[Haskell-cafe] How to send signal from Xmonad to my app?

Hi, I have a little program, which shows a message box when gets 'USR1' signal. This works fine when I `kill -USR1 $PID`. But my code in xmonad.hs does not work. No error throws out. And as I inspected, the "pid" is right. Only nothing happened in my app. So I wonder what is wrong? 28 , ((mod4Mask, xK_x), sendSignal2PIDinFile sigUSR1 ".sidebar.pid") 31 sendSignal2PIDinFile signal pidfile = io $ do 32 pid <- readFile pidfile 33 signalProcess signal $ read pid -- 竹密岂妨流水过 山高哪阻野云飞