The following one-liner hangs in Scala REPL
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.{Future, future, Await}
import scala.concurrent.duration._
def fib(n: Int): Int = {println(n + " is handeled by " + Thread.currentThread.getName) ; if (n < 2) n else Await.result( Future.sequence(List(1,2) map (i => Future{println(n-i); n-i} map fib)) map (_.sum), Duration.Inf)} ; fib(3)
Its core is
val lf: List[Future[Int]] = List(1,2) map (i => Future{n-i} map fib)
val f: Future[List[Int]] = Future.sequence(lf)
Await.result(f, Inf)
It also hangs in ammonite interpreter. It is more advanced and allows to break the execution and probably stack trace will tell you something
def fib(n: Int): Int = {println(n + " is handeled by " + Thread.currentThread.getName) ; if (n < 2) n else Await.result( Future.sequence(List(1,2) map (i => Future{println(n-i); n-i} map fib)) map (_.sum), Duration.Inf)} ; fib(3)
3 is handeled by main
1
2
^C
Interrupted!
java.lang.NoClassDefFoundError: Could not initialize class ammonite.session.cmd3$
at ammonite.session.cmd3$$anonfun$fib$1$$anonfun$apply$2.apply$mcII$sp(cmd3.scala:1)
at ammonite.session.cmd3$$anonfun$fib$1$$anonfun$apply$2.apply(cmd3.scala:1)
at ammonite.session.cmd3$$anonfun$fib$1$$anonfun$apply$2.apply(cmd3.scala:1)
at scala.util.Success$$anonfun$map$1.apply(Try.scala:237)
at scala.util.Try$.apply(Try.scala:192)
at scala.util.Success.map(Try.scala:237)
at scala.concurrent.Future$$anonfun$map$1.apply(Future.scala:237)
at scala.concurrent.Future$$anonfun$map$1.apply(Future.scala:237)
at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:32)
at scala.concurrent.impl.ExecutionContextImpl$AdaptedForkJoinTask.exec(ExecutionContextImpl.scala:121)
at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.pollAndExecAll(ForkJoinPool.java:1253)
at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1346)
at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)
java.lang.NoClassDefFoundError: Could not initialize class ammonite.session.cmd3$
at ammonite.session.cmd3$$anonfun$fib$1$$anonfun$apply$2.apply$mcII$sp(cmd3.scala:1)
at ammonite.session.cmd3$$anonfun$fib$1$$anonfun$apply$2.apply(cmd3.scala:1)
at ammonite.session.cmd3$$anonfun$fib$1$$anonfun$apply$2.apply(cmd3.scala:1)
at scala.util.Success$$anonfun$map$1.apply(Try.scala:237)
at scala.util.Try$.apply(Try.scala:192)
at scala.util.Success.map(Try.scala:237)
at scala.concurrent.Future$$anonfun$map$1.apply(Future.scala:237)
at scala.concurrent.Future$$anonfun$map$1.apply(Future.scala:237)
at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:32)
at scala.concurrent.impl.ExecutionContextImpl$AdaptedForkJoinTask.exec(ExecutionContextImpl.scala:121)
at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)
ubuntu-workspace@
As you see, the code says that n=3 is handeled by main, prints 1 and 2 and hangs. This means that futures have started but do not finish. They do not proceed to fib as mandated by map. I can replace Future{n-i} map fib with Future{fib(n-i)} but nothing changes. Meantime, the code proceeds normally in Scastie. This suggests that behaviour strongly depends on the context.
