Hi
I'm trying to understand Monad Transformers. The code below works as
expected but I have the following questions:
- why can I use liftIO but not lift in the doSomething function?
- why is there no liftSTM function?
now to the code:
module Main where
import Control.Monad.Reader
import Control.Monad.Writer
import Control.Concurrent.STM
type MyM a = WriterT [Entry] (ReaderT MyData IO) a
data MyData = MyData {myData::TVar String}
data Entry = Log String deriving (Eq, Show)
logMsg :: String -> MyM ()
logMsg s = tell [Log s]
doSomething :: MyM Int
doSomething = do
dataRef <- asks myData
logMsg "Writing"
liftIO $ do
--lift $ do
mv <- atomically $ readTVar dataRef
putStrLn mv
logMsg "Written"
return 1
main :: IO ()
main = do
i <- atomically $ newTVar "2"
log <- runReaderT (runWriterT doSomething) (MyData i)
print log
regards and thanks
Stefan
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