Proposal: Improve the API for TChan, TMVar, and TVar

[wren ng thornton]

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.

     -- | Non-blocking version of 'readTChan'.
     tryReadTChan :: TChan a -> STM (Maybe a)

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

     -- | Non-blocking version of 'peekTChan'.
     tryPeekTChan :: TChan a -> STM (Maybe a)

     -- | Non-blocking version of 'readTMVar'.
     tryReadTMVar :: TMVar a -> STM (Maybe a)

     -- | Like 'modifyIORef' but for 'TVar'.
     modifyTVar :: TVar a -> (a -> a) -> STM ()

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

     -- | Like 'swapTMVar' but for 'TVar'.
     swapTVar :: TVar a -> a -> STM a

Discussion period: 2 weeks.

-- 
Live well,
~wren
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