# File lib/money/bank/variable_exchange.rb, line 180
      def export_rates(format, file=nil)
        raise Money::Bank::UnknownRateFormat unless
          RATE_FORMATS.include? format

        s = ""
        @mutex.synchronize {
          s = case format
              when :json
                JSON.dump(@rates)
              when :ruby
                Marshal.dump(@rates)
              when :yaml
                YAML.dump(@rates)
              end

          unless file.nil?
            File.open(file, "w").write(s)
          end
        }
        s
      end