How to handle state in a event trigger program ?

I know that state monad can use to handle state by returning an updated state. But most case in my job, we use event trigger, message bus and mediator pattern a lot. Like below example, Handler is a entry point and some events trigger them. How to use state monad and avoid side-effects? Or I shouldn’t struggle with this issue because it’s not worth it ?

namespace Application
{
    static class Robot
    {
        public enum RobotStateEnum
        {
            Up,
            Down,
            Unknown
        }

        public static RobotStateEnum state = RobotStateEnum.Unknown;
    }

    public class Robot_Up : INotification { }

    public class Robot_Down : INotification { }

    public class Robot_Up_Handler : INotificationHandler<Robot_Up>
    {
        public Task Handle(Robot_Up notification, CancellationToken cancellationToken)
        {
            Robot.state = Robot.RobotStateEnum.Up; // change state
            // Do something
            return Task.CompletedTask;
        }
    }

    public class Robot_Up_Handler2 : INotificationHandler<Robot_Up>
    {
        public Task Handle(Robot_Up notification, CancellationToken cancellationToken)
        {
            // Do something
            return Task.CompletedTask;
        }
    }

    public class Robot_Down_Handler : INotificationHandler<Robot_Down>
    {
        public Task Handle(Robot_Down notification, CancellationToken cancellationToken)
        {
            Robot.state = Robot.RobotStateEnum.Down; // change state
            // Do something
            return Task.CompletedTask;
        }
    }
}

[orthoxerox]

@qwas368

1. enums are not guaranteed to be restricted to the variants you’ve defined, (RobotEventEnum)42 is a valid enum value
2. DUs can have properties, e.g., MoveUp(int distance)

[qwas368]

Thanks for your detail explanation! I need more practice, now. 😃

The reason I suggest this, is because a lot of people use this library think that monads begin and end with Option or the other in-built monads.

You are right. The reason is we don’t need to reinvent the wheel. We can use Option such as IEnumerable.