Call By Value vs Call By Name

Call By Value is just like any other programming language where we use the static value of the argument directly

     def calledByValue(x: Long): Unit = {
         println("by value: " + x)
         println("by value: " + x)
     }
     calledByValue(System.nanoTime())

Call By Name
Here instead of the value, the expression is passed as is and it will be evaluated by the compiler everytime. Call by Name is lazily evaluated.

     def calledByName(x: => Long): Unit = {
         // the arrow above is going to tell the compiler to evaluate the parameter by NAME
         // x is evaluated for every use
         println("by name: " + x)
         println("by name: " + x)
     }
     calledByName(System.nanoTime())

Lazy Evaluation example

def infinite(): Int = 1 + infinite()
def printFirst(x: Int, y: => Int) = println(x)

// println(infinite(), 34) // errors with stackoverflow

println(34, infinite()) // runs fine as second parameter is lazily evaluated and is never executed