haskell - 消息框错误 : foreign import unsafe

Question

import Graphics.Win32
import System.Win32.DLL
import Control.Exception (bracket)
import Foreign
import System.Exit
main :: IO ()
main = do
    mainInstance <- getModuleHandle Nothing
    hwnd <- createWindow_ 200 200 wndProc mainInstance
    createButton_ hwnd mainInstance
    messagePump hwnd
wndProc :: HWND -> WindowMessage -> WPARAM -> LPARAM -> IO LRESULT
wndProc hwnd wmsg wParam lParam
    | wmsg == wM_DESTROY = do
        sendMessage hwnd wM_QUIT 1 0
        return 0
    | wmsg == wM_COMMAND && wParam == 1 = do
        messageBox nullPtr "Yahoo!!" "Message box" 0 -- Error! Why? :(
        return 0
    | otherwise = defWindowProc (Just hwnd) wmsg wParam lParam
createWindow_ :: Int -> Int -> WindowClosure -> HINSTANCE -> IO HWND
createWindow_ width height wndProc mainInstance = do
    let winClass = mkClassName "ButtonExampleWindow"
    icon <- loadIcon Nothing iDI_APPLICATION
    cursor <- loadCursor Nothing iDC_ARROW
    bgBrush <- createSolidBrush (rgb 240 240 240)
    registerClass (cS_VREDRAW + cS_HREDRAW, mainInstance, Just icon, Just cursor, Just bgBrush, Nothing, winClass)
    w <- createWindow winClass "Button example" wS_OVERLAPPEDWINDOW Nothing Nothing (Just width) (Just height) Nothing Nothing mainInstance wndProc
    showWindow w sW_SHOWNORMAL
    updateWindow w
    return w
createButton_ :: HWND -> HINSTANCE -> IO ()
createButton_ hwnd mainInstance = do
    hBtn <- createButton "Press me" wS_EX_CLIENTEDGE (bS_PUSHBUTTON + wS_VISIBLE + wS_CHILD) (Just 50) (Just 80) (Just 80) (Just 20) (Just hwnd) (Just (castUINTToPtr 1)) mainInstance
    return ()
messagePump :: HWND -> IO ()
messagePump hwnd = allocaMessage $ \ msg ->
    let pump = do
        getMessage msg (Just hwnd) `catch` \ _ -> exitWith ExitSuccess
        translateMessage msg
        dispatchMessage msg
        pump
    in pump

这是带有按钮的简单 win32 gui 应用程序，但是当我单击按钮时，必须有一个消息框(22 行)，但出现错误:

buttons.exe: schedule: re-entered unsafely. Perhaps a 'foreign import unsafe' should be 'safe'?

如何修复它？

Answer 1

正如 Daniel Wagner 评论的那样，这是 Win32 软件包中的一个错误。 MessageBoxW 必须安全导入，因为它有很多副作用。

messageBox 函数是“不安全”导入的 MessageBoxW 函数的包装器。当一个不安全导入的函数函数被不安全导入时，Haskell 假定该线程在返回之前不会调用任何 Haskell 代码。但是，如果您调用 MessageBoxW，Windows 会向您在第 30 行创建的窗口抛出相当多的窗口消息，因此当您处于不安全的外部函数中时，Haskell 代码将会运行。这也是为什么对 messageBox 的调用将起作用直到该窗口被创建。

一个可能的解决方法是简单地自行更正该函数。首先，改变

import Graphics.Win32

至

import Graphics.Win32 hiding (messageBox, c_MessageBox)

然后，从模块 Graphics.Win32.Misc 中复制 messageBox 和 c_MessageBox 的定义，其中 unsafe 删除和/或添加安全:

messageBox :: HWND -> String -> String -> MBStyle -> IO MBStatus
messageBox wnd text caption style =
  withTString text $ \ c_text ->
  withTString caption $ \ c_caption ->
  failIfZero "MessageBox" $ c_MessageBox wnd c_text c_caption style
foreign import stdcall safe "windows.h MessageBoxW"
  c_MessageBox :: HWND -> LPCTSTR -> LPCTSTR -> MBStyle -> IO MBStatus