class Hashie::Dash

API for defining properties as well as per-property defaults.
It is preferrable to a Struct because of the in-class
resources than something like a DataMapper resource.
lightweight data object that needs even fewer options and
Dashes are useful when you need to create a very simple
optional defaults) and only those keys may be set or read.
that has a set of defined keys that are accessible (with
A Dash is a ‘defined’ or ‘discrete’ Hash, that is, a Hash

def self.inherited(klass) 

def self.inherited(klass)
  super
  (@subclasses ||= Set.new) << klass
  klass.instance_variable_set('@properties', self.properties.dup)
  klass.instance_variable_set('@defaults', self.defaults.dup)
  klass.instance_variable_set('@required_properties', self.required_properties.dup)
end

def self.property(property_name, options = {})


existing Dash.
property, to raise an error if a value is unset in a new or
* :required - Specify the value as required for this

Dash.
to be returned before a value is set on the property in a new
* :default - Specify a default value for this property,

as follows:
Defines a property on the Dash. Options are 
def self.property(property_name, options = {})
  property_name = property_name.to_sym
  self.properties << property_name
  if options.has_key?(:default)
    self.defaults[property_name] = options[:default]
  elsif self.defaults.has_key?(property_name)
    self.defaults.delete property_name
  end
  unless instance_methods.map { |m| m.to_s }.include?("#{property_name}=")
    class_eval <<-ACCESSORS
      def #{property_name}(&block)
        self.[](#{property_name.to_s.inspect}, &block)
      end
      def #{property_name}=(value)
        self.[]=(#{property_name.to_s.inspect}, value)
      end
    ACCESSORS
  end
  if defined? @subclasses
    @subclasses.each { |klass| klass.property(property_name, options) }
  end
  required_properties << property_name if options.delete(:required)
end

def self.property?(name)

defined.
Check to see if the specified property has already been 
def self.property?(name)
  properties.include? name.to_sym
end

def self.required?(name)

required.
Check to see if the specified property is 
def self.required?(name)
  required_properties.include? name.to_sym
end

def [](property)

property's default value if it hasn't been set).
Retrieve a value from the Dash (will return the 
def [](property)
  assert_property_exists! property
  value = super(property.to_s)
  yield value if block_given?
  value
end

def []=(property, value)

on pre-existing properties.
Set a value on the Dash in a Hash-like way. Only works 
def []=(property, value)
  assert_property_required! property, value
  assert_property_exists! property
  super(property.to_s, value)
end

def assert_property_exists!(property) 

def assert_property_exists!(property)
  unless self.class.property?(property)
    raise NoMethodError, "The property '#{property}' is not defined for this Dash."
  end
end

def assert_property_required!(property, value) 

def assert_property_required!(property, value)
  if self.class.required?(property) && value.nil?
    raise ArgumentError, "The property '#{property}' is required for this Dash."
  end
end

def assert_property_set!(property) 

def assert_property_set!(property)
  if send(property).nil?
    raise ArgumentError, "The property '#{property}' is required for this Dash."
  end
end

def assert_required_properties_set! 

def assert_required_properties_set!
  self.class.required_properties.each do |required_property|
    assert_property_set!(required_property)
  end
end

def initialize(attributes = {}, &block)

just like you would many other kinds of data objects.
You may initialize a Dash with an attributes hash 
def initialize(attributes = {}, &block)
  super(&block)
  self.class.defaults.each_pair do |prop, value|
    self[prop] = value
  end
  attributes.each_pair do |att, value|
    self[att] = value
  end if attributes
  assert_required_properties_set!
end