Nathan has posted Simple State Machine to CodePlex, it is the first project that I am aware of that uses Rhino DSL and the techniques that I am talking about in the book.

What is impressive about this is the level of professionalism that is involved in the project. It is a full scale DSL, with all the supporting infrastructure. I spent half an hour or so going through the entire thing, and I am impressed.

Put simply, this is how I think state based work flows should be defined. I could easily see myself extending this a bit to add persistence support & integration with NServiceBus, and be done with it.

Like most state machines, it has the ideas of states, events that can cause the state to be changed, and legal transitions from state to state. You can define tasks which will be executed upon changing a state, or upon entering / leaving a certain state.

Enough talking, let us look at a reasonably complex work flow:

workflow "Order Lifecycle"

#Event & State Identifier Targets.
#This section controls which Types will be used
#to resolve Event or State names into strongly typed CLR objects.
#--------------------------------------------------------
state_identifier_target @OrderStatus
event_identifier_target @OrderEvents

#Global Actions
#--------------------------------------------------------
on_change_state      @WriteToHistory, "on_change_state"
on_workflow_start    @WriteToHistory, "on_workflow_start"
on_workflow_complete @WriteToHistory, "on_workflow_complete"

#Event Definitions
#--------------------------------------------------------
define_event  @OrderPlaced
define_event  @CreditCardApproved
define_event  @CreditCardDenied
define_event  @OrderCancelledByCustomer
define_event  @OutOfStock
define_event  @OrderStocked
define_event  @OrderShipped
define_event  @OrderReceived
define_event  @OrderLost

#State & Transition Definitions
#--------------------------------------------------------
state @AwaitingOrder:
       when @OrderPlaced              >> @AwaitingPayment

state @AwaitingPayment:
       when @CreditCardApproved       >> @AwaitingShipment
       when @CreditCardDenied         >> @OrderCancelled
       when @OrderCancelledByCustomer >> @OrderCancelled

state @AwaitingShipment:
       when @OrderCancelledByCustomer >> @OrderCancelled
       when @OutOfStock               >> @OnBackorder
       when @OrderShipped             >> @InTransit

       #Individual states can define transition events as well
       on_enter_state @WriteToHistory, "on_enter_state(AwaitingShipment)"

state @OnBackorder:
       when @OrderCancelledByCustomer >> @OrderCancelled
       when @OrderStocked             >> @AwaitingShipment

state @InTransit:
       when @OrderReceived            >> @OrderComplete
       when @OrderLost                >> @AwaitingShipment

#NOTE: State definitions without any transitions will cause
#the state machine to Complete when they are reached.
#------------------------------------------------------------
state @OrderComplete
state @OrderCancelled

Here is the demo application UI, for the order processing life cycle:

As I said, impressive.