F# / Argu -如何显示两级命令树的帮助而不抛出异常

open Argu

type Command =
    | Level1 of string
    | Level2 of string

let parseArgs args =
    let parser = new ArgParser<Command>()
    parser.Parse(args)

let displayHelp() =
    let parser = new ArgParser<Command>()
    printfn "%s" (parser.FormatUsage())

[<EntryPoint>]
let main args =
    try
        let command = parseArgs args
        match command with
        | Level1 param ->
            // 处理一级命令
            printfn "Level 1 command: %s" param
        | Level2 param ->
            // 处理二级命令
            printfn "Level 2 command: %s" param
    with
    | :? ArgParserException as ex ->
        // 显示帮助文档
        displayHelp()
        printfn "Error: %s" ex.Message
        1 // 返回错误码
    | _ ->
        0 // 返回成功码