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


 
fmod FACTORIAL is
 
	protecting INT .
 
	op undefined : -> Int .
	op _! : Int -> Int .
 
	var n : Int .
 
	eq 0 ! = 1 .
	eq n ! = if n < 0 then undefined else n * (sd(n, 1) !) fi .
 
endfm
 
red 11 ! .
 