TMVars

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.

Create a TMVar which contains the supplied value.

Create a TMVar which is initially empty.

IO version of newTMVar. This is useful for creating top-level TMVars using unsafePerformIO, because using atomically inside unsafePerformIO isn't possible.

IO version of newEmptyTMVar. This is useful for creating top-level TMVars using unsafePerformIO, because using atomically inside unsafePerformIO isn't possible.

Return the contents of the TMVar. If the TMVar is currently empty, the transaction will retry. After a takeTMVar, the TMVar is left empty.

Put a value into a TMVar. If the TMVar is currently full, putTMVar will retry.

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 takeTMVar that does not retry. The tryTakeTMVar function returns Nothing if the TMVar was empty, or Just a if the TMVar was full with contents a. After tryTakeTMVar, the TMVar is left empty.

A version of putTMVar that does not retry. The tryPutTMVar function attempts to put the value a into the TMVar, returning True if it was successful, or False otherwise.

Check whether a given TMVar is empty.

Notice that the boolean value returned is just a snapshot of the state of the TMVar. By the time you get to react on its result, the TMVar may have been filled (or emptied) - so be extremely careful when using this operation. Use tryTakeTMVar instead if possible.