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 p ERB.new(template).result

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

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

class Product def initialize @title = "My First Ruby Book" @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

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

template = "Product <%= @id %> has title <%= @title %>." product = Product.new 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| puts "I am rule /oo/!" puts "I am processing #{item.inspect}!" 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 puts "I am rule /oo/!" puts "I am processing #{item.inspect}!" end

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

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

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

process /oo/ do puts "I am rule /oo/!" 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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