Some of the State exercises

State.scala

trait RNG {
  def nextInt: (Int, RNG) // Should generate a random `Int`. We'll later define other functions in terms of `nextInt`.
}

object RNG {
  type Rand[+A] = RNG => (A, RNG)

  // ?? What does _ mean in this syntax? Access to the RNG trait?
  val int: Rand[Int] = _.nextInt 

  // Define ints with a non-tail recursion
  // Why does rng go up by 2??
  // Does this therefore have a bug, where we wind up using `r2` "by accident" in the next (recursive) call to `ints`?
  def ints(count: Int)(rng: RNG): (List[Int], RNG) =
    if (count == 0) (Nil, rng)
    else {
      val (i, r1) = rng.nextInt
      val (l, r2) = ints(count - 1)(r1)
      (i :: l, r2)
    }

  // Unpacking values (a pattern match doesn't make sense here)
  def map2[A,B,C](ra: Rand[A], rb: Rand[B])(f: (A, B) => C): Rand[C] =
    rng => {
      val (a, r1) = ra(rng)
      val (b, r2) = rb(r1)
      (f(a,b), r2)
    }


  def flatMap[A,B](f: Rand[A])(g: A => Rand[B]): Rand[B] =
    // Destructured assignments require 2 lines of code
    // 2 lines of code require being wrapped in braces
    rng => {
      val (a, r2) = f(rng)
      g(a)(r2)
    }

}