Method contracts for Ruby. No more `if object.respond_to? :waddle and object.respond_to? :quack and object.respond_to?...`

quack.gemspec

Gem::Specification.new do |s|
  s.name         = 'quack'
  s.summary      = ''
  s.description  = ''
  s.version      = '0.0.1'
  s.platform     = Gem::Platform::RUBY
 
  s.files        = ['quack.rb']
  s.require_path = '.'
 
  s.author      = 'Chris Keele'
  s.email       = 'dev@chriskeele.com'
  s.homepage    = ''
 
  s.test_file    = 'quack_test.rb'
end

quack.rb

class Duck < BasicObject
  attr_reader :requirements
  def initialize(*args)
    @requirements = args
  end
  def === other
    requirements.all? do |requirement|
      other.respond_to? requirement
    end
  end
end

class Object
  def quacks_like?(contract)
    contract === if self.respond_to? :const_defined? and self.const_defined? :Duck
      self::Duck
    else
      self
    end
  end
end

quack_test.rb

require 'minitest/autorun'

class QuackTest < MiniTest::Unit::TestCase
  
  Hash::Duck = Duck.new(:[], :[]=, :key?, :keys, :values)
  
  class Hash::Sub < Hash; end
    
  class Hash::Like
    def [](_); end
    def []=(_); end
    def key?(_); end
    def keys; end
    def values; end
  end
  
  def test_hash
    assert {}.quacks_like? Hash
    assert {}.quacks_like? Hash.new
  end
  
  def test_hash_subclass
    assert Hash::Sub.new.quacks_like? Hash
    assert Hash::Sub.new.quacks_like? Hash.new
  end
  
  def test_hash_like
    assert Hash::Like.new.quacks_like? Hash
    assert Hash::Like.new.quacks_like? Hash.new
  end
  
  def test_unlike_hash
    refute Array.new.quacks_like? Hash
    refute Array.new.quacks_like? Hash.new
  end
  
  def test_duckless_fallback
    assert Array.new.quacks_like? Array
    assert [1,2,3].quacks_like? [1,2,3]
    refute Hash.new.quacks_like? Array
    refute [1,2,3].quacks_like? [1,2]
  end
  
  def test_hash_case
    assert case {}
    when Hash::Duck
      true
    end
    refute case []
    when Hash::Duck
      true
    end
  end
      
end