A solution to SftI Ex 21.3

gistfile1.scala

trait Factorial {
  def ! : BigInt
  //   ^ space required!
}

object Factorial {
  implicit def Int2Factorial(n: Int): Factorial = new Factorial {
    def ! = {
      if (n < 0) throw new RuntimeException("unable to compute factorial of negative number")
      @annotation.tailrec
      def fact(n: Int, prevRes: BigInt): BigInt =
        if (n == 0) prevRes else fact(n - 1, prevRes*n)
      fact(n, 1)
    }
  }
}

object test extends App {
  import Factorial._
  assert((0!) == BigInt(1))
  assert((1!) == BigInt(1))
  assert((2!) == BigInt(2))
  assert((3!) == BigInt(2*3))
  assert((4!) == BigInt(2*3*4))
  assert((5!) == BigInt(2*3*4*5))

  for (n <- 0 to 5) println(n!)
}