Protocol implementation in Ruby

protocol.rb

class Protocol
  attr_reader :name, :ancestors
  
  def initialize(name, ancestors, block)
    @name = name
    @ancestors = ancestors
    @methods = []
    instance_exec(&block)
  end
  
  def method(name)
    @methods << name
  end
  
  def check_type!(type, _proto = self)
    @ancestors.all? { |protocol| protocol.check_type!(type, _proto) }
    @methods.each do |name|
      type.instance_methods.include?(name) or raise "Incomplete protocol '#{_proto.name}' implementation (missing methd '#{name}')"
    end
  end
  
  def check_object!(object)
    object.class._protos.include?(name) or raise "#{object} does not conform to protocol '#{name}'"
  end
  
  alias_method :&, :check_object!
end

def protocol(name, *ancestors, &block)
  Protocol.new(name, ancestors, block).tap do |proto|
    Object.const_set(name, proto)
  end
end

module ProtocolSupport
  def _protos
    @_protos ||= {}
  end
  
  def implements(protocol)
    protocol.check_type!(self)
    _protos[protocol.name] = true
  end
end

Class.send :include, ProtocolSupport

#------------------------------------------------------------

protocol :FullyNamed do
  method :full_name
end

protocol :Addressable, (FullyNamed) do
  method :street_address
  method :city
end

Person = Struct.new(:first_name, :last_name)

class Person
  def full_name
    [first_name, last_name].join ' '
  end
end

DeliveryAddress = Struct.new(:person, :street_address, :city)

class DeliveryAddress
  def full_name
    person.full_name
  end
end

class Pet
  def initialize(name)
    @name = name
  end
end

def print_full_name(o)
  FullyNamed & o
  puts o.full_name
end

def print_full_address(o)
  Addressable & o
  puts o.full_name
  puts o.street_address
  puts o.city
end

Person.implements FullyNamed
DeliveryAddress.implements Addressable

print_full_name Person.new('John', 'Smith')
puts '---'
print_full_address DeliveryAddress.new(Person.new('Jane', 'Doe'), 'Via Larga 1', 'Milano')
puts '---'
print_full_name Pet.new('Cuddles')