fsharp-learning/exercism/fsharp/armstrong-numbers/ArmstrongNumbers.fs

module ArmstrongNumbers

let intFromChar a = int a - int '0'

// a ^ b where they are both ints
let rec intPow (a: int) (b: int) =
    if b = 0 then 1 else a * (intPow a (b - 1))

let isArmstrongNumber (number: int): bool =
    let len = String.length (string (number))
    number =
        (Seq.toList (string (number))
         |> Seq.fold (fun a c -> a + (intPow (intFromChar c) len)) 0)