Simple reduce

worksheet.scala

import scala.annotation.tailrec

def sum(numbers: Seq[Int]): Int =
  loopRight(numbers, 0, _ + _)

def product(numbers: Seq[Int]): Int =
  loopRight(numbers, 1, _ * _)

def sumTail(numbers: Seq[Int]): Int =
  loopLeft(numbers, 0, (acc, i) => acc + i)

def productTail(numbers: Seq[Int]): Int =
  loopLeft(numbers, 1, _ * _)

def loopLeft(numbers: Seq[Int], init: Int, f: ((Int, Int) => Int)) = {

  @tailrec
  def loop(acc: Int, seq: Seq[Int]): Int =
    if (seq.isEmpty) acc
    else loop(f(acc, seq.head), seq.tail)

  loop(init, numbers)
}

def loopRight(numbers: Seq[Int], init: Int, f: ((Int, Int) => Int)): Int =
  if (numbers.isEmpty) init
  else f(numbers.head, loopRight(numbers.tail, init, f))

sum((1 to 10).toList)

product((1 to 5).toList)

sumTail((1 to 10).toList)

productTail((1 to 5).toList)