strange_scala_futures_in_repl.scala

Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_45).
Type in expressions for evaluation. Or try :help.

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

scala> scala.concurrent.Future( { println("foo"); 4 })
foo
res0: scala.concurrent.Future[Int] = Success(4)

scala> res0.map(_ + 1)
res1: scala.concurrent.Future[Int] = List()      //It's a List. Wait. What?

scala> res0.map(_ + 1)
res2: scala.concurrent.Future[Int] = Success(5)  //OK, so it's a Success. Phew

scala> res0.map(_ + 1)
res3: scala.concurrent.Future[Int] = Success(5)  //Thanks goodness!

scala> res0.map(_ + 1)
res4: scala.concurrent.Future[Int] = List()      //It's a List again. What is going on?

scala> res0.map(_ + 1)
res5: scala.concurrent.Future[Int] = List()      //Still a List

scala> res0.map(_ + 1)
res6: scala.concurrent.Future[Int] = Success(5)  //um. Hang on, why *is* it a Success?

Yes, You are right.

Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_101).
Type in expressions for evaluation. Or try :help.

scala> 

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

scala>  scala.concurrent.Future( { println("foo"); 4 })
foo
res0: scala.concurrent.Future[Int] = Success(4)

scala> res0.map(_ + 1)
res1: scala.concurrent.Future[Int] = List()

scala> res0.map(_ + 1)
res2: scala.concurrent.Future[Int] = Success(5)

scala> res0.map(_ + 1)
res3: scala.concurrent.Future[Int] = Success(5)

scala> res0.map(_ + 1)
res4: scala.concurrent.Future[Int] = Success(5)

scala> res0.map(_ + 1)
res5: scala.concurrent.Future[Int] = List()

scala> res0.map(_ + 1)
res6: scala.concurrent.Future[Int] = List()

scala> res0.map(_ + 1)
res7: scala.concurrent.Future[Int] = List()

scala> res0.map(_ + 1)
res8: scala.concurrent.Future[Int] = List()

scala> res0.map(_ + 1)
res9: scala.concurrent.Future[Int] = Success(5)

scala> res0.map(_ + 1)
res10: scala.concurrent.Future[Int] = Success(5)

scala> res0.map(_ + 1)
res11: scala.concurrent.Future[Int] = Success(5)

but on 2.11.7 It' work fine

Welcome to Scala version 2.11.7 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_101).
Type in expressions to have them evaluated.
Type :help for more information.

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

scala>  scala.concurrent.Future( { println("foo"); 4 })
foo
res0: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@4d28aed2

scala>  scala.concurrent.Future( { println("foo"); 4 })
foo
res1: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@4daa513

scala> res1.map(_ + 1)
res2: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@179ac8cd

scala> res1.map(_ + 1)
res3: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@8aaab63

scala> res1.map(_ + 1)
res4: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@4c061956

scala> res1.value
res6: Option[scala.util.Try[Int]] = Some(Success(4))