gistfile1.rb

module ObjectSpace
  def each_instance_of(klass)
    each_object do |obj|
      yield(obj) if obj.instance_of? klass
    end
  end

  def each_kind_of(klass)
    each_object do |obj|
      yield(obj) if obj.kind_of? klass
    end
  end

  def each_such_that(query)
    each_object do |obj|
      yield(obj) if query[obj]
    end
  end
end

include ObjectSpace

class Foo; def whatever; end; end
x, y, z = Foo.new, Foo.new, Foo.new

class Bar; end
a = Bar.new

each_instance_of(Foo) { |x| p x }
each_such_that(->(x) { x.respond_to? :whatever }) { p x }