Implementation and simple trace of a foldRight (from Functional Programming in Scala, chapter 3)

foldRight_TakeOne_ImplAndSimpleTrace.scala

def foldRight[A,B](as: List[A], b: B)(f: (A, B) => B): B =
  as match {
    case Nil => b
    case Cons(x, xs) => f(x, foldRight(xs, b)(f))
  }

foldRight(List(1, 2, 3))                      ((x,y) => x + y)
foldRight(Cons(1, Cons(2, Cons(3, Nil))), 0)  ((x,y) => x + y)
1 + foldRight(Cons(2, Cons(3, Nil)), 0)       ((x,y) => x + y)
1 + (2 + foldRight(Cons(3, Nil), 0)           ((x,y) => x + y))
1 + (2 + (3 + (foldRight(Nil, 0)              ((x,y) => x + y)))) 
1 + (2 + (3 + 0))