Great! Did you add to the user documentation? Simon | -----Original Message----- | From: Tim Harris (RESEARCH) | Sent: 10 October 2006 11:45 | To: haskell@haskell.org | Cc: Simon Peyton-Jones; Simon Marlow | Subject: ANNOUNCE: STM invariants and exceptions | | Hi, | | I've checked a number of changes to the transactional memory support into GHC head: | | 1. The main change is to add support for dynamically checked data invariants of the kind described in | http://research.microsoft.com/~tharris/papers/2006-transact.pdf. There are two operations: "always | X :: STM Bool -> STM ()" and "alwaysSucceeds X :: STM a -> STM ()". Invariants are expressed as | STM actions which must hold (i) when passed to always/alwaysSucceeds, and (ii) at the point that | every subsequent transaction commits. An invariant passed to "always" signals failure by returning | False or by raising an exception. An invariant passed to "alwaysSucceeds" signals failure just by raising | an exception. Tests conc062 and conc063 try out some corner cases, and the paper discusses the | details (what if an invariant calls "retry", what if it updates TVars, what if it loops, and so on etc...) | | 2. If an atomic block contains "catch X Y" and X raises an exception then the exception is passed to Y | after rolling back the effects of X. Previously we would retain the effects of X, requiring the | programmer to manually compensate for the portion of X that had run before raising the exception. | Rolling back X seems more consistent with the treatment of exceptions that propagate out of atomic | blocks, and the argument that transactional memory can bring ease-of-programming benefits by | automating the construction of compensating code for memory operations. The post-publication | version of the "Composable memory transactions" paper has more details | http://research.microsoft.com/~tharris/papers/2005-ppopp-composable.pdf. Test conc061 tests some | of the corner cases. | | 3. There are some bug-fixes to the implementation of "X `orElse` Y" in which, under high contention, X | and Y may not appear to execute against the same view of the heap. I'm not sure if this problem has | ever come up in practice! | | We'd love to hear about it if you're using these features, | | Cheers, | | Tim |