Home > Articles

This chapter is from the book

26.6 Functional-Concurrent Programming Patterns

Both futures, on the one hand, and higher-order functions, on the other hand, are powerful abstractions. Together, they form a potent combination, though one that can take some effort to master. Even so, it is a worthwhile effort. This section illustrates a few guidelines you should keep in mind as you venture into functional-concurrent programming.

flatMap as an Alternative to Blocking

Higher-order functions are abstractions for code you don’t have to write. They are convenient but could often be replaced with handwritten implementations. If opt is an option, for instance, opt.map(f) could also be written:


opt match
   case Some(value) => Some(f(value))
   case None        => None

In the case of futures, however, higher-order functions are an alternative to computations that would be hard to implement directly. If fut is a future, what can you replace fut.map(f) with? A future cannot simply be “opened” to access its value, since the value may not yet exist. Short of creating—and blocking—additional threads, there is no alternative to using higher-order functions to act inside a future.

You can leverage your functional programming skills with higher-order functions when working with futures. Earlier, for instance, we used flatMap on options to chain computations that may or may not produce a value. You can use flatMap in a similar way on futures to chain computations that may or may not be asynchronous. Instead of “optional” stages, from A to Option[B], you define asynchronous stages, as functions from A to Future[B].

As an illustration, the three optional functions used in Section 10.3 can be changed to represent asynchronous steps:


def parseRequest(request: Request): Future[User] = ...
def getAccount(user: User): Future[Account] = ...
def applyOperation(account: Account, op: Operation): Future[Int] = ...

The steps can then be chained using flatMap:


Listing 26.9: A pipeline of futures using flatMap.
   .flatMap(user => getAccount(user))
   .flatMap(account => applyOperation(account, op))

The expression in Listing 26.9 is exactly the same as that in Listing 10.5, except that it produces a value of type Future[Int] instead of Option[Int].

Uniform Treatment of Synchronous and Asynchronous Computations

You could mix synchronous and asynchronous operations by combining steps of type A => B—using map—and steps of type A => Future[B]—using flatMap. Instead, it is often more convenient to use only steps of the form A => Future[B] combined with flatMap. When needed, synchronous steps can be implemented as already completed futures. This design increases flexibility: It makes it easier to replace synchronous steps with asynchronous steps, and vice versa.

For instance, if accounts are simply stored in a map, the getAccount function from the earlier example can be implemented synchronously, within the calling thread:


val allAccounts: Map[User, Account] = ...
def getAccount(user: User): Future[Account] = Future.successful(allAccounts(user))

This function returns an already completed future and does not involve any additional thread. If a need to fetch accounts asynchronously then arises, you can reimplement the function without modifying its signature, and leave all the code that uses it—such as Listing 26.9—unchanged.

Functional Handling of Failures

Exceptions are typically thrown and caught within a thread. They don’t naturally travel from thread to thread, and they are ill suited for multithreaded programming. Instead, you are better off following the functional approach to error handling discussed in Chapter 13.

An added benefit of relying on computations of type A => Future[B] instead of A => B is that futures can also carry failures—in Scala, you can think of Future as an asynchronous Try. For example, you can improve the getAccount function by making sure it always produces a future, even when a user is not found:


def getAccount(user: User): Future[Account] = Future.fromTry(Try(allAccounts(user)))

This way, an expression like getAccount(user).onComplete(...) still executes a callback action, which is not true if getAccount throws an exception. Failed futures can be handled functionally, using dedicated functions such as recover in Scala or exceptionally in Java.

For simplicity, the connection-handling function from Listing 26.8 does not deal with errors. You could use standard future functions to add robustness to the server. For instance, failure to create a page could be handled by transforming the pageF future:


val safePageF: Future[Page] = pageF.recover { case ex: PageException => errorPage(ex) }

or by adding a failure callback:


pageF.failed.foreach { ex =>

Either the callback actions specified using pageF.foreach or those specified using pageF.failed.foreach will run, but not both.

Non-Blocking “Join” Pattern

In the server example, pageF is created by combining two futures, dataF and adF, using flatMap. You can use the same approach to combine three or more futures:


val f1: Future[Int]    = ...
val f2: Future[String] = ...
val f3: Future[Double] = ...

val f: Future[(Int, String, Double)] =
   f1.flatMap(n => f2.flatMap(s => f3.map(d => (n, s, d))))

This won’t scale to larger numbers of futures, though. An interesting and not uncommon case is to combine N futures of the same type into a single one, for an arbitrary number N. In the server example, a client might obtain data from N database queries, which are executed in parallel:


def queryDB(requests: List[Request]): Future[Page] =
   val futures: List[Future[Data]] = requests.map(request => Future(dbLookup(request)))
   val dataListF: Future[List[Data]] = Future.sequence(futures)

The first line uses map to create a list of database-querying tasks, one for each request. These tasks, which run in parallel, form a list of futures. The key step in queryDB is the call to Future.sequence. This function uses an input of type List[Future[A]] to produce an output of type Future[List[A]]. The future it returns is completed when all the input futures are completed, and it contains all their values as a list (assuming no errors). Invoking Future.sequence serves the same purpose as the “join” part of a fork-join pattern, but does so without blocking. The last step uses a function makeBigPage from List[Data] to Page to build the final page.

As of this writing, there is no standard sequence function for CompletableFuture, but you can implement your own using thenCompose (equivalent to flatMap) and thenApply (equivalent to map):


Listing 26.10: Joining a list of CompletableFutures into one without blocking.
def sequence[A](futures: List[CompletableFuture[A]]): CompletableFuture[List[A]] =
   futures match
      case Nil => CompletableFuture.completedFuture(List.empty)
      case future :: more =>
         future.thenCompose(first => sequence(more).thenApply(others => first::others))

This function uses recursion to nest calls to thenCompose (flatMap). In the recursive branch, sequence(more) is a future that will contain the values of all the input futures, except the first. This future and the first input future are then combined using thenCompose and thenApply (flatMap and map), according to the pattern used earlier to merge two futures (as in Listings 26.5, 26.7, and 26.8).

Non-Blocking “Fork-Join” Pattern

Function queryDB uses a fork-join pattern in which sequence implements the “join” part without blocking. Fork-join is a common enough pattern that Scala defines a function traverse that implements both the “fork” and “join” parts of a computation. You can use it for a simpler implementation of queryDB:


def queryDB(requests: List[Request]): Future[Page] =
   Future.traverse(requests)(request => Future(dbLookup(request))).map(makeBigPage)

Instead of working on a list of futures, as sequence does, function traverse uses a list of inputs and a function from input to future output. It “forks” a collection of tasks by applying the function to all inputs, and then “joins” the tasks into a single future, as in sequence, without blocking.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020