Haskell: generic IORef, MVar?

Question

Hi,

I made the following function which is specific for the IO monad:

memoIO :: MonadIO m => m a -> IO (m a)
memoIO action = do
  ref <- newMVar Nothing
  return $ do
    x <- maybe action return =<< liftIO (takeMVar ref)
    liftIO . putMVar ref $ Just x
    return x

Example usage:

main :: IO ()
main = do
  p <- memoIO $ putStrLn "hello"
  p
  p

Prints "hello" once.

I would like (a pet peeve) to make it work for as many cases as possible (not just in IO).

I found stateref on hackage and with it my code looks like this:

{-# LANGUAGE FlexibleContexts, FlexibleInstances, MultiParamTypeClasses, Rank2Types, UndecidableInstances #-}

import Data.MRef

class (NewMRef r m a, DefaultMRef r m a, PutMRef r m a, TakeMRef r m a) => MRef r m a
instance (NewMRef r m a, DefaultMRef r m a, PutMRef r m a, TakeMRef r m a) => MRef r m a

memo :: (MRef r m (Maybe a), Monad s) => (forall x. m x -> s x) -> s a -> m (s a)
memo liftFunc action = do
  ref <- newDefaultMRef Nothing
  return $ do
    x <- maybe action return =<< liftFunc (takeDefaultMRef ref)
    liftFunc . putDefaultMRef ref $ Just x
    return x

Is there an alternative for stateref or a better way to use it than I did?

Answer 1

I've rewritten a cheesy little MonadRef class on a few separate occasions for my own personal use and someone probably has one on Hackage, but I can't find one that is unencumbered with other baggage.

class Monad m => MonadRef m where
    type Ref m :: * -> *
    newRef :: a -> Ref m a
    writeRef :: Ref m a -> -> m ()
    readRef :: Ref m a -> m a

instance MonadRef IO where
    type Ref IO = IORef
    newRef = newIORef
    writeRef = writeIORef
    readRef = writeIORef

instance MonadRef STM where
    type Ref STM = TVar
    ...


instance MonadRef (ST s) where
    type Ref (ST s) = STRef s
    ...

Then it is easy to abstract away your memoization routine (though you probably want to replace IORef in this context with an MVar.)

[Edit: clarified verbage]