The “Tell, don’t ask” principle

This is one of my favorite principles because it is easy to spot when violated and because it helps in having proper API surfaces when applied.

Tell, don’t ask

In essence this principle proposes that instead of asking from an instance for its values in order to decide how the same instance will execute something, just tell the instance to execute it. It knows its own state, it can make its own decisions!

Asking

By asking we actually refer to accessing many of a class’s properties. For example:

	// somewhere in the code
	if (task.status == Status.NotStarted && task.createdAt < LocalDate.of(2021, 1, 14) && task.subscribers.isEmpty()) {
	task.close()
	}
	// which might have led to this API
	class Task(
	initialStatus: Status,
	val createdAt: LocalDate
	) {
	var status: Status = initialStatus
	private set
	private val innerSubscribers = mutableListOf<Subscriber>()
	val subscribers: List<Subscriber> = innerSubscribers
	fun add(subscriber: Subscriber) {
	innerSubscribers.add(subscriber)
	}
	fun close() {
	status = Status.Resolved
	}
	}

view raw tell_dont_ask__violation.kt hosted with ❤ by GitHub

in the code above we ask the task to provide the values of three of its properties in order to decide if we are going to close it or not.

In the process we (a) might had to expose those properties just for this code (creation date and subscribers could be private) and (b) inevitably leaked business logic that involves a task (when a task is eligible for closing).

Telling

Lets change the code in order to tell the task to close itself if possible:

	// somewhere in the code
	task.closeIfUnclaimed(createdBefore = LocalDate.of(2021, 1, 14))
	// task's better API
	class Task(
	initialStatus: Status,
	private val createdAt: LocalDate
	) {
	var status: Status = initialStatus
	private set
	private val subscribers = mutableListOf<Subscriber>()
	fun add(subscriber: Subscriber) {
	subscribers.add(subscriber)
	}
	fun closeIfUnclaimed(createdBefore: LocalDate): Boolean {
	if (status == Status.NotStarted && createdAt < createdBefore && subscribers.isEmpty()) {
	status = Resolved
	return true
	}
	return false
	}
	}

view raw tell_dont_ask__applied.kt hosted with ❤ by GitHub

There are two things that we gain from this change:

1. we moved the “closing” logic in the Task itself which allows us to contain future logic alterations in just one place.
2. the class’s API exposes only the necessary helping us to avoid accidental couplings.