closures in scala

Write a “factory function” that returns a function which implements a counter.

We can do this using Scala’s amazingly concise syntax, using a closure to encapsulate the mutable counter state.

val counterF = () => {
  var c = 0
  () => {
    c += 1
    c
  }
}

val counter = counterF()
counter()
// 1
counter()
// 2

val counter2 = counterF()
counter2()
// 1
counter()
// 3

Compare to an object-oriented approach:

class Counter() {
  private var count: Int = 0

  def inc: Int = {
    count += 1
    count
  }

  def dec: Int = {
    count -= 1
    count
  }
}

val counterF = () => new Counter()

val counter = counterF()

counter.inc
// 1
counter.inc
// 2
counter.dec
// 1