Re: Inaccurate docs for atomically

12 Nov 2017


      This code works, so I don't think that's the case either:

    import Control.Concurrent.STM
    import Data.IORef
    import System.IO.Unsafe

    main :: IO ()
    main = do
      ref <- newIORef (6 :: Int)
      i <- atomically $ do
        var <- newTVar (unsafePerformIO (readIORef ref))
        readTVar var
      print i

On Sun, Nov 12, 2017 at 11:28 AM, Henning Thielemann <
lemming@henning-thielemann.de> wrote:
...
On Sun, 12 Nov 2017, Andrew Martin wrote:
In the stm package, the docs for atomically read:
...
...
You cannot use 'atomically' inside an 'unsafePerformIO' or >
'unsafeInterleaveIO'. Any attempt to do so will result in a runtime >
error.  (Reason: allowing this would effectively allow a transaction >
inside a transaction, depending on exactly when the thunk is > evaluated.)
I always thought that it would be the other way round, i.e. that you
cannot call 'unsafePerformIO' inside an 'atomically'. Maybe I mixed
something up.
-- 
-Andrew Thaddeus Martin