Unicode-aware string reverse in F#

reverse.fs

open System.Text
open System
open ICU4N.Text

let reverse (str: string) =
    if str.Length <= 0 then
        str
    else
        let span = str.AsSpan ()
        let breaks = seq {
            let iterator = BreakIterator.GetCharacterInstance ()
            iterator.SetText str
            let mutable previous = 0
            while iterator.Next () <> BreakIterator.Done do
                yield (previous, iterator.Current - previous)
                previous <- iterator.Current
        }
        let breaks = breaks |> Seq.fold (fun xs x -> x :: xs) []
        let builder = StringBuilder ()
        for (idx, len) in breaks do
            let span = span.Slice (idx, len)
            builder.Append span |> ignore
        builder.ToString ()

[<EntryPoint>]
let main args =
    match args |> Array.tryHead with
    | Some str -> printfn "“%s” (len: %d) reverses to “%s”" str (str |> String.length) (reverse str)
    | _ -> printfn "Usage: reverse <string to reverse>"
    0