def product(f: Int => Int)(a: Int, b: Int): Int = mapReduce(f, (x, y) => x * y, 1)(a, b)

  def factorial(x: Int) = product(x => x)(1, x)

  def mapReduce(f: Int => Int, combine: (Int, Int) => Int, zero: Int)(a: Int, b: Int): Int =
    if (a > b) zero
    else combine(f(a), mapReduce(f, combine, zero)(a + 1, b))