Ruby

class Foo
  def subject(&blk)
    if block_given?
      blk.call(@subject) # How do I pass by reference and not by value here?
    else
      @subject
    end
  end
end

f = Foo.new

p f.subject #=> nil

f.subject do |s|
  p s.object_id
  s = ['one', 'two', 'three']
  s.pop
  p s #=> ['one', 'two']
end

p f.subject #=> nil... want it to be ['one, 'two']