Simple, yet function implementation of MonadState that can be shared across multiple threads (unlike simple State Monad) The implementation uses MonadReader with mutable environment (instance of MutableWrapper class):
class (Monad m) => MutableWrapper w m where
update :: (s -> (a , s)) -> w s -> m a
Here is instance for STM TVar and MonadIO:
instance (MonadIO m) => MutableWrapper TVar m where
update f ts =
liftIO $ atomically $ do
s <- readTVar ts
let (a, s1) = f s
writeTVar ts s1
return a
Main.hs demonstrates usage of SharedState. It uses the following newtype that auto derives all required classes (thanks to GeneralizedNewtypeDeriving):
newtype TVarMonadState s a = TVarMonadState (ReaderT (TVar s) IO a)
deriving (Functor, Applicative, Monad, MonadReader (TVar s), MonadIO)
as result it also became instance of MonadState s IO so the following functions are available:
- modify
- get
- put
Inherently, when using TVar based MutableWrapper update is thread safe, but if when using get and then put value can be overridden from different threads.
While currently not provided here additional implementation of MutableWrapper are possible including IORef and MVar