arrow-fx-stm/arrow.fx.stm/STM/read

read

abstract fun <A> TVar<A>.read(): A(source)

Read the value from a TVar.

import arrow.fx.stm.TVar
import arrow.fx.stm.atomically

suspend fun main() {
  //sampleStart
  val tvar = TVar.new(10)
  val result = atomically {
    tvar.read()
  }
  //sampleEnd
  println(result)
}
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This comes with a few guarantees:

  • Any given TVar is only ever read once during a transaction.

  • When committing the transaction the value read has to be equal to the current value otherwise the transaction will retry

open fun <A> TMVar<A>.read(): A(source)

Read a value from a TMVar without removing it.

import arrow.fx.stm.TMVar
import arrow.fx.stm.atomically

suspend fun main() {
  //sampleStart
  val tmvar = TMVar.new(30)
  val result = atomically {
    tmvar.read()
  }
  //sampleEnd
  println("Result $result")
  println("New value ${atomically { tmvar.tryTake() } }")
}
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This retries if the TMVar is empty but does not take the value out if it succeeds.

See also

for a version that does not retry.

for a version that leaves the TMVar empty after reading.

open fun <A> TQueue<A>.read(): A(source)

Remove the front element from the TQueue or retry if the TQueue is empty.

import arrow.fx.stm.TQueue
import arrow.fx.stm.atomically

suspend fun main() {
  //sampleStart
  val tq = TQueue.new<Int>()
  val result = atomically {
    tq.write(2)
    tq.read()
  }
  //sampleEnd
  println("Result $result")
  println("Items in queue ${atomically { tq.flush() }}")
}
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See also

for a version that does not retry.

for a version that does not remove the element.