Definitions

   The factorial of   0   (zero)   is defined as being   1   (unity).
   The   Factorial Function   of a positive integer,    n,    is defined as the product of the sequence:
                 n,   n-1,   n-2,   ...   1 



Task

Write a function to return the factorial of a number.

Solutions can be iterative or recursive.

Support for trapping negative    n    errors is optional.



Related task

   Primorial numbers


shared void run() {
 
	Integer? recursiveFactorial(Integer n) => 
			switch(n <=> 0)
			case(smaller) null
			case(equal) 1
			case(larger) if(exists f = recursiveFactorial(n - 1)) then n * f else null;
 
 
	Integer? iterativeFactorial(Integer n) =>
			switch(n <=> 0)
			case(smaller) null
			case(equal) 1
			case(larger) (1:n).reduce(times);
 
	for(Integer i in 0..10) {
		print("the iterative factorial of     ``i`` is ``iterativeFactorial(i) else "negative"``
		       and the recursive factorial of ``i`` is ``recursiveFactorial(i) else "negative"``\n");
	}
}
 