An expression of type IOSpec MVarS a corresponds to an IO computation that uses shared, mutable variables and returns a value of type a.

By itself, MVarS is not terribly useful. You will probably want to use IOSpec (ForkS :+: MVarS).

An MVar is a shared, mutable variable.

The newEmptyMVar function creates a new MVar that is initially empty.

The takeMVar function removes the value stored in an MVar. If the MVar is empty, the thread is blocked.

The putMVar function fills an MVar with a new value. If the MVar is not empty, the thread is blocked.