either.fs

open FSharpx
open FSharpx.Choice

type Error = Error of string
type AuthLevel = Admin | Normal | Guest

let getSetting = function
    | "user" -> Choice1Of2 "user1" 
    | name -> name |> sprintf "setting %s was not found" |> Error |> Choice2Of2

let getAuth = function
    | "user1" -> Choice1Of2 Normal
    | user -> user |> sprintf "user %s was not found" |> Error |> Choice2Of2 

let log = printfn "%s"
let getAuthorizationLevel = getSetting >=> getAuth

let authLevel =
    getAuthorizationLevel "user"
    |> Choice.choice id (fun (Error error) -> log error; Guest)