factorial.go

func Factorial(num int) int {
   if num == 0 {
      return 1;	
   } else {
      return num * Factorial(num - 1);
   }
}

factorial.hs

factorial n = product [1..n]

factorial.php

<?php

function factorial($num) {
	return ($num == 0) ? 1 : $num * factorial($num - 1);
}

factorial.pl

sub factorial {
  my $arg = shift;
  return $arg == 1 ? 1 : $arg * factorial($arg - 1);
}

factorial.py

def factorial(num): 
	return (1 if num == 0 else num * factorial(num - 1))

Math.factorial.rb

module Math
  def self.factorial(f)
    f == 0 ? 1 : f * factorial(f - 1)
  end
end

# Or we can extend an instance of Fixnum so we can do 5.factorial :)
class Fixnum
  def factorial
    self == 0 ? 1 : self * (self - 1).factorial
  end
end