Hello, I've a short question about interruptible operations. In the following program is it possible for 'putMVar' to re-throw asynchronous exceptions even when asynchronous exception are blocked/masked? newEmptyMVar >>= \mv -> block $ putMVar mv x The documentation in Control.Exception about interruptible operations[1] confused me: "Some operations are interruptible, which means that they can receive asynchronous exceptions even in the scope of a block. Any function which may itself block is defined as interruptible..." Do I need to interpret this as: 1) 'putMVar' is always interruptible because it's an operation that has the ability to block. For example when applied to a full MVar. or: 2) 'putMVar' is only interruptible when it actually blocks and not interruptible when it is sure that it will not block. For example when applied to an empty MVar like in the given program. The reason I'm asking is that in my threads library[2] I previously applied 'putMVar' to an empty MVar to signal the termination of a thread. However due to some comments in the topic about "Asynchronous Exception Wormholes" I got a bit scared that 'putMVar' might throw exceptions even in the scope of a block. Because this could cause dead-locks in my threads library, I rewrote it using TMVars which don't have interruptible operations. However, I'm still not sure about the exact semantics so I would like to clear this up. Regards, Bas [1] http://haskell.org/ghc/docs/6.12.1/html/libraries/base-4.2.0.0/Control-Excep... [2] darcs get http://code.haskell.org/~basvandijk/code/threads/