Scala Basics

Val vs. Var

• vals are immutable; cannot be re-assgin
• compiler can infer types
• vars can be re-assign

Expressions

val x = 1 + 2 // EXPRESSION

• instruction vs. expression
• scala forces everything to be an Zexpression

val condition = true
val conditionValue =. if condition 5 else 3 // IF EXPRESSION

Called-by-Value VS. Called-by-Name

object CBNvsCBV extends App {
def calledByValue(x: Long): Unit = {
    println("by value: " + x) // timestamp
    println("by value: " + x) // timestamp
  }

  def calledByName(x: => Long): Unit = {
    println("by name: " + x) // timestamp1
    println("by name: " + x) // timestamp2
  }

  calledByValue(1257387745764245L)
  calledByName(System.nanoTime())

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

  //  printFirst(infinite(), 34) // stack overflow
  printFirst(34, infinite())
}

Difference:

By Value: evaluate System.nanoTime() first, and use the value throughout the execution

By Name: evaluate System.nanoTime() each time

• lazy evaluation: if the parameter called by name is never used, it will never be evaluated

Useful When:

• Lazy stream
• Exception handling with the Try type