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TQueue.scala
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TQueue.scala
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/*
* Copyright 2017 TimWSpence
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package io.github.timwspence.cats.stm
import scala.collection.immutable.Queue
/**
* Convenience definition of a queue in the `STM` monad.
*/
trait TQueueLike[F[_]] extends STMLike[F] {
final class TQueue[A] private[stm] (private val tvar: TVar[Queue[A]]) {
/**
* Enqueue a value.
*/
def put(a: A): Txn[Unit] = tvar.modify(_.enqueue(a))
/**
* Dequeue the first element. Retries if currently empty.
*/
def read: Txn[A] =
tvar.get.flatMap {
case q if q.isEmpty => retry
case q =>
val (head, tail) = q.dequeue
tvar.set(tail) >> pure(head)
}
/**
* Peek the first element. Retries if empty.
*/
def peek: Txn[A] =
tvar.get.flatMap {
case q if q.isEmpty => retry
case q => pure(q.head)
}
/**
* Attempt to dequeue the first element. Returns
* `None` if empty, `Some(head)` otherwise.
*/
def tryRead: Txn[Option[A]] =
tvar.get.flatMap {
case q if q.isEmpty => pure(None)
case q =>
val (head, tail) = q.dequeue
tvar.set(tail) >> pure(Some(head))
}
/**
* Attempt to peek the first element. Returns
* `None` if empty, `Some(head)` otherwise.
*/
def tryPeek: Txn[Option[A]] = tvar.get.map(_.headOption)
/**
* Check if currently empty.
*/
def isEmpty: Txn[Boolean] = tryPeek.map(_.isEmpty)
}
object TQueue {
/**
* Create a new empty `TQueue`.
*/
def empty[A]: Txn[TQueue[A]] = TVar.of(Queue.empty[A]).map(tvar => new TQueue[A](tvar))
}
}