arrow-fx-coroutines / arrow.fx.coroutines / Promise

Promise

interface ~~Promise~~<A> Deprecated: Use CompletableDeferred

A Promise is commonly used to provide and receive a value from 2 different threads. Since Promise can only be completed once unlike ConcurrentVar, we can consider it a synchronization primitive.

When made, a Promise is empty, and it remains in this state until it is fulfilled, which can only happen once.

Let’s say we wanted to await a Fiber, we could complete a Promise latch to signal it finished. Awaiting the latch Promise will now prevent main from finishing early.

import arrow.fx.coroutines.*

suspend fun main(): Unit {
  val await = Promise<Unit>()

  ForkConnected {
    println("Fiber starting up!")
    sleep(3.seconds)
    println("Fiber finished!")
    await.complete(Unit)
  }

  await.get() // Suspend until fiber finishes
}

Types

AlreadyFulfilled object AlreadyFulfilled

Functions

complete	Completes, or fulfills, the promise with the specified value A. Returns Promise.AlreadyFulfilled in Either.Left if the promise is already fulfilled.`abstract suspend fun complete(a: A): Either<AlreadyFulfilled,` `Unit>`
get	Gets the promised value or throws. Use `attempt` when throwing is not required. Suspends until the promised value is available.`abstract suspend fun get(): A`
tryGet	Tries to get the promised value, returning `null` if promise is not fulfilled yet. Returns A if promise is fulfilled.`abstract suspend fun tryGet(): A?`

Companion Object Functions

invoke	`suspend operator fun <A> ~~invoke~~():` `Promise<A>`
unsafe	`fun <A> ~~unsafe~~():` `Promise<A>`