Why do we need both Future and Promise?

Question

As I know, Future is read-only and Promise is write-once data structure.

We need a Promise to complete a Future

For example,

object Lie extends Throwable

val lie = Future { throw Lie } 

val guess = Promise[String]()     

lie.onComplete { case Success(s) => guess.success("I knew it was true!") 
                 case Failure(t) => guess.failure("I knew it was lie")} 
// return type: Unit 

guess.future.map(println) 
// res12: scala.concurrent.Future[Unit] = List()
// I knew it was lie!
// Requires Promise to chain Future with exception 

But, I can't understand why we need to have both Future and Promise

I guess Promise is required because of Future.onComplete signature

Since Future.onComplete return type is Unit,Future with possible exceptions cannot be chained

I assume Promise was introduced to overcome this limitation

But why not just change the signature of Future.onComplete ?

Changing the return type of Future.onComplete as Future[T] will enable chaining on Future with exception

and then, Future does not need Promise

For example, code above can be changed into

val lie = Future { throw Lie } 

lie.onComplete { 
   case Success(s) => "I knew it was true!"
   case Failure(t) => "I knew it was lie!"
}.map(println) 

//onComplete return type is Future[String]

My question is

1) am I right? does Future not need Promise , If onComplete signature is changed from Unit to Future[T]?

2) Why Future and Promise are divided in the first place ?

UDPATE

Thanks to the repliers, Now I understand the purpose of Promise. It wasn't actually for Future chaining

If I may, can I ask you

Why onComplete returns Unit ??

It can actually return Future[T] to enable chaining Future easily

For example

Future { throw Error }.onComplete {
  case Success(s) => "Success" 
  case Failure(t) => throw Error
}.onComplete {
  case Success(s) => "Success"
  case Failure(t) => throw Error 
}. ... 

Answer 1

Future.apply[T](block: => T): Future[T] is syntactic sugar for Future.unit.map(_ => block)[1]

A Future represents a value which may or may not be currently available.

A Promise represents the obligation to provide such a value at some point.

Having separate entities for Future (for reads) and Promise (for writes) means that it is easy to reason about capabilities:

• When a Future is a parameter, it is a request to have some value at some point and when it is used as a return type, it's a response which may not be currently available.

• When a Promise is a parameter, it is the "consumption" of responsibility of producing some value at some point, and when it is used as a return type it is the "production" of responsibility to produce the value at some point.

All in all, being able to reason about capabilities, especially in asynchronous, or even concurrent programs, is extremely valuable.

Most of the time Promises need not be used, since that is transparently handled by the Future-combinators—but when integrating with third party software or networking libraries it can be extremely useful.

For more information about interesting new features in Scala 2.12, have a look here.

1: Where Future.unit is defined as: val unit: Future[Unit] = Future.successful(())

Answer 2

am I right? does Future not need Promise , If onComplete signature is changed from Unit to Future[T]?

You're mixing things up a little. Let's clarify.

A Future[T] represents a computation which will complete in the future. That is, you pass Future.apply a function which will execute on a thread assigned by some ExecutionContext you define.

Now, on the other hand, a Promise[T] is a way to create a Future[T], without actually creating a Future[T]. A good example for this would be the Future.successful method (which will internally consume Promise.successful):

def successful[T](result: T): Future[T] = Promise.successful(result).future

This requires no ExecutionContext and no queuing if any additional resources. It's merely a convenience wrapper that allows you to "artificially" create a Future[T].

Answer 3

No.

Futurecannot cover the use cases of Promise. Promise has its own value. You cannot replace Future with Promise.

Future represents a computation which is available at a later point of time. You can get the result using onComplete once the future execution is complete and you can compose futures using map, flatMap, recover and recoverWith.

Promise is one time write container, clients can subscribe to the container. When subscribed client gets a future to wait until value is written into that container called promise.

Future is read-only

You cannot use future as one time write container. You can only read from future.

But Promise is different.

What if you want to give user something, which you do not have right now but you think you will have it soon ?

That means you are promising user something which you do not have right now.

So you want to keep the user waiting till you have the thing which you will give it to user. Thats when you do p.future and generate a future so that user can wait for the result using the future.

Once you have the result which you promised to give the user, You give to user by making the future a success or failure when something bad happens (i.e by doing p.complete).

Even if onComplete return type is changed to Future[T]. Future cannot act like or serve purpose of Promise.

Creating future using Future companion object

Future can be created by using Future.apply also. In this case future created would be complete once the computation inside the future finishes.

Future is used to subscribe to the result of the time-consuming computation, whereas Promise can be a publish and subscribe model.