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?

Found it: -Ywarn-value-discard

So it turns out you can solve this problem with an implicit def converting Unit to your own special MyUnit and then using MyUnit everywhere you'd otherwise use Unit

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

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