Multiple late initialisation

问题:

I could not find a natural way to do something like that in scala :

class Car {
  var speed: Int
  var color: String
}

var myCar = new Car()

myCar.set {
  speed = 5
  color = "green"
}

I know it is possible in other languages as Groovy. I also know I can do it with a constructor like this :

val myCar = new Car { 
  speed = 5
  color = "green"
}

I am interested in a way to do the same, not at the object construction but later, once the object has already been created

This is what I have been doing so far :

class Car (var speed: Int, var color: String) {

  def set(f: (Car) => Unit) = {
    f(this)
  }

}

val myCar = new Car(5, "red")
myCar.set { c =>
  c.speed = 12
  c.color = "green"
}

But I do not like the need to write the 'c' var for every attribute

Any idea on how I can do it or if there is an easier way ??

回答1:

You should avoid mutable classes unless absolutely necessary. You would do normally this in Scala:

case class Car(speed: Int, color: String)

val c1 = Car(5, "red")
val c2 = c1.copy(speed = 12, color = "green")

(Then c2 is a new version of the car, while c1 remains unchanged.)

---

If you want to stick with your mutable type, why not just

class Car(var speed: Int, var color: String)

val myCar = new Car(5, "red")
import myCar._
speed = 12
color = "green"

---

Going with a dedicated set method:

class Car(var speed: Int, var color: String) {
  def set(speed: Int = this.speed, color: String = this.color): Unit = {
    this.speed = speed
    this.color = color
  }
}

val myCar = new Car(5, "red")
myCar.set(speed = 12, color = "green")
myCar.set(color = "blue")

回答2:

Although we all agree to say var reassignement is ugly, this is a possible solution

object DoTo { 
  def apply[T](that: T)(functions: (T) => Unit*): T = {
    functions foreach { _.apply(that) }
    that
  }    
}


class Car (var speed: Int, var color: String) {
  def move() = println("moving")
  def stop() = println("stop")
}


val myNewCar = DoTo(new Car(12, "red")) (
  _.move(),
  _.stop(),
  _.speed = 15,
  _.color = "green"
)

This is not exactly what I wanted initially but I cannot find anything simpler whithout using macros :-(

回答3:

You can do it by importing a variable:

class Car {
  var speed: Int = _
  var color: String = _
}

// ...
val myCar = new Car();
// a blocks that works with myCar:
{
    import myCar._
    // access the content without any prefix
    speed = 5
    color = "green"
}