Scala future traverse. In Scala, it’s said that a future returns “eventually.
Scala future traverse sleep the reason for this is that Future. sequence(userLocations. traverse(myList)(myFun) . 15 - scala. Implicits. traverse() function is fairly similar to the Future. Futures can't be cancelled. On your second snippet, you are calling filter over a Future, which doesn't work as you expect. val future = Future() future. I'm going to give your answer a try. Today it is commonly used in popular languages from Java to Dart. sequence to transform your Set[Future[_]] into a Future[Set], so you can wait for them all to finish:. So, you code. traverse lets us combine the Future. In this case, you need the proofs for List and Future. fromExecutor(Executors Introduction Scala Future represents a result of an asynchronous computation that may or may not be available yet. g. List(Option(1), Option(2), Option(3)). Futures are a pattern for concurrent code execution present in many languages (Java, Scala, Clojure to name a few). (I had to invent many details because your questions was very vague) Usually cats will pack specific evidence implicits under different imports. Let's implement traverseOption (you can implement sequenceOption in terms of traverseOption). You can do something as follows: Create a helper object which will use to aggregate the logic and log on failures, also maybe you would like to get a result of this component which will tell if Traverse. foreach(println) Please note that i'm calling foreach separately, using it in the call chain would lead to a different result type. You will see it in Future. EDIT: Added doc link for traverse the typesignature is a tad scary though. The reason why I want to do this is as follows: I send a message to an actor which uses Slick 3 to query a database. Re: Future. Developers want more, more, more: the 2024 results from Stack I agree with @Kolmar 's idea. data. Consider how callbacks on futures (map, flatMap, onComplete, fold, etc) are processed: they are placed in an executor's queue and are executed when the results of their parent futures are completed. test("execute futures sequentially") { import scala. So in essence you are just chaining Future. Putting it A non-blocking, asynchronous left fold over the specified futures, with the start value of the given zero. for { // create persistent records of the cloned client articles, and discard the response _ <- clonedArticles. Option has a Traverse instance, Thanks @Dima. sequence on Tuples. x it has a different signature. concurrent. 12. EDIT 2: to make this answer complete. lazy val futureC: Future[C] lazy vals in scala will be compiled in to the code which uses a synchronized block for thread safety. Even after removing the [and ] surrounding the Either (in the definition of Result) and changing mylist to myList I'm getting "type mismatch" errors in the 1st and 3rd cases. Both Future. traverse(promises)(_. traverse(seq) { case (f1, f2) => f1. In this situation, we can use the Future. Commented Feb 5, You cannot usually combine different monads in a single for comprehension (except for scala's collection-likes). Here you want to combine the Future and List monads, which cannot be done this way. userId) } This would leave you with a Future[Future[Vector[User]]], which can be fixed by changing the map to a flatMap. The Scalaz traverse Travis mentions does immediately give you a Future[Stream[String]], where the (single) Future does not complete until the entire Stream has had toFutureStream applied and run to completion. Computations are executed using an ExecutionContext, which is usually supplied implicitly, and which is import scala. Regex and import scala. generic. higherKinds /** * Linearize asynchronously applies a given function in Executing N times a Scala Future. global def getFuturePeople = Future { List("Steven", So, instead of just mapping on list of Original flatTraverse actually works in that use case since cats 2. sequence functions, traverse expects the return type of f to be G[B] whereas map just wants B. 2. Also note that using Future with cats is broken beyond repair for time being, and it's officially recommended in the docs to use cats-effect's IO instead. traverse: Future. scala; future; Share. You can think of an Option like a list of 1 or 0 elements but since is technically not It sounds like you want to create a NewsArticle instance based on the NewsCategory returned in the future from buildNewsCategory, which means that you were on the right track. As per the Scala API documentation, the traverse function also In this situation, we can use the Future. std. Improve this question. sequence needs a CanBuildFrom to build the collection inside the Future it returns. Many other methods in the standard library require a CanBuildFrom, for example most map methods in the collections API. sequence or Future. successful (obj) } then you can use the chained syntax shown below: As a side note, there is also a dedicated method Future. However, as modern applications are becoming I'm looking for a way to convert a Vector[(Future[TypeA], TypeB)] to a Future[Vector[(TypeA, TypeB)]]. import cats. traverse will find these names and signatures familiar. Their results might be ignored/discarded but they will run their course. Ask Question Asked 2 years, 5 months ago. Sign in Product Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company You can use Future. These effects tend to show up in functions working on a single piece of data - for instance parsing a single String into an Int, validating a login, or Before Scala 2. I believe that each just creates a new Promise that is completed in the onComplete for the preceeding Future and the Future from that Promise is what's returned as the result of the combinator. final def delegate[T](body: => Future Simple version of Future. Scala - Future. As a note: input. No, you can't. Validation[Throwable, T], it also works this way: Then that collapsed Future can be mapped back to a Map. By exemple items. Reveal more information for comprehension is a combination of flatMap and map. We then wait for this futureSequenceResults to complete as like we do for individual future. Seq or Scala 2. 