InMemoryKeyValueStorage.hs

{-# LANGUAGE OverloadedStrings #-}

import           Control.Concurrent (forkIO)
import           Control.Monad      (void)
import           Data.IORef         (IORef, atomicModifyIORef, newIORef,
                                     readIORef)
import           Data.Map.Strict    (Map)
import qualified Data.Map.Strict    as Map
import           Data.Text          (Text)

type Storage = IORef (Map Text Integer)

main :: IO ()
main = do
    storage <- newIORef Map.empty
    void $ forkIO $
        writer storage
    void $ forkIO $ do
        answer <- reader storage
        print answer

writer :: Storage -> IO ()
writer r = atomicModifyIORef r $ \m -> (Map.insert "answer" 42 m, ())

reader :: Storage -> IO (Maybe Integer)
reader r = Map.lookup "answer" <$> readIORef r