So now you can write
  fileSystems =
    [ { mountPoint = "/";
        device = "/dev/sda1";
      }
    ];
as
  fileSystems."/".device = "/dev/sda1";
		
	
So now you can write
  fileSystems =
    [ { mountPoint = "/";
        device = "/dev/sda1";
      }
    ];
as
  fileSystems."/".device = "/dev/sda1";