F# attach interface

gistfile1.fs

open System

let getComparableUri uri = { 
    new Uri( uri) 
        interface IComparable<string> with
            member this.CompareTo other = 
                compare (string this) other
}

let url = "http://news.google.com/"
let x = getComparableUri url

let x': IComparable<string> = x |> box |> unbox

printfn "Result: %i" (x'.CompareTo(url))