Console app that listens for keypress events even in the packground

Keypress.fs

namespace Native

type HookProc = delegate of int * nativeint * nativeint -> nativeint

module User32 =
    open System.Runtime.InteropServices

    [<DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)>]
    extern nativeint SetWindowsHookEx(int idHook, HookProc lpfn, nativeint hMod, uint32 dwThreadId)


    [<DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)>]
    [<MarshalAs(UnmanagedType.Bool)>]
    extern bool UnhookWindowsHookEx(nativeint hhk)

    [<DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)>]
    extern nativeint CallNextHookEx(nativeint hhk, int nCode, nativeint wParam, nativeint lParam)

module Kernal32 =
    open System.Runtime.InteropServices

    [<DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)>]
    extern nativeint GetModuleHandle(string lpModuleName)

module Program =
    open System.Runtime.InteropServices
    open System.Windows.Forms
    open System.Diagnostics

    // Credit: https://blogs.msdn.microsoft.com/toub/2006/05/03/low-level-keyboard-hook-in-c/
    [<Literal>]
    let WH_KEYBOARD_LL = 13
    [<Literal>]
    let WM_KEYDOWN = 0x0100
    let mutable hookId = nativeint 0

    [<EntryPoint>]
    let main argv =
    
        let sprintKey k = KeysConverter().ConvertToString(k)

        // installs a hook from WH_KEYBOARD_LL events to callback. 
        // See: https://msdn.microsoft.com/en-us/library/windows/desktop/ms644990(v=vs.85).aspx
        let setHook hookProc : nativeint =
            using (Process.GetCurrentProcess()) (fun curProcess ->
            using (curProcess.MainModule) ( fun curModule ->
                let handle = Kernal32.GetModuleHandle(curModule.ModuleName)
                User32.SetWindowsHookEx(WH_KEYBOARD_LL, hookProc, handle, (0 |> uint32))
            ))
        
        let callback = fun nCode wParam lParam ->
            if (nCode >= 0 && wParam = (WM_KEYDOWN |> nativeint)) then
                let vkCode = Marshal.ReadInt32(lParam)
                printfn "%s" (sprintKey vkCode)
            User32.CallNextHookEx(hookId, nCode, wParam, lParam)
        
        hookId <- setHook(new HookProc(callback))
        Application.Run()
        User32.UnhookWindowsHookEx(hookId) |> ignore    
        0