Call by Value, Name, Need

Call by

• value
• name
• need

Call by Value

Evaluated on function call

def functionByValue(x: Long) = 
  println(x)
  println(x)

The param x is evaluated on the function call
From then on the value of x is fixed
So the two print statements will print the same value

Call by Name

Evaluated on usage of x

def functionByName(x: => Long) = 
  println(x)
  println(x)

The param x is evaluated on the moment of usage
So the two print statements will print the different values

Call by Need

Evaluated on the first time usage of x1

def functionByNeed(x: => Long) = {
  lazy val x1 = x
  println(x1)
  println(x1)

The param x is evaluated on the first println usage
From then on the value of lazy x1 is fixed
So the two print statements will print the same value

main

def main(args: Array[String]): Unit = 
  functionByValue(System.nanoTime())
  functionByName(System.nanoTime())
  functionByNeed(System.nanoTime())

// 118920267089041
// 118920267089041

// 118920327472000
// 118920327493583

// 118920327830750
// 118920327830750