(template).result

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

template = "Product <%=
@id %> has title <%= @title %>."

product = Product.new

# TODO use the product details

class Product
def initialize
@id = "39t8zfeg"
end

def print

puts ERB.new(template).result(binding)
end
end

class Product

def initialize
@id = "39t8zfeg"
end

end

class Product

def initialize
@id = "39t8zfeg"
end

def get_binding
binding
end

end

def get_binding
binding
end

def binding
binding
end

def binding
binding
end

SystemStackError: stack level too deep



p ERB.new(template).result(product.get_binding)

compile '/articles/*/' do
filter :erb
filter :bluecloth

layout 'article'

filter :rubypants
end

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

… simpler

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

class Item

attr_reader :identifier

def initialize(identifier)
@identifier = identifier
end

end

items = [
Item.new('foo'),
Item.new('foobar'),
Item.new('quxbar'),
Item.new('moo')
]

magically_load_rules

items.each do |item|
magically_process(item)
end

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

class Application

def initialize
@rules = []
end

def load_rules
rules_content = File.read('Rules')
dsl = DSL.new(@rules)
dsl.instance_eval(rules_content)
end

⋮

class Bob

def initialize
@secret = "abc"
end

end

bob = Bob.new
p bob.secret

NoMethodError: undefined method `secret'
for #<Bob:0x574324>

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

abc

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

abc

⋮

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

rule.apply_to(item)
end

end

class DSL

def initialize(rules)
@rules = rules
end

def process(pattern, &block)
@rules << Rule.new(pattern, block)
end

end

process /oo/ do |item|
end

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

process /oo/ do
end

class RuleContext

def initialize(item)
@item = item
end

end

class Rule

@pattern = pattern
@block = block
end

end

def apply_to(item)
# original way:
@block.call(item)
end

end

class Rule

@pattern = pattern
@block = block
end

end

def apply_to(item)
rule_context = RuleContext.new(item)
rule_context.instance_eval(&@block)
end

end

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

class RuleContext

def initialize(item)
@item = item
end

def item
@item
end

end

‣ 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/

Metaprogramming + Ds Ls

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