STM package:relude

A monad supporting atomic memory transactions.

Exception handling within STM actions. catchSTM m f catches any exception thrown by m using throwSTM, using the function f to handle the exception. If an exception is thrown, any changes made by m are rolled back, but changes prior to m persist.

A variant of throw that can only be used within the STM monad. Throwing an exception in STM aborts the transaction and propagates the exception. If the exception is caught via catchSTM, only the changes enclosed by the catch are rolled back; changes made outside of catchSTM persist. If the exception is not caught inside of the STM, it is re-thrown by atomically, and the entire STM is rolled back. Although throwSTM has a type that is an instance of the type of throw, the two functions are subtly different:

throw e    `seq` x  ===> throw e
throwSTM e `seq` x  ===> x

The first example will cause the exception e to be raised, whereas the second one won't. In fact, throwSTM will only cause an exception to be raised when it is used within the STM monad. The throwSTM variant should be used in preference to throw to raise an exception within the STM monad because it guarantees ordering with respect to other STM operations, whereas throw does not.

Monadic version of whenJust.

>>> whenJustM (pure Nothing) $ \b -> print (not b)

>>> whenJustM (pure $ Just True) $ \b -> print (not b)
False

Lift a Maybe to the MaybeT monad