On this page:
5.1 The event Structure
5.2 Event Lists
5.3 Example – Functions as Events
Version: 4.2.1

5 Events

    5.1 The event Structure

    5.2 Event Lists

    5.3 Example – Functions as Events

In a simulation model, an event represents an action that will take place in the (simulated) future.

In the PLT Scheme Simulation Collection, an event represents the (smulated) future application of a procedural object to a list of arguments.

5.1 The event Structure

  (struct event
    time              : (<=/c 0.0)
    priority          : real?
    process           : (or/c process? false/c)
    function          : (or/c procedure? false/c)
    arguments         : list?
    event-list        : (or/c event-list? false/c)
    linked-event-list : (or/c event-list? false/c)

Defines an event. Note that since the function field can contain any procedural object, including a continuation, any event can call the wait/work function. This is a slight extension to the definition of event given above in that an event may represent a sequence of actions that take place over a (simulated) duration.

(make-event time    
  arguments)  event?
  time : (<=/c 0.0)
  priority : real?
  process : (or/c process? false/c)
  function : (or/c procedure? false/c)
  arguments : list?
Returns a newly created event with the corresponsing fields set to the values of time, priority, function, and arguments. The event-list and linked-event-list fields are initialized to #f.

Note that make-event is not typically used in user code. Instead, the schedule macro is used to create and schedule events.

5.2 Event Lists

It isn’t generally necessary to explicitly manipulate the event lists in user code. See Chapter 13 Simulation Control (Advanced).

An event list stores a list of events. Events are stored in order of their time and priority values.

  (struct event-list (events))
  events : list?

Defines an event list. The current implementation just encapsulates a list that stores the events.

(make-event-list)  event-list?
Returns a new, empty event list.

(event-list-empty? event-list)  boolean?
  event-list : event-list?
Returns true, #t, if the event-list is empty, and false, #f, otherwise.

(event-list-add! event-list event)  any
  event-list : event-list?
  event : event?
Adds event to event list. Currently, the event list is ordered by time and priority.

(event-list-remove! event-list event)  any
  event-list : event-list?
  event : event?
Removes event from event-list. No error is signaled if the specified event is not on the event list.

(event-list-pop! event-list)  event?
  event-list : event-list?
Remove the first event from event-list and returns it.

5.3 Example – Functions as Events

This example is a simulation model of a simple system. Customer arrivals are exponentialy distributed with an interrival time of four minutes. The time a customer remains in the system is uniformly distributed between two and ten minutes. The output is a simple trace of customer arrivals and departures.

The random distribution functions are provided by the PLT Scheme Science Collection.

  #lang scheme/base
  ; Example 0 - Functions as Events
  (require (planet williams/simulation/simulation))
  (require (planet williams/science/random-distributions))
  (define (generator n)
    (for ((i (in-range n)))
      (wait (random-exponential 4.0))
      (schedule now (customer i))))
  (define (customer i)
    (printf "~a: customer ~a enters~n"
            (current-simulation-time) i)
    (work (random-flat 2.0 10.0))
    (printf "~a: customer ~a leaves~n"
            (current-simulation-time) i))
  (define (run-simulation n)
     (schedule (at 0.0) (generator n))
  (run-simulation 10)

Produces the following output.

0.6153910608822503: customer 0 enters
5.599485116393393: customer 1 enters
6.411843645405005: customer 2 enters
8.48917994426752: customer 0 leaves
10.275428842274628: customer 1 leaves
14.749397986170655: customer 2 leaves
23.525886616767437: customer 3 enters
27.18604340910279: customer 3 leaves
32.1644631797164: customer 4 enters
33.14558760001698: customer 5 enters
39.67682614849173: customer 4 leaves
40.486553934113665: customer 6 enters
41.168084930967424: customer 5 leaves
45.72670063299798: customer 6 leaves
46.747675912143016: customer 7 enters
49.212327970772435: customer 8 enters
50.556538752352886: customer 9 enters
51.46738784004611: customer 8 leaves
52.514846525674855: customer 7 leaves
56.11635302397275: customer 9 leaves

The require forms load the simulation collection and the random-distributions subcollection of the science collection from PLaneT – downloading them if necessary.

The generator function generates customers into the system. It takes a single argument, n, which is the total number of customers to generate. It provides the appropriate interrival time by waiting between each customer. After the wait, it schedules an event to represent the customer in the system. It does this n times to generate the appropriate number of customers.

The customer function represents a unique customer in the system. It takes a single argument, i, which is a unique integer between 0 and n-1, inclusive. The customer prints it’s arrival, works a random length of time, prints it’s departure, and ends.

The run-simulation function sets up and runs the simulation model. It creates a new simulation environment for the simulation model, schedules the generator to start execute as soon as the model starts (i.e. at time 0.0), and starts the simulation main loop by calling start-simulation. Note that the with-new-simulation-environment isn’t actually required in this case. However, it is a good idea to always use it to ensure the simulation model begins execution with a clean simulation environment.

Finally, the (run-simulation 10) form executes the simulation model, specifying a total of ten customers.

We will build on this trivial simulation model as we define more advanced simulation models. A few things to note at this point are: