TRef

A TRef[A] is a mutable reference to an immutable value, which can participate in transactions in STM. The mutable reference can be retrieved and set from within transactions, with strong guarantees for atomicity, consistency, and isolation from other transactions.

TRef provides the low-level machinery to create transactions from modifications of STM memory.

Create a TRef

Creating a TRef inside a transaction:

import zio._
import zio.stm._

val createTRef: STM[Nothing, TRef[Int]] = TRef.make(10)

Or creating a TRef inside a transaction, and immediately committing the transaction, which allows you to store and pass along the reference.

import zio._
import zio.stm._

val commitTRef: UIO[TRef[Int]] = TRef.makeCommit(10)

Retrieve the value out of a TRef

Retrieving the value in a single transaction:

import zio._
import zio.stm._

val retrieveSingle: UIO[Int] = (for {
  tRef <- TRef.make(10)
  value <- tRef.get
} yield value).commit

Or on multiple transactional statements:

import zio._
import zio.stm._

val retrieveMultiple: UIO[Int] = for {
  tRef <- TRef.makeCommit(10)
  value <- tRef.get.commit
} yield value

Set a value to a TRef

Setting the value overwrites the existing content of a reference.

Setting the value in a single transaction:

import zio._
import zio.stm._

val setSingle: UIO[Int] = (for {
  tRef <- TRef.make(10)
  _ <- tRef.set(20)
  nValue <- tRef.get
} yield nValue).commit

Or on multiple transactions:

import zio._
import zio.stm._

val setMultiple: UIO[Int] = for {
  tRef <- TRef.makeCommit(10)
  nValue <- tRef.set(20).flatMap(_ => tRef.get).commit
} yield nValue

Update the value of the TRef

The update function A => A allows computing a new value for the TRef using the old value.

Updating the value in a single transaction:

import zio._
import zio.stm._

val updateSingle: UIO[Int] = (for {
  tRef <- TRef.make(10)
  nValue <- tRef.updateAndGet(_ + 20)
} yield nValue).commit

Or on multiple transactions:

import zio._
import zio.stm._

val updateMultiple: UIO[Int] = for {
  tRef <- TRef.makeCommit(10)
  nValue <- tRef.updateAndGet(_ + 20).commit
} yield nValue

Modify the value of the TRef

The modify function A => (B, A): B works similar to update, but allows extracting some information (the B) out of the update operation.

Modify the value in a single transaction:

import zio._
import zio.stm._

val modifySingle: UIO[(String, Int)] = (for {
  tRef <- TRef.make(10)
  mValue <- tRef.modify(v => ("Zee-Oh", v + 10))
  nValue <- tRef.get
} yield (mValue, nValue)).commit

Or on multiple transactions:

import zio._
import zio.stm._

val modifyMultiple: UIO[(String, Int)] = for {
  tRef <- TRef.makeCommit(10)
  tuple2 <- tRef.modify(v => ("Zee-Oh", v + 10)).zip(tRef.get).commit
} yield tuple2

Example usage

Here is a scenario where we use a TRef to hand-off a value between two Fibers

import zio._
import zio.stm._

def transfer(tSender: TRef[Int],
             tReceiver: TRef[Int],
             amount: Int): UIO[Int] = {
  STM.atomically {
    for {
      _ <- tSender.get.retryUntil(_ >= amount)
      _ <- tSender.update(_ - amount)
      nAmount <- tReceiver.updateAndGet(_ + amount)
    } yield nAmount
  }
}

val transferredMoney: UIO[String] = for {
  tSender <- TRef.makeCommit(50)
  tReceiver <- TRef.makeCommit(100)
  _ <- transfer(tSender, tReceiver, 50).fork
  _ <- tSender.get.retryUntil(_ == 0).commit
  tuple2 <- tSender.get.zip(tReceiver.get).commit
  (senderBalance, receiverBalance) = tuple2
} yield s"sender: $senderBalance & receiver: $receiverBalance"

In this example, we create and commit two transactional references for the sender and receiver to be able to extract their value. On the following step, we create an atomic transactional that updates both accounts only when there is sufficient balance available in the sender account. In the end, we fork to run asynchronously. On the running fiber, we suspend until the sender balance suffers changes, in this case, to reach zero. Finally, we extract the new values out of the accounts and combine them in one result.