TSet

A TSet is a concurrent transactional implementation of a hashset.

Based on a Hash-Array-Mapped-Trie implementation. While this does mean that a read may take up to 5 steps to be resolved (depending on how well distributed the hash function is), it also means that structural changes can be isolated and thus do not increase contention with other transactions. This effectively means concurrent access to different values is unlikely to interfere with each other.

Hash conflicts are resolved by chaining.

Creating a TSet

Depending on whether or not you are in a transaction you can use either STM.newTSet or TSet.new to create a new TSet.

There are a few alternatives because TSet can be supplied a custom hash strategy. If no argument is given it defaults to Any.hashCode.

Adding elements to the set

Adding an element can be achieved by using either STM.insert or its alias STM.plusAssign:

import arrow.fx.stm.TSet
import arrow.fx.stm.atomically

suspend fun main() {
  //sampleStart
  val tset = TSet.new<String>()
  atomically {
    tset.insert("Hello")
    tset += "World"
  }
  //sampleEnd
}

Removing an element from the set

import arrow.fx.stm.TSet
import arrow.fx.stm.atomically

suspend fun main() {
  //sampleStart
  val tset = TSet.new<String>()
  atomically {
    tset.insert("Hello")
    tset.remove("Hello")
  }
  //sampleEnd
}

Checking for membership

import arrow.fx.stm.TSet
import arrow.fx.stm.atomically

suspend fun main() {
  //sampleStart
  val tset = TSet.new<String>()
  val result = atomically {
    tset.insert("Hello")
    tset.member("Hello")
  }
  //sampleEnd
  println("Result $result")
}

Where are operations like isEmpty or size?

This is a design tradeoff. It is entirely possible to track size however this usually requires one additional TVar for size and almost every operation would modify that. That will lead to contention and thus decrease performance.

Should this feature interest you and performance is not as important please open an issue. It is most certainly possible to add another version of TSet that keeps track of its size.