12, the transform method had a different signature:. If all of your futures share the same In the Applicative tutorial we saw a more polymorphic version of the standard library Future. I want a tolerant/forgiving version of this method which on occurrence of errors carries on with the rest of the sequence. Ask Question Asked 6 years, 10 months ago. list. Since, it doesn't remove that element from the collection of futures, but rather returns a failed Future with a NoSuchElementException, which then makes all the code to fail with such exception. That means that when you reach the first println, the execution inside analyticsService. future) This will give you a Future[List[Unit]], which doesn't exactly qualify as "lots of intermediate collections", but isn't necessarily ideal See the following example Scala console interaction. Scalaz's sequence (and Haskell's sequenceA in Data. AbstractSeq[Int] with elements of type Future is an abstraction to represent the completion of an asynchronous operation. . The assignment val a = analyticsService. foldLeft(Future()){(x,y) => x. publishMagicNumber) It calls the publishMagicNumber method for each of the given magic numbers and combines them into a single Future. – jwvh I have a case class defined in scala as. sequence and map operations together. In = Future. If you really want to stop calling methodA() after the first Left result or Future failure (not the same thing) then you'll need to go synchronous. failed[Int](new Exception("42")) import scala. - btw, since you probably have that list of futures after Scala Standard Library 2. x's fallbackTo. Future import scala. 13. map(f). Every line with <-is converted into a flatMap but the last line which is converted to a map. - As the two answers suggest, you can use Future. duration. In your case, if you have a type Future[Future[]], you need to call flatten to get a flat Future[], but i guess you can double Labs The future of collective knowledge sharing; Scala: how to traverse stream/iterator collecting results into several different collections. id) returns immediately, because (and I'm taking a small leap here) analyticsService. andThen(v => println(v)) Future. Just the transform() in his solution is new the Scala 2. Asynchronous computations are created by calling Future. x version while in 2. A solution using Future. The fold is performed asynchronously in left-to-right order as the futur Let each Future return your partial result and then merge them into a list, in fact Future. I think the following will do what you want: Future. 2k bronze badges. If something returns a A it's Future[A]. Future won't something like this work: def sequenceSides[A](sides: Sides[Future[A]])(implicit exec: ExecutionContext): Future[Sides[A]] (There exists no Traverse instance for Future, unless you are willing to eagerly evaluate the Future. sequence is the same as input. If you want to make the function inside Future . sequence with the list of individual futures. tupled(x)) As noted in the comments you may want to run them in serial (although it's not 100% clear from the question), in that case you can use foldLeft and flatMap to chain future execution: scala Future to run sequential jobs. To specify one explicitly and the other then you can change this to a Future[Future[Seq[MyObject]]] with result. publishMagicNumber) It Here are a couple examples to show you how to use futures in scala code. apply, which yields instances of Future. ” The following examples show a variety of ways to create futures and work with their eventual results. io. scala> Future. implicit val singleThreadEc: ExecutionContext = ExecutionContext. So I'm looking for something that implements the Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Visit the blog Run code in Future block; Return Future from function; Wait for future; map; flatmap; for-comprehensions; Catch errors; Updated for Scala 2. val s: Set[scala. The Future. toList in my answer, but posting to show the usage of Future#traverse: import scala. traverse(paramList)(x => (myMethod _). With the identity function x => x as a parameter it behaves like the sequence Both code snippets delegate the execution of fatMatrix. The correct approach (simplified) I show the correct approach to using multiple Traverse provides a convenience method sequence that does exactly this. map((i, _))}. This will execute a function returning a Future for every item in the list (or other collection) and combine the results in one Future. I'm curious how global context is shutdown after all executions. – once more the solution is traverse. case class Node(key: String, value: String, var left: Node, var right: Node) and am trying to traverse it using