I am struggling to understand how this code 'strips out' the Future.
getFutureResult() returns a Future[String]. So how come the return from the yield is only a String?
def getFutureResult:Future[String] = { ... }
def getMyResult:String = {
for {
myFutureResult <- getFutureResult()
} yield {
myFutureResult
}
}
It is translated according to Scala for-comprehension translation rules:
for (x <- <expr>) yield f(x)
becomes:
<expr>.map(x => f(x))
This is a desugaring done by the compiler irregardless if <expr> has the type of a collection, a future or something else.
Future in Scala has a method map, so it uses the myFutureResult String from above to construct another Future[String].
You never know if the resulting future getMyResult is completed -- you should install an onComplete callback that will be called by the resulting future once it completes:
getMyResult onComplete {
case Success(x) => println("Yes! " + x)
case Failure(t) => println("An error: " + t)
}
The code for the onComplete method is executed asynchronously -- it might happen on a different thread, much later or simultaneously.
If you really need to know if the Future has completed, use the:
Await(getMyResult, 0 nanos)
pattern to block the calling thread until the getMyResult future is completed. In general, you should avoid this and compose your Future code using for-comprehensions and callback.
Related
I'm a beginner in scala and I'm trying to understand this syntax :
val f = Future {
Thread.sleep(Random.nextInt(500))
42
}
println("2- before onComplete")
f.onComplete {
case Success(value) => println(s"Got the callback, meaning = $value")
case Failure(e) => e.printStackTrace
}
Please explain these 2 blocks inside Future{...} and onComplete{...}
Thanks a lot
The first block is equal to something like that:
def code(): Int = {
Thread.sleep(Random.nextInt(500))
42
}
val f = Future(code)
Block acts like an anonymous function but it not reuqires to declare a function, it is just a syntax sugar.
So future f eagerly evaluates contents of a block asynchronously starting at the point of declaration. I.e. if random will yield a number n which is 0 < n < 500, future execution will be suspended for n milliseconds and then resumed and number 42 will be returned and set as a result of a Future.
As futures are non-blocking, while execution context of your future working on your future f, the main thread will run line
println("2- before onComplete")
and you will see this line in the console.
Then, the second block uses pattern matching to create a function that acts from Try[Int] to Unit, as it defined on both Success and Failure constructors of Try, it is an equivalent to:
f.onComplete { ttry =>
ttry match {
case Success(value) => println(s"Got the callback, meaning = $value")
case Failure(e) => e.printStackTrace
}
}
Note that you must await your future till application termination to ensure onComplete block execution.
In scala every statement is expression. Blocks are expressions also. Every expression has return value. These blocks will do something in blocks and return some values.
In Future block, it creates Future object witch contains 2 operations: sleep for random milliseconds and returns 42. In onComplete method block means handling two cases of Future evaluation, on success and on failure statements. If future will throw an error or evaluation will interrupted, Failure case will evaluate. If future evaluated successfully Success case will evaluate. Return value in both cases is Unit it's analogue of void.
Hi I have two Futures FutureA and FutureB i want to take a value from FutureB and use it in FutureA but all i am getting is null
here is code
futureA = ask(ActorA, MessageA).mapTo[Int]
FutureB = ask(ActorB, MessageB).mapTo[Int]
var someResult=0
futureA.map {
result =>
FutureB.map { x =>
someResult=x//suppose it will retun 5
}
someResult=someResult+1//it should give 6
println(someResult)//here i am getting 1
}
I want someResult to return 6 but it is returning 1
how can I achieve that
I do not want to use blocking call await.result
Don't use var, especially with futures. That's code smell and asking for trouble.
futureA
.flatMap { _ => FutureB }
.map { _ + 1 }
.onSuccess(println)
Dima's answer is absolutely correct. You're not assigning the result of FutureB.map so it's basically a side effect potentially executed in a background thread without your code knowing. You're likely to reach the print statement at runtime before the variable gets incremented, but maybe not, you can't know.
The right way of doing it is to use immutable variables and to chain your futures using flatMap/map.
Say you have a function called someFunction that takes two Ints and return a Result type : def someFunction(a: Int, b: Int): Result. You can then write the following code:
val result: Future[Result] =
futureA.flatMap { a =>
FutureB.map { b =>
someFunction(a, b)
}
}
You can also write it using a for-comprehension which compile to exactly the same thing but is the standard way of doing it:
val result: Future[Result] =
for {
a <- futureA
b <- FutureB
} yield someFunction(a, b)
Hope this answers your question.
I have the following snippet in Scala using Scalaz and Task/EitherT:
def myMethod(request: MyRequest) : EitherT[Task, Failure, Success] =
EitherT[Task, Failure, Success](
for {
aamOpt <- async.findUser(request)
aamEith <- Task.now(aamOpt.\/>(NoUserFound()))
result <- aamEith.fold(err => Task.now(-\/(err)), aam => process(aam)).run)
} yield result)
Where async.findUser returns a Task[Option[MyUser]] and process(aam) returns EitherT[Task, Failure, Success]. These return types are needed as these methods interact (down the line) with external services.
Is there any way to simplify the last line of the for comprehension into something nicer? I mean this one:
result <- aamEith.fold(err => Task.now(-\/(err)), aam => process(aam)).run)
I would probably immediately lift the result from async.findUser into the relevant EitherT monad and then just flatMap:
def myMethod(request: MyRequest): EitherT[Task, Failure, Success] =
EitherT.eitherT[Task, Failure, MyUser](
async.findUser(request).map(_.\/>(NoUserFound()))
).flatMap(process)
This is untested but something like it should more or less work.
