F# program to print Fibonacci Series using Anonymous Recursion

Program

open System
// using Anonymous Recursion to find fibonacci
let fibonacci = function
    | x when x < 0 -> None
    | x -> let rec fibo2 = function
               | 0 | 1 -> 1
               | x -> fibo2 (x-1) + fibo2 (x-2)
            in Some (fibo2 x)
Console.Write("Enter the number to get the fibonacci series:\t")
let number = Int32.Parse(Console.ReadLine())
let series =  fibonacci number
Console.WriteLine("Fibonnaci series of {0} using Anonymous Recursion is {1}\t", number, series); 

Output

Enter the number to get the fibonacci series: 9
Fibonnaci series of 9 using Anonymous Recursion is Some(55)