fsharp-learning/exercism/fsharp/grains/Grains.fs

module Grains

let rec safeSqaure n: uint64 =
    if n = 1 then uint64 (1) else uint64 (2) * safeSqaure (n - 1)

let square (n: int): Result<uint64, string> =
    if n < 1 || n > 64 then Error "square must be between 1 and 64" else Ok(safeSqaure n)

let total: Result<uint64, string> =
    Ok
        (List.sumBy (fun x ->
            let s = square x
            match s with
            | Ok (n) -> n
            | _ -> uint64 (0)) [ 1 .. 64 ])