tail recursion and a match case rather than loops and if statements. For your currency list this wil give you : val getForCurrency: String => Future[List[HistoricalCurrency]] = getccyHistory(_, "2015-07-12","2016-01-11") Future. - typelevel/cats Future. The Overflow Blog Generative AI is not going to build your engineering team for you. I'm extending little bit the answer, but I think you can find it useful. scala; future; or ask your own question. scalaFuture. traverse(f). map(_. ExecutionContext. The foldLeft combines all the logic from filter and minBy and produces the appropriate Option. traverse does just that. sequence's implicit parameter list consists of two parameters, and both must be present in any invocation. Here when the func(A) is called, it will obtain the lock for the lazy val and that thread will go to sleep. in a for-comprehension or directly in a map. Example, 1) Using global executionContext, I think they are similar. And those only work on the same monad. { Future, Promise } val promises = List(Promise[Unit], Promise[Unit], Promise[Unit]) Future. def getCommentListWithDetail( targetId: Long, sortOrder: Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company The reason is that traverse expects an Applicative from the nested effect, which cats provides for IO but not for Future. global def runFunction: Int = { val inputListOfInts = 1 to 1000 val fut: Future[List[Int]] = Future First, this is not a proper benchmark, you actually haven't show formal proof that this is sequential and not parallel (although is "obvious" from the source code that it isn't). 2k 1. Stan : Future. traverse as suggested by @IonutG. NOTE. {Await, Future} import scala. trait Data def writeToStore(data: Data): As the other answers have already stated: no, traverse doesn't (necessarily[1]) apply the transformation sequentially, to completion, for the elements. Because if I create my own ExecutionContext I have to manually shutdown. I want to apply the function f onto a sequence of Strings, such that I get a Seq[Future[Unit]]. futureInstance import scalaz. {Failure, Success, Try} val f1 = Future(Try(1 / 1)) val f2 = Future(Try Scala Knowledge Bits - Scala Traversable To implement traverse we re-use what we built in the previous episodes: out into F[List[A]]. Run code in Future block. This can be made slightly shorter using Future. A. traverse which will work on arbitrary many Future[A] instances: val f1 = Future. global import scala. global val x: Users of the standard library Future. traverse would work more succinctly here: Future. CanBuildFrom import language. traverse method, which performs a parallel map of multiple elements: val published: Future[List[Boolean]] = There is an alternate operator called traverse, which works similar but takes a function as an extra argument. If you want to do this in a "sequential" way (waiting for previous future operations to finish before starting new ones) you need to use a ListT monad transformer from My issue is that when I use map on a this collection of Futures, the type being mapped is Future[Int]. traverse(m){ case (i, f) => f. Another useful function here is Future. sequence List(Option(1), None, Option(3)). First way is: import scalaz. If you need to pass a value of type A to a function you need to do it in the context of Future, I. result(f, 1 second)) will return a scalaz. Some imports are required: import java. global val alphabet = 'a' to 'z' val text for { result <- Future. id) is a Future. To convert it into a single Future, we are using Future. Asynchronously and non-blockingly transforms, in essence, a IterableOnce[Future[A]] into a Future[IterableOnce[A]]. reduce/traverse is generic and therefore does not run faster with an associative operator, map on Nested works if there is an instance of Functor for both types. Modified 2 years, 5 months ago. util. EitherT import cats. map Lightweight, modular, and extensible library for functional programming. Using foldLeft/foldRight plus flatMap does work as expected. 11. Right now, I have some compilation errors. successful) ^ error: Cannot construct a collection of type scala. ? After you finish reading this post, you will feel more confident about these scala Futures things. collections. Builder import scala. You could however, make something equivalent to linearize. future. The reason why it does not work in your case is that you are using a helper method serialize where the right hand future (f2) is passed by value, thereby forcing its When you have a list (or any TraversableOnce) of futures and want a single future for computing the whole list, you use Future. 13 where it became the default Seq. option. A for comprehension is just sugar syntax for calls to flatMap & map. So, the reason is that due the eager and cached nature of Future , we can not really reason about its behavior. Implicits. global def f[A, B](x: Array[A], f: A => Future[Seq[B]]): Future[Seq[B]] = Future. global // ^ first implicit ExecutionContext class MyClass { def myMethod(e: EitherT[Future, _, _])(implicit ec: I know that a Future. ids. sequence call can convert a List[Future[T]] to a Future[List[T]], but what if I want to go the other way around?. id) is slow, most likely. Inf import scala. I, instead, want an Int so that I can asynchronously access my array. global You will get your expected result. traverse using just getCommentDetail you will get a Future[Seq[Option[CommentDetailDTO]]] which then you can map and use collect with the Seq to remove the Option. My current traversal method is as follows: Follow the types. import scala. myExecutionContext def Future. 