CODEWITHSUNDEEP
scala
Mojo
Mojo

Reputation: 1202

Future Not Completed But Looks Like it Has

I am a Scala newbie trying to understand Futures. I typed in the following in the REPL:

scala> import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.ExecutionContext.Implicits.global

scala> import scala.concurrent.Future
import scala.concurrent.Future

scala> val m = Future(println("Message"))
Message
m: scala.concurrent.Future[Unit] = Future(<not completed>)

scala> Future(println("Another Message"))
res4: scala.concurrent.Future[Unit] = Future(<not completed>)
Another Message

In the first case I am assigning a Future computation to a variable m. I haven't called m yet but the Future actually produces the "Message" string output. But I also get a Future(<not completed>) message. What is going on here? Has the future completed or not? The string output tells me it has. Can somebody clear this up for me?

Upvotes: 0

Views: 152

Answers (2)

pme
pme

Reputation: 14803

It is completed, what you see Future(<not completed>) is just the `toString() from the Future.

try:

import scala.concurrent.ExecutionContext.Implicits.global

val f = scala.concurrent.Future{
  println("Message")
}

println(f.toString())

Future execute always eagerly.

Upvotes: 1

Terry Dactyl
Terry Dactyl

Reputation: 1868

A Future will start executing as soon as it is created (Depending on the ExecutionContext implementation and assuming resources are available for it to run.)

The fact that you see your message printed to the console indicates your Futures have completed.

Try the following and see what happens:

Future{
  Thread.sleep(5000)
  println("Message")
}

Upvotes: 2

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