TPromise

TPromise is a mutable reference that can be set exactly once and can participate in transactions in STM.

Create a TPromise

Creating a TPromise:

import zio._
import zio.stm._

val tPromise: STM[Nothing, TPromise[String, Int]] = TPromise.make[String, Int]

Complete a TPromise

In order to successfully complete a TPromise:

import zio._
import zio.stm._

val tPromiseSucceed: UIO[TPromise[String, Int]] = for {
  tPromise <- TPromise.make[String, Int].commit
  _        <- tPromise.succeed(0).commit
} yield tPromise

In order to fail a TPromise use:

import zio._
import zio.stm._

val tPromiseFail: UIO[TPromise[String, Int]] = for {
  tPromise <- TPromise.make[String, Int].commit
  _        <- tPromise.fail("failed").commit
} yield tPromise

Alternatively, you can use done combinator and complete the promise by passing it Either[E, A]:

import zio._
import zio.stm._

val tPromiseDoneSucceed: UIO[TPromise[String, Int]] = for {
  tPromise <- TPromise.make[String, Int].commit
  _        <- tPromise.done(Right(0)).commit
} yield tPromise

val tPromiseDoneFail: UIO[TPromise[String, Int]] = for {
  tPromise <- TPromise.make[String, Int].commit
  _        <- tPromise.done(Left("failed")).commit
} yield tPromise

Once the value is set, any following attempts to set it will result in false.

Retrieve the value of a TPromise

Returns the result if the promise has already been completed or a None otherwise:

import zio._
import zio.stm._

val tPromiseOptionValue: UIO[Option[Either[String, Int]]] = for {
  tPromise <- TPromise.make[String, Int].commit
  _        <- tPromise.succeed(0).commit
  res      <- tPromise.poll.commit
} yield res

Alternatively, you can wait for the promise to be completed and return the value:

import zio._
import zio.stm._

val tPromiseValue: IO[String, Int] = for {
  tPromise <- TPromise.make[String, Int].commit
  _        <- tPromise.succeed(0).commit
  res      <- tPromise.await.commit
} yield res