compile

# File activesupport/lib/active_support/callbacks.rb, line 323
      def compile
        method =  ["value = nil", "halted = false"]
        callbacks = "value = !halted && (!block_given? || yield)"
        reverse_each do |callback|
          callbacks = callback.apply(callbacks)
        end
        method << callbacks

        method << "value"
        method.join("\n")
      end