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An MVar (pronounced "em-var") is a synchronising variable, used for communication between concurrent threads. It can be thought of as a a box, which may be empty or full.

Monads in which IO computations may be embedded. Any monad built by applying a sequence of monad transformers to the IO monad will be an instance of this class.

Instances should satisfy the following laws, which state that liftIO is a transformer of monads:

• liftIO . return = return

• liftIO (m >>= f) = liftIO m >>= (liftIO . f)

A monad supporting atomic memory transactions.

A TMVar is a synchronising variable, used for communication between concurrent threads. It can be thought of as a box, which may be empty or full.

Shared memory locations that support atomic memory transactions.

Perform a series of STM actions atomically.

You cannot use atomically inside an unsafePerformIO or unsafeInterleaveIO. Any attempt to do so will result in a runtime error. (Reason: allowing this would effectively allow a transaction inside a transaction, depending on exactly when the thunk is evaluated.)

However, see newTVarIO, which can be called inside unsafePerformIO, and which allows top-level TVars to be allocated.