Digit Power Sum with BigInteger

Question

This is my 1st F# program so I'm open to any and all criticism on my syntax and the way I approached the problem.

Challenge

Project Euler 16: Power Digit Sum

2^15 = 32768 and the sum of its digits is 3 +たす 2 +たす 7 +たす 6 +たす 8 =わ 26. What is the sum of the digits of the number 2^1000?

open System
open System.Numerics
let digitPower (n:bigint) p = pown n p
let digitToPowerSum (n:bigint) p = 
 (digitPower n p) |> 
 string |> 
 (fun s -> s.ToCharArray()) |> 
 Array.map (fun x -> Int32.Parse (x.ToString())) |> 
 Array.sum
[<EntryPoint>]
let main argv = 
 let n = new bigint(2)
 let p = 1000
 let value = digitPower n p
 let sum = digitToPowerSum n p
 Console.WriteLine(String.Format("{0}^{1} = {2:n}", n, p, value))
 Console.WriteLine()
 Console.WriteLine(String.Format("Its digit sum is {0}", sum))
 Console.ReadKey() |> ignore
 0

I'm primarily concerned with

Answer 1

It's quite common to convert a number to a string to get the digits, and most of the times, it ought to work, but it's a hack, instead of modelling what should happen.

Here's another way to do it, which deliberately separates integers into its digits.

let rec digits i =
 let tens = i / 10I
 if tens = 0I
 then [i]
 else
 let ones = i % 10I
 (digits tens) @ [ones]

The rest is easy:

let euler16 x y = pown x y |> digits |> List.sum

Some examples from F# Interactive:

> euler16 2I 15;;
val it : System.Numerics.BigInteger = 26 {IsEven = true;
 IsOne = false;
 IsPowerOfTwo = false;
 IsZero = false;
 Sign = 1;}
> euler16 2I 1000;;
val it : System.Numerics.BigInteger = 1366 {IsEven = true;
 IsOne = false;
 IsPowerOfTwo = false;
 IsZero = false;
 Sign = 1;}

• Shelby115: Could you explain the 10I and 0I? I assume their some sort of byte representation?
• Shelby115: Oh nevermind, it's a bigint literal