0. ) import scalaz. Future implicit class ToFutureSuccessful [T](obj: T) { def asFuture: Future [T] = Future. In Scala, it’s said that a future returns “eventually. sequence() but cannot find out a way to manage the step from the tuple with a future to a future of tuple. Haskell's sequence isn't as generic as it could be, or as generic as Scalaz's (and I'm assuming you're okay with a Scalaz solution since you mention sequence). foreach(u => /* your import scala. flatMap(identity)", in fact, you can do this for any M[M[X]] to create a M[X] as long as M has flatMap. printSlow is a function that If you want to create multiple Scala Futures and merge their results together to get a result in a for comprehension, the correct approach is to (a) first create the futures, (b) merge their results in a for comprehension, then (c) extract the result using onComplete or a similar technique. _ val fofo: When you want to write parallel and concurrent applications in Scala, you could still use the native Java Thread — but the Scala Future makes parallel/concurrent programming much simpler, and it’s preferred. Future and Promises revolve around Simple version of Future. I found my way around using the 2. The question still stands: given a collection of Future[Int], how do I use the value generated by the future? – import scala. inverse() to an ExecutionContext and embody the result of the computation in inverseFuture. Navigation Menu Toggle navigation. Here’s a description of Future from its Scaladoc: “A Future represents a value which may or may not currently be available, but will be available at some point, or an I have a general query on future concepts of Scala. traverse and Future. _ import cats. Type parameters A Seems like causes of this issue can be various; I just ran into this because I had two implicit ExecutionContexts available in scope, so cats was unable to select one to provide the Functor[Future]:. successful) val res5: scala. Whenever there is any exception to any of the futures, it would finally execute the onComplete callback Failure block of code. getData(lesson. def transform[S](s: (T) ⇒ S, f: (Throwable) ⇒ Throwable): Future[S] The above method takes two functions as input that allows transforming a If you are using scala. Future[IndexedSeq[Int]] = Future(<not completed>) scala> Future. Follow asked Dec 24, 2015 at 18:15. Random import scala. Example, import scala. toMap) This will build a new collection within a Future from m, using the same function provided earlier to map tuples with futures to future tuples. If you do a simple Future. Second, and Iterator of Futures is probably a bad idea; at this point, it may make sense to look into a streaming solution like Akka-Streams, fs2, Monix or ZIO. collection. When it comes to scala futures, it seems You'll have to wait for all of the Futures to finish. Future. sequence produce a failed future if any of the futures they are built from fails, so you already have proper failure handling. traverse But I am not sure how I would use the result in this case to pass to my other service. Simple version of Future. – Randall Schulz. g:. map(users => users. A Future gives you a simple way to run an algorithm concurrently. Traversable) only requires that the outer type constructor have a Traverse instance—it doesn't necessarily have to be a list. 3 - scala. traverse. Sometimes our effectful functions return a Unit value in cases where there is no interesting value to return (e. id) has not completed, but A Future represents a value which may or may not be currently available, but will be available at some point, or an exception if that value could not be made available. traverse(1 to 5)(Future. title val pdf = doc. optionInstance import scalaz. scala. I tried to upgrade but ran into dependency problems. How to combine 2 Futures in Scala. Say I have a list of elements and foreach element present in the list i have to invoke a method context call-site simplifies to implicit val ec = tenThreadsEc Future(42) . traverse to execute parallel, you can do Future . When we create a new Future, Scal. sequence for instance that is very convenient to wait or combine many Future together. I don't like the x. In Scala, how do I share data between two We have 3 future methods here each of which returns different type. concurrent. sequence(_)) Then to collapse a Future[Future[X]] for any X, you can run "result. So if you add a line like this (or any other form of putting an implicit ec in scope):. catID val title = doc. Useful for reducing many Futures Basic use of futures is easy with the factory method on Future, which executes a provided function asynchronously, handing you back a future result of that function without blocking the current Future. traverse(magicNumbers)(publisher. I have a a list of ids in Scala and for each id I launch a future to perform a database operation like below: import myPackage. userId))) Or use Future. Viewed 199 times 1 I have a method, which may throw an Exception depends on passed value: private def transform If you are using scalaZ or cats, you can use more generic approach, which would work for more types ( Future would require to have Traverse typeclass, and Option just need to be a Monad). Future traverse. File, import scala. traverse[A, Seq[B], Seq](x Future. Asynchronously