defining a singleton method on a attribute of an instance of a class

singleton_foo.rb

# defining a singleton method on a attribute of an instance of a class
# which method is recommended, define_singleton_methods is short-and-sweet. benchmark.

class FooNestedDef
  attr_accessor :info

  def initialize
    @info = []

    # NOTEL mindbending - def inside a def works. is this legal?
    def @info.to_s
      self.join(',')
    end
  end
end

# TODO: works on 1.9 Defined in SOAP::Mapping::define_singleton_method in 1.8
class FooDefineSingleton
  attr_accessor :info

  def initialize
    @info = []
    @info.define_singleton_method(:to_s) { self.join(',') }
  end
end

class FooInstEval
  attr_accessor :info

  def initialize
    @info = []
    @info.instance_eval do
      class << self
        def to_s
          self.join(',')
        end
      end
    end
  end
end

class FooExplicit
  attr_accessor :info

  def initialize
    @info = []
    class << @info
      def to_s
        self.join(',')
      end
    end
  end
end

__END__

f = FooNestedDef.new
f.info << "x" << "y"
p f.info.to_s
p f.info.method(:to_s).owner

f = FooDefineSingleton.new
f.info << "x" << "y"
p f.info.to_s
p f.info.method(:to_s).owner

f = FooInstEval.new
f.info << "x" << "y"
p f.info.to_s
p f.info.method(:to_s).owner

f = FooExplicit.new
f.info << "x" << "y"
p f.info.to_s
p f.info.method(:to_s).owner