Why ConcurrentHashMap

import java.util.concurrent.ConcurrentHashMap

object Why {
  val data = new ConcurrentHashMap[Int, String]

  def update(int: Int, str: String): Unit = {
    data.put(int, str)
  }
}

// This compiles! But the return type of ConcurrentHashMap#put for our hash map `data` should be String
// Why does Scala let the `update` function return Unit?

WartRemover also catches value discarding - pretty much the first thing I put in 🐱

I strongly recommend using -Xfatal-warnings -Ywarn-value-discard!