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


 
{ let rec
  { Fact = -- compile-time constant binding
    { proc { N } as -- precondition: N.IsI48[] & (N >= 0)
    : if N == 0 then 1 else
      N * Fact[N - 1]
    }
  }
  in -- use Fact here or just make the whole expression to evaluate to it:
  Fact
}
 