make a lazy var in scala

Question

Scala does not permit to create laze vars, only lazy vals. It make sense.

But I\'ve bumped on use case, where I\'d like to have similar capability. I need a lazy var

Answer 1

This works:

var value: () => Int = _
val calc1: () => Int = () => { println("calc1"); 47 }
val calc2: () => Int = () => { println("calc2"); 11 }
value = calc1
value = calc2
var result = value + 1 /* prints "calc2" */

implicit def invokeUnitToInt(f: () => Int): Int = f()

Having the implicit worries me slightly because it is widely applicable, which might lead to unexpected applications or compiler errors about ambiguous implicits.

Another solution is using a wrapper object with a setter and a getter method that implement the lazy behaviour for you:

lazy val calc3 = { println("calc3"); 3 }
lazy val calc4 = { println("calc4"); 4 }

class LazyVar[A] {
  private var f: () => A = _
  def value: A = f() /* getter */
  def value_=(f: => A) = this.f = () => f /* setter */
}

var lv = new LazyVar[Int]
lv.value = calc3
lv.value = calc4
var result = lv.value + 1 /* prints "calc4 */