TMVar: Transactional MVars, for use in the STM monad (GHC only)
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.
Check whether a given TMVar is empty.
Create a TMVar which is initially empty.
Create a TMVar which contains the supplied value.
This is a combination of takeTMVar and putTMVar; ie. it takes the value from the TMVar, puts it back, and also returns it.
Swap the contents of a TMVar for a new value.
A version of readTMVar which does not retry. Instead it returns Nothing if no value is available.