[Haskell-cafe] STM semantics

[Alberto G. Corona]

According with the philosophy of STM, readTChan does not block, but retries:

readTChan :: TChan a -> STM areadTChan (TChan read _write) = do
listhead <- readTVar read  head <- readTVar listhead  case head of
TNil -> *retry****    *TCons a tail -> do	writeTVar read tail	return a

This retry forces the execution of the whole atomic block, so it do readTVar
again, readTVar suppossedly "wait" without locking as well, until new data
is available. Because it is True, the atomic block terminates.

The readTVar behaviour is not clear form me,  althoug it is convenient.
Otherwise any atomic block with readTVar and retries would loop
continuously.

So I think that this execution is convenient, if not correct

2009/10/24 Paolino <paolo.veronelli at gmail.com>

> I have a doubt that this code works like I want because actual STM
> implementation exposes its effects
>
> import Control.Concurrent
> import Control.Concurrent.STM
>
> main = do
>     c <- atomically $ newTChan :: IO (TChan ())
>     r <- atomically $ newTVar False
>     forkIO $ do
>         atomically $ do
>             r0 <- readTVar r
>             case r0 of
>                 True -> return ()
>                 False -> readTChan c
>         myThreadId >>= killThread
>     threadDelay 1000000
>     atomically (writeTVar r True)
>
> The thread stops on readTChan, but exits when I change the TVar, which
> happens before the readTChan.
>
> Should I trust this is the correct STM behaviour , and will not change
> in different implementations ?
>
> thanks
>
> paolino
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