[Haskell-cafe] How does RTS judge that a STM transaction is blocked?

Hi, I met an issue while I was using xhb. When a certain property of root window being changed, it was supposed to give me an AccessError. But actually I got "thread blocked indefinitely in an STM transaction" when I waitForEvent. AFAIK, in xhb, it forks a thread to read what X server sends. Then parse and put it into a TChan. waitForEvent reads from the TChan. It is reasonable to me that if X server did not send anything, then waitForEvent would be blocked. But apparently there are some other policies in RTS. So what is that? And how should I debug it? -- 竹密岂妨流水过 山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.