Improve the API for TChan, TMVar, and TVar

Apply the patch for the proposal, reproduced at bottom. This proposal was discussed on the mailing list, http://www.haskell.org/pipermail/libraries/2011-March/016086.html. Supported by Edward Z. Yang, Bas van Dijk(*), Antoine Latter. No dissent.

Bas van Dijk's support is generally contingent on further improving the overall API by adding similar functions for the non-STM concurrency types. However, doing so requires a deprecation cycle in order to bring modifyMVar in sync with modifyTVar, modifyIORef, and the rest of the Haskell ecosystem. And therefore should be pursued in a separate proposal. He offered a patch, http://www.haskell.org/pipermail/libraries/2011-March/016096.html, which still requires verification of correctness. These changes to the base package are not included in the current proposal.

---

I've found the following functions helpful for working with STM. Some of them are just filling out the API so that TVars, TMVars, and IORefs match better. And all the non-TVar functions can be optimized considerably by including them in the STM library rather than defining them externally. The implementations are obvious, so I'll just include the types here; see the patch if you're interested.

    -- | A version of 'readTChan' which does not retry. Instead it
    -- returns @Nothing@ if no value is available.
    tryReadTChan :: TChan a -> STM (Maybe a)

    -- | Get the next value from the @TChan@ without removing it,
    -- retrying if the channel is empty.
    peekTChan :: TChan a -> STM a

    -- | A version of 'peekTChan' which does not retry. Instead it
    -- returns @Nothing@ if no value is available.
    tryPeekTChan :: TChan a -> STM (Maybe a)

    -- | A version of 'readTMVar' which does not retry. Instead it
    -- returns @Nothing@ if no value is available.
    tryReadTMVar :: TMVar a -> STM (Maybe a)

    -- Like 'modifyIORef' but for @TVar@.
    -- | Mutate the contents of a @TVar@. /N.B./, this version is
    -- non-strict.
    modifyTVar :: TVar a -> (a -> a) -> STM ()

    -- | Strict version of 'modifyTVar'.
    modifyTVar' :: TVar a -> (a -> a) -> STM ()

    -- Like 'swapTMVar' but for @TVar@.
    -- | Swap the contents of a @TVar@ for a new value.
    swapTVar :: TVar a -> a -> STM a

Trac metadata

Trac field	Value
Version	7.0.3
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries (other)
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