FML in ten minutes

This page reads one complete model, line by line, then runs it. The model is a to-do list: a list holds tasks, a task has a label and can be completed, and the list can tell which tasks are still open. It uses no external resource.

The model

import java.util.List;

@URI("http://openflexo.org/test/TestResourceCenter/TaskList.fml")
public model TaskList {

	Task[0,*] tasks;
	int taskCount values tasks.size;

	public Task addTask(String label) {
		Task task = new Task(parameters.label);
		tasks.add(task);
		return task;
	}

	public List<Task> openTasks() {
		return select Task from (this) where (selected.done == false);
	}

	public concept Task {

		String label;
		boolean done;
		String status values done ? "done" : "open";

		create(String label) {
			label = parameters.label;
			done = false;
		}

		public complete() {
			done = true;
		}
	}
}

Reading it

The virtual model. model TaskList { … } declares a virtual model, the unit that holds concepts. @URI(…) gives it its address, the one other models use to refer to it. import java.util.List; imports the Java type List, used by the return type of openTasks().

A property that holds other objects. Task[0,*] tasks; declares a property named tasks, whose values are instances of the concept Task, from zero to any number ([0,*] is the cardinality). The properties of a concept are the data of its instances.

A computed property. int taskCount values tasks.size; is read-only: its value is the result of the expression after values, and follows the properties that expression reads. tasks.size is a path, made of a property followed by what that property offers.

A behaviour. public Task addTask(String label) { … } is a behaviour of the model, the counterpart of a method. Its body is a sequence of instructions:

• new Task(parameters.label) creates an instance of the concept Task. The instance is created inside the instance of the model on which the behaviour runs.
• tasks.add(task); adds it to the property.
• return task; returns it.

A behaviour reads its arguments through parameters: write parameters.label, not label. A bare label does not designate the argument.

A query. select Task from (this) where (selected.done == false) looks for the instances of Task contained in the current instance and keeps those for which the condition holds; selected designates the instance being examined. The result is a List<Task>, the return type of the behaviour.

A concept. public concept Task { … } declares a concept nested in the model. Its properties are label, done and status, the last one computed with a conditional expression: done ? "done" : "open".

A creation scheme. create(String label) { … } is what runs when an instance of the concept is created with new Task(…): it plays the part of a constructor. It reads its argument through parameters.label and assigns it to the property label.

A behaviour of the concept. complete() has no return type: it only changes state.

Running it

An FML-script loads the model, creates an instance, calls the behaviours and checks the values they give. The model above and this script (an excerpt of it) are part of the automated tests of the openflexo-core repository, which run every assert.

virtualModel = load -r ["http://openflexo.org/test/TestResourceCenter/TaskList.fml"];
service ResourceCenterService add_temp_rc;
list = new TaskList() with (name="groceries");
assert list.taskCount == 0;

milk = list.addTask("Buy milk");
bread = list.addTask("Buy bread");
assert milk.status == "open";
assert milk.container == list;
assert list.taskCount == 2;
assert list.openTasks().size == 2;

milk.complete();
assert milk.status == "done";
assert list.openTasks().size == 1;
assert list.openTasks().get(0) == bread;

load reads the model from its address, new TaskList() with (name="groceries") creates an instance of it, and every assert states a value that must hold, or the run fails.

What you have seen

A model is made of concepts, which have properties (data, possibly computed) and behaviours (instructions, possibly querying), and are instantiated by the runtime.