How to compose Future and Nullable in Kotlin

May 17, 2016

In a previous post, we’ve seen how to compose Future and Option in Scala. Let’s try to do the same with Kotlin using nullables types and Java 8 completable futures.

This is how you can compose nullable functions :

  fun giveInt(x: Int):Int? = x+1

  fun giveInt2(x: Int):Int? = x+2

  fun combine(x: Int): Int? = giveInt(x)?.let(::giveInt2)

  combine(1) //4

The result of giveInt is passed to giveInt2. But let only applies if the first part is not null. Either a nullable is returned.

This is how you can compose futures (using Java 8 CompletableFuture API):

  fun giveInt(x: Int): CompletableFuture<Int> = CompletableFuture.supplyAsync({ x + 1 })

  fun giveInt2(x: Int): CompletableFuture<Int> = CompletableFuture.supplyAsync({ x + 2 })

  fun combine(x: Int): CompletableFuture<Int> =  giveInt(x).thenCompose(::giveInt2)

  combine(1).get() //4

This is how you can compose futures of nullable types :

  fun giveInt(x: Int): CompletableFuture<Int?> = CompletableFuture.supplyAsync({ x + 1 })

  fun giveInt2(x: Int): CompletableFuture<Int?> = CompletableFuture.supplyAsync({ x + 2 })

  fun combine(x: Int): CompletableFuture<Int?> = giveInt(x).thenCompose({ it?.let(::giveInt2) })

  combine(1).get() //4

And this how we would have done it with Scala and with the Hamsters library :

def giveInt(x: Int): Future[Option[Int]] = Future.successful(Some(x+1))

def giveInt2(x: Int): Future[Option[Int]] = Future.successful(Some(x+2))

def combine(x: Int): Future[Option[Int]] = for {
  y <- FutureOption(giveInt(x))
  z <- FutureOption(giveInt2(y))
} yield z

Await.result(combine(1), 1 second) //Some(4)

Finally even if it’s not possible to use monad transformers as we would do in Scala, if think Kotlin is giving out of the box a very simple and readable solution.

And this may be even simpler in the future using Kotlin coroutines.