scala - Dealing with a Failed `Future` -

given following 2 methods:

def f: future[int] = future { 10 } def g: future[int] = future { 5 } 

i'd compose them:

scala> import scala.concurrent.future import scala.concurrent.future  scala> import scala.concurrent.future._ import scala.concurrent.future._  scala> import scala.concurrent.executioncontext.implicits.global import scala.concurrent.executioncontext.implicits.global  scala> {       |   <- f      |   b <- g      | } yield (a+b) res2: scala.concurrent.future[int] = scala.concurrent.impl.promise$defaultpromise@34f5090e 

now, i'll call await.result block until it's finished.

scala> import scala.concurrent.duration._ import scala.concurrent.duration._ 

as expected, 15, since await.result took future[int] , returned int.

scala> await.result(res2, 5.seconds) res6: int = 15 

defining recoverfn failed future:

scala> val recoverfn: partialfunction[throwable, future[int]] =      { case _ => future{0} } recoverfn: partialfunction[throwable,scala.concurrent.future[int]] = <function1> 

i try define failedfuture:

scala> def failedfuture: future[int] = future { 666 }.failed.recoverwith{ recoverfn } <console>:20: error: type mismatch;  found   : scala.concurrent.future[any]  required: scala.concurrent.future[int]        def failedfuture: future[int] = future { 666 }.failed.recoverwith{ recoverfn }                                                                         ^ 

but, above compile-time error.

specifically, how can fix error? generally, future#recoverwith typically how failed future's handled?

the problem future#failed always returns future[throwable]. it's purpose is not fail future, instead return failed projection of future. means if original future failed, converted successful future holds exception. , if original future succeeded, becomes failed, , holds nosuchelementexception. error getting because you're recovering future[throwable] future[int], has least upper-bound of future[any].

if you're trying play failed futures, try instead:

scala> future.failed[int](new exception("???")).recoverwith(recoverfn) res4: scala.concurrent.future[int] = scala.concurrent.impl.promise$defaultpromise@6933711b  scala> res4.value.get res5: scala.util.try[int] = success(0) 

there nothing wrong recoverwith.