Clarification needed about futures and promises in Scala

Question

I am trying to get my head around Scala\'s promise and future constructs.

I\'ve been reading Futures and Promises in Scala Documentation and am a bit confused as I\'

Answer 1

Note that under the hood Future is implemented in terms of Promise and this Promise is completed with the body you passed to your Future:

def apply[T](body: =>T): Future[T] = impl.Future(body)  //here I have omitted the implicit ExecutorContext

impl.Future is an implementation of Future trait:

def apply[T](body: =>T)(implicit executor: ExecutionContext): scala.concurrent.Future[T] =
{
  val runnable = new PromiseCompletingRunnable(body)
  executor.prepare.execute(runnable)
  runnable.promise.future
}

Where PromiseCompletingRunnable looks like this:

class PromiseCompletingRunnable[T](body: => T) extends Runnable {
val promise = new Promise.DefaultPromise[T]()

override def run() = {
  promise complete {
    try Success(body) catch { case NonFatal(e) => Failure(e) }
  }
} }

So you see even though they are seperate concepts that you can make use of independently in reality you can't get Future without using Promise.