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

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