3 ms·

I've seen them created a few times. I'm pretty sure there are at least a few implementations in Hackage of such divided IO monads. More generally, the haute met

• dllthomas 14y ago
  I think optimally, with a set of typeclasses, one could define instances for IO and just use things with no interpretation or build a datastructure with free monads to perform whatever other inspections or manipulations you want. There could be some incompatability between the two notions that I'm missing, though...
  • tel 14y ago
    I think I've seen the typeclass route as well before. It may have just been a blog post and not a library implementation, though. The two methods have different epistemological ideas, but could be combined. For instance: {-# LANGUAGE DeriveFunctor, GeneralizedNewtypeDeriving, FlexibleInstances #-} module CrWr where import Control.Monad.Free import Data.IORef data RWRefF t a = NewRef t (IORef t -> a) | PeekRef (IORef t) (t -> a) | PutRef (IORef t) t a deriving Functor newtype RWRef t a = RWRef (Free (RWRefF t) a) deriving Monad newRef :: t -> RWRef t (IORef t) newRef t = RWRef $ liftF $ NewRef t id peekRef :: IORef t -> RWRef t t peekRef r = RWRef $ liftF $ PeekRef r id putRef :: IORef t -> t -> RWRef t () putRef r t = RWRef $ liftF $ PutRef r t () interpret :: RWRef t a -> IO a interpret (RWRef (Pure a)) = return a interpret (RWRef (Free (NewRef t f))) = newIORef t >>= interpret . RWRef . f interpret (RWRef (Free (PeekRef r f))) = readIORef r >>= interpret . RWRef . f interpret (RWRef (Free (PutRef r t next))) = writeIORef r t >> interpret (RWRef next) -- Highly specialized class of IO monads class Monad m => RWIntRefMonad m where new :: Int -> m (IORef Int) peek :: IORef Int -> m Int put :: IORef Int -> Int -> m () toIO :: m a -> IO a instance RWIntRefMonad (RWRef Int) where new = newRef peek = peekRef put = putRef toIO = interpret incr :: RWIntRefMonad m => IORef Int -> m () incr r = peek r >>= put r . (+1) main = do r <- newIORef 0 -- We need to specialize before the m gets erased toIO (incr r :: RWRef Int ()) readIORef r >>= print