test.rb

class Note
  attr_accessor :message
  def initialize(attrs)
     @message = attrs[:message]
  end

  def save
    Note.all << self
  end

  def self.all
    @all ||= []
  end
end

class ShoesApp ; include Shoes
  url '/', :index

  def index
    para 'Say something...'
    flow do
      @note = edit_line
      button 'OK' do
        Note.new(:message => @note.text).save
        @note.text = ''
        @@result.replace get_notes    # error
      end
    end
    @@result = para get_notes # Works fine
  end

  def get_notes
    Note.all.map(&:message).join("\n")
  end
end

Shoes.app :title => 'Notes', :width => 260, :height => 350