grammar Shell
  rule command
    (interpolation / simple)+ {
      def execute(&block)
        yield elements.map do |e|
          e.execute(&block)
        end.join
      end
    }
  end

  rule interpolation
    '$(' command ')' {
      def execute(&block)
        yield command.execute(&block)
      end

      def node_type
        :interpolation
      end
    }
  end

  rule simple
    (!'$(' !')' .)+ {
      def execute(&block)
        text_value
      end

      def node_type
        :simple
      end
    }
  end
end