De-referencing Either back into the object

Question

I'm trying to use an Either return type from my function to either get back an object or a string. In the case it is an object, I'd like to start calling methods from this object. In the case it is a string, I'd like to call some other functions elsewhere. I keep getting hung up because the thing being returned is not the object I'm returning, it is of type "left" and I can't seem to get that object out of the "Left" type back into the "Player" type I'd like. This is contained in an object that is extending the mutable queue. Here is my function which looks up the Player object in a Map based off the key which is in my ActionQueue object:

def popCurrentAction : Either[Player, String] = {
  val currentAction = this.dequeue
  this.enqueue(currentAction)

  if (playerMap.get(currentAction) != None) {
    Left((playerMap.get(currentAction).get))
  }
  else {
    Right(currentAction)
  }
}

Here is my function which is trying to use the function which returns Either a "Player" object or a String.

def doMove = {
  var currentAction = ActionQueue.popCurrentAction
  if (currentAction.isLeft) {
    var currentPlayer = currentAction.left
    var playerDecision = currentPlayer.solicitDecision() // This doesn't work

    println(playerDecision)

  }
  else {
    // Do the stuff if what's returned is a string.
  }
}

I have tried using the .fold function, which does allow me to call the solicitDecision function and get what it returns, but I would like to use the Player object directly. Surely this is possible. Can someone help?

  var currentPlayer = currentAction
  var playerDecision = currentPlayer.fold(_.solicitDecision(), _.toString())
  // This is close but doesn't give me the object I'm trying to get!
  println(playerDecision)

What is the return type of `solicitDecision`? What do you expect to get back from `currentPlayer.fold(...)`? — danielnixon, Mar 14 '17 at 22:30
You cannot get the object out of the `Either`, because there might be no object inside. — ziggystar, Mar 15 '17 at 08:15

score 2 · Accepted Answer · answered Mar 14 '17 at 21:57

2

You haven't stated what error you're getting, just "this doesn't work", and the code you've posted isn't complete enough to be compiled and tested. What is the currentPlayer type when the compiler fails it?

Having said that, you might consider refactoring your code.

def doMove = ActionQueue.popCurrentAction.fold(getDecision, stringStuff)

def getDecision(player: Player) = ....
def stringStuff(str: String) = ....

answered Mar 14 '17 at 21:57

jwvh

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It's complaining because "solicitDecision is not a member of Either[Player, String]." currentPlayer is of type Either[Player, String], and solicitDecision is a method in the Player class. – shutch Mar 14 '17 at 22:05
1

Ah, the `.left` and `.right` methods are not what you think they are. They produce left/right _projections_ of the `Either`. They are not extractors. For that you need `currentAction.left.get`, which is workable but poor Scala style. – jwvh Mar 14 '17 at 22:19

score 2 · Answer 2 · answered Mar 15 '17 at 07:57

The best way for making case distinctions and extracting things at the same time is to use a pattern match.

In your case, that would be roughly the following code:

def doMove = {
  val currentAction = ActionQueue.popCurrentAction
  currentAction match {
    case Left(currentPlayer) => 
      val playerDecision = currentPlayer.solicitDecision()
      println(playerDecision)
    case Right(string) =>
      println(string)
  }
}

Note also that I used val instead of var. When you have a variable that you are not planning to change, val is better style (roughly speaking, it makes a read-only variable).

De-referencing Either back into the object

2 Answers2