从回调更改gtk窗口状态

Question

我有一个简单的GTK窗口，其菜单栏有Open项。它在menuItemActivate：

的回调中获取文件名

openPressed = do
    chooser <- fileChooserDialogNew

...
    filename <- performResponse res chooser
        widgetHide chooser
        where
            performResponse ResponseCancel _ = return ("")
            performResponse ResponseOk chooser = do
                Just fname <- fileChooserGetFilename chooser
                putStrLn fname 
                --how can i put filename in label inside MainForm from here?
                return fname

如果openPressed没有嵌套在初始化主窗口的主函数中，如何将文件名放到主窗口中的标签上？