Support duplicating a TChan with the current content available.

I propose adding a function to the Control.Concurrent.STM.TChan that duplicates the channel, but with all current content available in the duplicate channel too. Suggestion:

-- |Clone a 'TChan': similar to dupTChan, but the cloned channel starts with the
-- same content available as the original channel.
cloneTChan :: TChan a -> STM (TChan a)
cloneTChan (TChan read write) = do
  readpos <- readTVar read
  new_read <- newTVar readpos
  return (TChan new_read write)

A little bit of context for the proposal. I was writing a function that started getting a bit complicated (I am waiting of either this message or two other messages in order, and there might be some uninteresting messages before I get any of those...) And I realized that this is a parsing problem and we have libraries for those! But, then I had to realize that there is no way to implement for example Parsec's Stream class with the currently available TChan methods. With the proposed cloneTChan it's trivial:

instance Stream (TChan a) STM a where
    uncons chan = do
      chan' <- cloneTChan chan
      x <- readTChan chan'
      return $ Just (x,chan')

And I can imagine some other applications where the new function would come handy.

The same thing also applies to Control.Concurrent.Chan, though there you have other possibilities (eg. you can "convert" a Chan a into an [a] with unsafeInterleaveIO and work on that, which is not a viable option for TChan.)

Trac metadata

Trac field	Value
Version	7.4.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries (other)
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