and non-blockingly transforms, in essence, a IterableOnce [Future [A]] into a Future [IterableOnce [A]]. traverse like so. What does following methods do over a scala Future. instances. map{ s2 => (s1, s2) } } I have a function f which returns a Future[Unit]. traverse in the Future companion object I'm playing with scala Futures with default global context and my own ExecutionContext. Future. traverse(a)(getForCurrency) // @ChrisGrimm, to the best of my knowledge the combinators on Future like map, flatMap, recover etc will not cause blocking behavior. A future starts running concurrently when you create it and returns a result at some point, well, in the future. Alternatively you can block the future and await for its result, but that's rarely a good strategy – It seems that your problem is not related to the number of futures you create, but the fairness with which they're executed. pdfLink val image = doc. Future[Some[User]]] = Set(Future(Some(User(1))), Future(Some(User(2)))) val f: Future[Set[Some[User]]] = Future. sequence(s) f. For-comprehension semi-parallel calls in scala. getById(location. Viewed 2k times The answer given here works fine, but I think Future. sequence to transform the list of futures into a future of a list, which is easier to use. val seq2 = Future. Duration. Since my Await. In this case, it is not. Future def validate(s: String): EitherT[Future, NumberFormatException, Int] = Don't try to do too many things at the same time, rather build the solution you need step by step. Now, since the function f can either fail or pass, we also convert it into a Try (to better handle the failures) using Scala Standard Library 2. traverse(seqTuple) { case (s1, s2Future) => s2Future. apply does not perform differently depending on whether it is called inside a map or a Future. zip(f2) } Or . traverse let's you take List[X] and X => Future[Y] and gives you Future[List[Y]] so it "flips" the containing types. then you can just map over the outer Future and filter the inner List. I removed the sequence call if that helps anything. BTW, here is a polished version of your working code which is a little bit more Planned maintenance impacting Stack Overflow and all Stack Exchange sites is scheduled for Wednesday, October 23, 2024, 9:00 PM-10:00 PM EDT (Thursday, October 24, 1:00 UTC - Thursday, October 24, 2:00 UTC). Here are a couple examples to show you how to use futures in scala code. Here is an example of how you may do that. You can use Future. sequence which converts it into a Future[Seq[Unit]]. global def runSequentially[A, B](lazyFutures: List[A ⇒ Future[B Try creating a pool with one thread and use Future. Future import scala. ExecutionContext. Blankman Blankman. In functional programming it is very common to encode "effects" as data types - common effects include Option for possibly missing values, Xor and Validated for possible errors, and Future for asynchronous computations. I'll try to figure them out myself and I'll give you some feedback so that, if some fixes are Future. writing to some sort of store). map(Future. Main Takeaway. All code can be found on this jupyter notebook. 3 and while using either scala. create(clonedArticle)) // add cloned I believe the most straightforward approach would be via a foreach function, e. Consider the following: import scala. The best you can do, if you can not change doSomeWork to return an F, is to convert your futures into Fs on the for. 3. sequence() function. traverse The return types are different. We then create a Future. mutable. immutable. I'm aware of the conversion of a collection of futures to a future of a collection using Future. syntax. When those blocked threads are run, the order cannot be guaranteed. map(location => userService. Useful for reducing many Futures into a single Future. Similarly the return type of traverse is G[F[B]] whereas The traverse method from Future object stops at first failure. Therefore func(B) & func(C) will blocked by the lock. The type signature of your tes value suggests that all Futures have already been launched. andThen(v => println(v )) } which Because analyticsService. traverse(userLocations) { location => userService. Currently we have added the following method to our utils: Scala: Future recover in Future traverse. traverse fits into executing a list that doesn’t depend upon each other (or side effects within your Future). I want to convert a Future[List[T]] into a List[Future[T]]. traverse(docs) { doc => val categoryID = doc. imageUrl val you need to traverse on List as Future. sequence Traversing for effect . traverse(1 until 5)(Future. _ import scala. Modified 3 years, 9 months ago. global def slowInt(i: Int) = { Thread. CanBuildFrom import However, you might as well not use Future . e. flatMap{_=>f(y)}} does exactly why you'd expect (executing f(y) serially for every y in items. And the Functor of Future requires and implicit ExecutionContext in scope, since all methods in Future requires it. matching. 267k 330 330 gold badges 794 794 silver badges 1. traverse method, which performs a parallel map of multiple elements: Future. traverse and use foldLeft and make the value sequential to create less IO in the function. Once you have a Future you have to keep it in one way or another, because otherwise you would need to await it which defeats the whole point of using it in the first place. Perhaps something like this: import scala. map(clonedArticle => clientArticleDAO.