Is it possible to declare a val before assignment/initialization in Scala?

Question

In general, can you declare a val in scala before assigning it a value? If not, why not? An example where this might be useful (in my case at least) is that I want to declare a val which will be available in a larger scope than when I assign it. If I cannot do this, how can I achieve the desired behavior?

And I want this to be a val, not a var because after it is assigned, it should NEVER change, so a var isn't ideal.

For example:

object SomeObject {
    val theValIWantToDeclare // I don't have enough info to assign it here

    def main(): Unit = {
        theValIWantToDeclare = "some value"
    }

    def someOtherFunc(): Unit {
        val blah = someOperationWith(theValIWantToDeclare)
    }
}

Andrey Tyukin · Accepted Answer · 2018-06-29 15:36:29Z

4

object SomeObject {
  private var tviwtdPromise: Option[Int] = None
  lazy val theValIWantToDeclare: Int = tviwtdPromise.get
  private def declareTheVal(v: Int): Unit = {
    tviwtdPromise = Some(v)
    theValIWantToDeclare
  }

  def main(args: Array[String]): Unit = {
    declareTheVal(42)
    someOtherFunction()
  }

  def someOtherFunction(): Unit = {
    println(theValIWantToDeclare)
  }
}

It will crash with a NoSuchElementException if you try to use theValIWantToDeclare before fulfilling the "promise" with declareTheVal.

edited Jun 29, 2018 at 15:36

answered Jun 29, 2018 at 15:23

Andrey Tyukin

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8 Comments

Michael Blankenship Over a year ago

This looks like it will fix my issue! One question, what does the: theValIWantToDeclare () do in the private def function? Don't you just need the tviwtdPromise = Some(v) line?

som-snytt Over a year ago

There's also Promise.

Andrey Tyukin Over a year ago

@MichaelBlankenship you definitely should force theValIWantToDeclare to evaluate the value right now by using it in statement position. Otherwise you could "redeclare" it multiple times, and each time, it would override the hidden tviwtdPromise value. The () in the very end is just to make clear that Unit is returned. It's not needed, actually, I've removed it.

Andrey Tyukin Over a year ago

@som-snytt Yes, but AFAIR it requires some strange imports with default executor pools etc. I decided that adding Promise into the mix would be more confusing rathen than more "DRY".

Michael Blankenship Over a year ago

@AndreyTyukin I cannot thank you enough for the solution as well as answering my second question. Great explanations! Thank you! :)

|

fusion · Accepted Answer · 2018-06-29 15:16:42Z

0

It sounds to me that you need a lazy val.

A lazy val is populated on demand and the result is cached for all subsequent calls.

https://blog.codecentric.de/en/2016/02/lazy-vals-scala-look-hood/

answered Jun 29, 2018 at 15:16

fusion

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1 Comment

Michael Blankenship Over a year ago

The problem is lazy doesn't help me because I don't have "some value" at declaration time. So, I don't have the 'expression' which would execute when the val is needed.

som-snytt · Accepted Answer · 2018-06-29 15:37:02Z

0

Why not define a SomeObjectPartial that is partially constructed, and class SomeObject(theVal) that takes the value as a parameter?

Then your program has two states, one with the partial object, and another with the completed object.

answered Jun 29, 2018 at 15:37

som-snytt

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