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