|Portability||non-portable (requires STM)|
Software Transactional Memory: a modular composable concurrency abstraction. See
- Composable memory transactions, by Tim Harris, Simon Marlow, Simon Peyton Jones, and Maurice Herlihy, in /ACM Conference on Principles and Practice of Parallel Programming/ 2005. http://research.microsoft.com/Users/simonpj/papers/stm/index.htm
data STM a
A monad supporting atomic memory transactions.
Perform a series of STM actions atomically.
You cannot use
atomically inside an
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.)
always is a variant of alwaysSucceeds in which the invariant is expressed as an STM Bool action that must return True. Returning False or raising an exception are both treated as invariant failures.
alwaysSucceeds adds a new invariant that must be true when passed to alwaysSucceeds, at the end of the current transaction, and at the end of every subsequent transaction. If it fails at any of those points then the transaction violating it is aborted and the exception raised by the invariant is propagated.
Retry execution of the current memory transaction because it has seen values in TVars which mean that it should not continue (e.g. the TVars represent a shared buffer that is now empty). The implementation may block the thread until one of the TVars that it has read from has been udpated. (GHC only)
Compose two alternative STM actions (GHC only). If the first action completes without retrying then it forms the result of the orElse. Otherwise, if the first action retries, then the second action is tried in its place. If both actions retry then the orElse as a whole retries.
Throwing an exception in
STM aborts the transaction and propagates the
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
throwSTM variant should be used in preference to
raise an exception within the
STM monad because it guarantees
ordering with respect to other
STM operations, whereas