Scala's sexy, sexy implicits...

gistfile1.txt

implicit class Factorial(val n: Int) extends AnyVal{
  def `!`: BigInt = n match {
  case _ if n < 0 => throw new IllegalArgumentException()
  case 0 => 1L
  case _ => (1 to n).foldLeft(1L)(_*_)
  }
  }
defined class Factorial

@ 5!
res3: BigInt = 120

@ 0!
res4: BigInt = 1

@ 2!
res5: BigInt = 2

@ 3!
res6: BigInt = 6

@ for(i <- 1 to 20){println(i!)}
1
2
6
24
120
720
5040
40320
362880
3628800
39916800
479001600
6227020800
87178291200
1307674368000
20922789888000
355687428096000
6402373705728000
121645100408832000
2432902008176640000