RIO.hs

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE TypeFamilies #-}
module RIO.Prelude.RIO
  ( RIO (..)
  , runRIO
  , liftRIO
  ) where

import RIO.Prelude.Reexports

-- | The Reader+IO monad. This is different from a 'ReaderT' because:
--
-- * It's not a transformer, it hardcodes IO for simpler usage and
-- error messages.
--
-- * Instances of typeclasses like 'MonadLogger' are implemented using
-- classes defined on the environment, instead of using an
-- underlying monad.
newtype RIO env a = RIO { unRIO :: ReaderT env IO a }
  deriving (Functor,Applicative,Monad,MonadIO,MonadReader env,MonadThrow)

runRIO :: MonadIO m => env -> RIO env a -> m a
runRIO env (RIO (ReaderT f)) = liftIO (f env)

liftRIO :: (MonadIO m, MonadReader env m) => RIO env a -> m a
liftRIO rio = do
  env <- ask
  runRIO env rio

instance MonadUnliftIO (RIO env) where
    askUnliftIO = RIO $ ReaderT $ \r ->
                  withUnliftIO $ \u ->
                  return (UnliftIO (unliftIO u . flip runReaderT r . unRIO))

instance PrimMonad (RIO env) where
    type PrimState (RIO env) = PrimState IO
    primitive = RIO . ReaderT . const . primitive