someThunks.scala

/**
 * Make a lazy value type as a thunk
 */


/**
 * Lazy value wihtout memoization
 */
case class LazyVal[A](thunk : () => A) {
  def value : A = this.thunk()
}

/**
 * Lazy value with memoization
 * Stores value as an Option[A]
 * which is None if the value has not been evaluated yet
 * and Some(value) if it has.
 */
case class LazyVal2[A](thunk : () => A, value : Option[A]) {
  def evaluate : LazyVal2[A] = 
    this.value match {
      case Some(v) => this
      case _ => LazyVal2(this.thunk, Some(this.thunk()))
    }
}
/**
 * Slow way to compute the nth Fibonacci number
 */
def slowFibs(n : Int) : Int = 
  n match {
    case n if n <= 2 => 1
    case _ => slowFibs(n - 1) + slowFibs(n - 2)
  }

/**
 * Fast way for reference
 */
def fastFibs(n : Int) : BigInt = {
  
  object fib {
    val fibSequence : Stream[BigInt] = 
      1 #:: 1 #:: fibSequence.zip(fibSequence.drop(1)).map{case (a,b) => a + b}
  }

  fib.fibSequence.take(n).drop(n-1).head

}

def lazyFibs(n : Int) : LazyVal[Int] =
  LazyVal(() => slowFibs(n))

def lazyFibs2(n : Int) : LazyVal2[Int] = 
  LazyVal2(() => slowFibs(n), None)

def lazyFibs3(n : Int) : LazyVal2[BigInt] = 
  LazyVal2(() => fastFibs(n), None)

val l1 = lazyFibs2(45)
val l2 = lazyFibs3(500)

/**
 * scala> l1
 * res38: LazyVal2[Int] = LazyVal2(<function0>,None)
 * (instant, unevaluated)
 *
 * scala> l1.evaluate
 * res39: LazyVal2[Int] = LazyVal2(<function0>,Some(1134903170))
 * (takes a few seconds to evaluate)
 *
 * scala> l2
 * res40: LazyVal2[BigInt] = LazyVal2(<function0>,None)
 * (instant, unevaluated)
 *
 * scala> l2.evaluate
 * res41: LazyVal2[BigInt] = LazyVal2(<function0>,Some(139423224561697880139724382870407283950070256587697307264108962948325571622863290691557658876222521294125))
 * (evalutes pretty fast)
 */