Use State Machines

Here's one way to do implement a simple state machine like this in Scheme, with a procedure representing a state, and using tail calls (function calls that don't return) for the transitions:

  (define (order-creating-state)
    (let loop-within-state ()
      (let ((event (get-event)))
        (case (event-type event)

          ((add-item)
           (add-item-to-order (event-data event))
           (loop-within-state))

          ((delete-item)
           (delete-item-from-order (event-data event))
           (loop-within-state))

          ((finalize)
           (order-finalizing-state))

          (else (error "order-creating-state: invalid event"))))))

In Scheme you could also decide on your state model, and make the code more maintainable in terms of that state model, by defining a minilanguage or domain-specific language for it:

  (state order-creating (event)
         (add-item    (add-item-to-order event))
         (delete-item (delete-item-from-order event))
         (finalize    (transition-to order-finalizing)))