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_ <- readAllFromChan 1 chan will drop messages already in queue.

------------------------------------------------------------------------------
-- | Write a message to the channel.
writeMessage :: Message -> User -> ChatRoom -> IO ()
writeMessage msg user cr = do
    atomically $ writeTChan chan msg
    _ <- readAllFromChan 1 chan
    TM.tickle timeoutHandle userTimeout

  where
    chan          = _chatChannel cr
    timeoutHandle = _timeoutHandle user
    userTimeout = _userTimeout cr

Hi,
I am a fresh man in Snap web developing and doing this tutorial right now.
While, when I run "cabal install" in the project root directory, an error generated like this:

CHENLINGXIs-MacBook-Air:chatroom Paprika$ cabal install
Resolving dependencies...
Configuring clientsession-0.8.1...
Building clientsession-0.8.1...
Preprocessing library clientsession-0.8.1...
[1 of 1] Compiling Web.ClientSession ( src/Web/ClientSession.hs, dist/build/Web/ClientSession.o )

src/Web/ClientSession.hs:93:50:
Module Crypto.Random.AESCtr' does not exportgenRandomBytes'
Failed to install clientsession-0.8.1
cabal: Error: some packages failed to install:
clientsession-0.8.1 failed during the building phase. The exception was:
ExitFailure 1
snap-chat-0.2 depends on clientsession-0.8.1 which failed to install.

I am a new in using cabal, and I check my current ClientSession package, the version is clientsession-0.9.0.3 Is this the problem? And how can I fix it?

Thanks a lot.