「fold書けないプログラマー」と聞いてドキっとしたので書いて確かめて気を落ち着かせる

fold.scala

def foldr[A, B](f:(A, B) => B)(z:B)(xs:List[A]):B = {
  def go(xs:List[A]):B = xs match {
    case Nil       => z
    case x :: rest => f(x, go(rest))
  }
  go(xs)
}

def foldl[A, B](f:(A, B) => A)(z:A)(xs:List[B]):A = {
  def go(acc:A, xs:List[B]):A = xs match {
    case Nil       => acc
    case x :: rest => go(f(acc, x), rest)
  }
  go(z xs)
}