Wrong scope of yield?

gistfile1.rb

# (Ruby version: 1.9.2)
#
##### A million-dollar question: why the following isn't working?
#
#

class A
  def self.metamethod(name)
    define_method(name) do
      yield
    end
  end
end

class B < A
  def m1
    "from m1"
  end
  
  metamethod :my_method do
    m1
  end
end

B.new.my_method #=> undefined local variable or method ‘m1’ for B:Class (* B:Class ??? *)

#
#
########### ...while the following does! #############
#
#

class A
  def self.metamethod(name)
    define_singleton_method(name) do
      yield
    end
  end
end

class B < A
  def self.m1
    "from m1"
  end
  
  metamethod :my_method do
    m1
  end
end

B.my_method #=> "from m1"

The solution was using instance_eval instead of yield:

class A
  def self.metamethod(name, &block)
    define_method(name) do
      instance_eval(&block)
    end
  end
end

class B < A
  def m1
    "from m1"
  end

  metamethod :my_method do
    m1
  end
end

B.new.my_method #=> "from m1"

Nice!