Is it possible and how to have var that can only be set once?

Question

Is there a native way to make sure that a variable can only be set once?

Currently, I am using this approach

class SetOnceVariable[T]
{
  private var value : T = _

  private var initialized = false

  def apply(_v : T = _) : T =
  {
    if (value != null && !initialized) {
      value = _v
      initialized = true
    }
    value
  }
}

class ClientInfo
{
  val userIP : SetOnceVariable[String] = new SetOnceVariable[String]
}

Answer 1

There's no such language construct, but I think I can clean up your code, at least.

class SetOnce[A](var toOption: Option[A] = None) {
    def set(a: A): Unit = if (toOption.isEmpty) toOption = Some(a)
    def get: A = toOption.get
}

Usage:

val x = new SetOnce[Int]
x.toOption // None

x.set(1)
x.get // 1

x.set(2)
x.get // 1

I omitted the null consideration because idiomatic Scala code tends to not use or consider null outside of Java compatibility. We mostly pretend that it doesn't exist.

Answer 2

Approach using lazy:

  class SetOnceVariable[T] {

    private var v: T = _
    private lazy val value: T = v

    def apply(_v: T = ???): T = {
      v = _v
      value
    }
  }

val info = new ClientInfo
println(info.userIP("IP")) // IP
println(info.userIP("IP2")) // IP
println(info.userIP("IP3")) // IP

To make it threadsafe you can use:

def apply(_v: T = ???): T =
  synchronized {
    v = _v
    value
  }