If I have some computation that takes a while I might place it in a scala.concurrent.Future:
val f = Future { someLongRunningFunction() }
and let's say I want to do something else asynchronously once that computation is completed:
f.flatMap{ _ => anotherLongRunningFunction() }
In the event that f's initial block fails, how do I "idiomatically" handle this when using flatMap or other combinators? Is it merely a case of using recover or onFailure before the flatMap?
I like the elegance and simplicity of using a flatMap but it seems failure scenarios get in the way of it.
Edit: the second future is reliant on the first, hence the flatMap. I'm looking for a solution that'll elegantly let me chain like I would with flatMap but also handle failures of the first.
To quote the scaladoc for flatMap:
Creates a new future by applying a function to the successful result
of this future, and returns the result of the function as the new
future. If this future is completed with an exception then the new
future will also contain this exception.
Notice the bold, meaning that whatever you pass to flatMap will only be executed if the initial future completes successfully.
So you should only handle the result of the entire execution:
val result = future1.flatMap {
result => functionReturningFuture2(result)
}
and then:
result.onFailure // or
result.onSuccess // or
result.recover
If you have several futures you can put them in a for comprehension.
val futureResult = for {
result1 <- future1
result2 <- future2
...
} yield {
//computation with results
}
You can add a recover at the end in case you want to process any exception you may find:
futureResult.recover{
case exceptionResult: Throwable => // Process exception
}
I think this is more clean that using flatMap.
I'm only starting with Future, but here are some ideas.
If you really want to use flatMap, you have to turn the failure into a success.
for{ a <- f recover r
b <- another(a)
} yield b
This works if the return type of r is :> the result type of f.
Or you can pass the problem of what to do with the failure on to the next process
for{ a <- f map (x => Success(x)) recover (ex => Failure(ex))
b <- another(a)
} yield b
Here the argument type of another would be Try[T] where the type of f is Future[T].
This is a follow-up to my previous question
Suppose I want to create a future with my function but don't want to start it immediately (i.e. I do not want to call val f = Future { ... // my function}.
Now I see it can be done as follows:
val p = promise[Unit]
val f = p.future map { _ => // my function here }
Is it the only way to create a future with my function w/o executing it?
You can do something like this
val p = Promise[Unit]()
val f = p.future
//... some code run at a later time
p.success {
// your function
}
LATER EDIT:
I think the pattern you're looking for can be encapsulated like this:
class LatentComputation[T](f: => T) {
private val p = Promise[T]()
def trigger() { p.success(f) }
def future: Future[T] = p.future
}
object LatentComputation {
def apply[T](f: => T) = new LatentComputation(f)
}
You would use it like this:
val comp = LatentComputation {
// your code to be executed later
}
val f = comp.future
// somewhere else in the code
comp.trigger()
You could always defer creation with a closure, you'll not get the future object right ahead, but you get a handle to call later.
type DeferredComputation[T,R] = T => Future[R]
def deferredCall[T,R](futureBody: T => R): DeferredComputation[T,R] =
t => future {futureBody(t)}
def deferredResult[R](futureBody: => R): DeferredComputation[Unit,R] =
_ => future {futureBody}
If you are getting too fancy with execution control, maybe you should be using actors instead?
Or, perhaps, you should be using a Promise instead of a Future: a Promise can be passed on to others, while you keep it to "fulfill" it at a later time.
It's also worth giving a plug to Promise.completeWith.
You already know how to use p.future onComplete mystuff.
You can trigger that from another future using p completeWith f.
You can also define a function that creates and returns the Future, and then call it:
val double = (value: Int) => {
val f = Future { Thread.sleep(1000); value * 2 }
f.onComplete(x => println(s"Future return: $x"))
f
}
println("Before future.")
double(2)
println("After future is called, but as the future takes 1 sec to run, it will be printed before.")
I used this to executes futures in batches of n, something like:
// The functions that returns the future.
val double = (i: Int) => {
val future = Future ({
println(s"Start task $i")
Thread.sleep(1000)
i * 2
})
future.onComplete(_ => {
println(s"Task $i ended")
})
future
}
val numbers = 1 to 20
numbers
.map(i => (i, double))
.grouped(5)
.foreach(batch => {
val result = Await.result( Future.sequence(batch.map{ case (i, callback) => callback(i) }), 5.minutes )
println(result)
})
Or just use regular methods that return futures, and fire them in series using something like a for comprehension (sequential call-site evaluation)
This well known problem with standard libraries Future: they are designed in such a way that they are not referentially transparent, since they evaluate eagerly and memoize their result. In most use cases, this is totally fine and Scala developers rarely need to create non-evaluated future.
Take the following program:
val x = Future(...); f(x, x)
is not the same program as
f(Future(...), Future(...))
because in the first case the future is evaluated once, in the second case it is evaluated twice.
The are libraries which provide the necessary abstractions to work with referentially transparent asynchronous tasks, whose evaluation is deferred and not memoized unless explicitly required by the developer.
Scalaz Task
Monix Task
fs2
If you are looking to use Cats, Cats effects works nicely with both Monix and fs2.
this is a bit of a hack, since it have nothing to do with how future works but just adding lazy would suffice:
lazy val f = Future { ... // my function}
but note that this is sort of a type change as well, because whenever you reference it you will need to declare the reference as lazy too or it will be executed.