Sample about using a Result to handle Function oriented in fable

Program.fs

namespace Weirdbird.Jdr.Browser

module Program =

    open Fable.Core
    open Fable.Import.Browser

    Fable.Import.Node.require.Invoke("core-js") |> ignore

    type AppModel = {
        ContextMenu: HTMLElement
    }

    type Result<'TResult, 'TError> =
        | Ok of 'TResult
        | Error of 'TError

    let clickInsideElement (e: PointerEvent, className) =
        let mutable el = unbox<Element> (unbox e.srcElement || unbox e.target)

        let rec matchClassName (el: Element) className =
            if (unbox el = null) then
                Error "Not inside an element with the given className"
            else
                if (unbox el.classList <> null && el.classList.contains className) then
                    Ok el
                else
                    matchClassName (unbox el.parentNode) className

        if (el.classList.contains className) then
            Ok el
        else
            matchClassName el.parentElement className

    let onContextMenu (e: PointerEvent) =
        console.log "oncontextmenu"

        match clickInsideElement (e, "w-container") with
        | Ok s -> console.log "inside"
        | Error error -> console.log error
        |> ignore
        null

    let model = {
        ContextMenu = document.querySelector """aside[data-context-menu="main"]""" :?> HTMLElement
    }

    document.addEventListener_contextmenu (fun e -> onContextMenu(e))