    --
    -- (c) The University of Glasgow 2002-2006
    --
    -- The IO Monad with an environment
    --
    {-# LANGUAGE UndecidableInstances #-}

    module IOEnv (
            IOEnv, -- Instance of Monad

            -- Monad utilities
            module MonadUtils,

            -- Errors
            failM, failWithM,
            IOEnvFailure(..),

            -- Getting at the environment
            getEnv, setEnv, updEnv,

            runIOEnv, unsafeInterleaveM,
            tryM, tryAllM, tryMostM, fixM,

            -- I/O operations
            IORef, newMutVar, readMutVar, writeMutVar, updMutVar,
            atomicUpdMutVar, atomicUpdMutVar'
      ) where

    import Exception
    import Panic

    import Data.IORef       ( IORef, newIORef, readIORef, writeIORef, modifyIORef,
                              atomicModifyIORef )
    import Data.Typeable
    import System.IO.Unsafe ( unsafeInterleaveIO )
    import System.IO        ( fixIO )
    import Control.Monad
    import MonadUtils

    ----------------------------------------------------------------------
    -- Defining the monad type
    ----------------------------------------------------------------------

    newtype IOEnv env a = IOEnv (env -> IO a)

    unIOEnv :: IOEnv env a -> (env -> IO a)
    unIOEnv (IOEnv m) = m

    instance Monad (IOEnv m) where
        (>>=)  = thenM
        (>>)   = thenM_
        return = returnM
        fail _ = failM -- Ignore the string

    instance Applicative (IOEnv m) where
        pure = returnM
        IOEnv f <*> IOEnv x = IOEnv (\ env -> f env <*> x env )

    instance Functor (IOEnv m) where
        fmap f (IOEnv m) = IOEnv (\ env -> fmap f (m env))

    returnM :: a -> IOEnv env a
    returnM a = IOEnv (\ _ -> return a)

    thenM :: IOEnv env a -> (a -> IOEnv env b) -> IOEnv env b
    thenM (IOEnv m) f = IOEnv (\ env -> do { r <- m env ;
                                             unIOEnv (f r) env })

    thenM_ :: IOEnv env a -> IOEnv env b -> IOEnv env b
    thenM_ (IOEnv m) f = IOEnv (\ env -> do { _ <- m env ; unIOEnv f env })

    failM :: IOEnv env a
    failM = IOEnv (\ _ -> throwIO IOEnvFailure)

    failWithM :: String -> IOEnv env a
    failWithM s = IOEnv (\ _ -> ioError (userError s))

    data IOEnvFailure = IOEnvFailure
        deriving Typeable

    instance Show IOEnvFailure where
        show IOEnvFailure = "IOEnv failure"

    instance Exception IOEnvFailure

    ----------------------------------------------------------------------
    -- Fundmantal combinators specific to the monad
    ----------------------------------------------------------------------

    ---------------------------
    runIOEnv :: env -> IOEnv env a -> IO a
    runIOEnv env (IOEnv m) = m env

    ---------------------------
    {-# NOINLINE fixM #-}
      -- Aargh!  Not inlining fixTc alleviates a space leak problem.
      -- Normally fixTc is used with a lazy tuple match: if the optimiser is
      -- shown the definition of fixTc, it occasionally transforms the code
      -- in such a way that the code generator doesn't spot the selector
      -- thunks.  Sigh.

    fixM :: (a -> IOEnv env a) -> IOEnv env a
    fixM f = IOEnv (\ env -> fixIO (\ r -> unIOEnv (f r) env))

    ---------------------------
    tryM :: IOEnv env r -> IOEnv env (Either IOEnvFailure r)
    -- Reflect UserError exceptions (only) into IOEnv monad
    -- Other exceptions are not caught; they are simply propagated as exns
    --
    -- The idea is that errors in the program being compiled will give rise
    -- to UserErrors.  But, say, pattern-match failures in GHC itself should
    -- not be caught here, else they'll be reported as errors in the program
    -- begin compiled!
    tryM (IOEnv thing) = IOEnv (\ env -> tryIOEnvFailure (thing env))

    tryIOEnvFailure :: IO a -> IO (Either IOEnvFailure a)
    tryIOEnvFailure = try

    -- XXX We shouldn't be catching everything, e.g. timeouts
    tryAllM :: IOEnv env r -> IOEnv env (Either SomeException r)
    -- Catch *all* exceptions
    -- This is used when running a Template-Haskell splice, when
    -- even a pattern-match failure is a programmer error
    tryAllM (IOEnv thing) = IOEnv (\ env -> try (thing env))

    tryMostM :: IOEnv env r -> IOEnv env (Either SomeException r)
    tryMostM (IOEnv thing) = IOEnv (\ env -> tryMost (thing env))

    ---------------------------
    unsafeInterleaveM :: IOEnv env a -> IOEnv env a
    unsafeInterleaveM (IOEnv m) = IOEnv (\ env -> unsafeInterleaveIO (m env))

    ----------------------------------------------------------------------
    -- MonadPlus
    ----------------------------------------------------------------------

    -- For use if the user has imported Control.Monad.Error from MTL
    -- Requires UndecidableInstances
    instance MonadPlus IO => MonadPlus (IOEnv env) where
        mzero = IOEnv (const mzero)
        m `mplus` n = IOEnv (\env -> unIOEnv m env `mplus` unIOEnv n env)

    ----------------------------------------------------------------------
    -- Accessing input/output
    ----------------------------------------------------------------------

    instance MonadIO (IOEnv env) where
        liftIO io = IOEnv (\ _ -> io)

    newMutVar :: a -> IOEnv env (IORef a)
    newMutVar val = liftIO (newIORef val)

    writeMutVar :: IORef a -> a -> IOEnv env ()
    writeMutVar var val = liftIO (writeIORef var val)

    readMutVar :: IORef a -> IOEnv env a
    readMutVar var = liftIO (readIORef var)

    updMutVar :: IORef a -> (a -> a) -> IOEnv env ()
    updMutVar var upd = liftIO (modifyIORef var upd)

    -- | Atomically update the reference.  Does not force the evaluation of the
    -- new variable contents.  For strict update, use 'atomicUpdMutVar''.
    atomicUpdMutVar :: IORef a -> (a -> (a, b)) -> IOEnv env b
    atomicUpdMutVar var upd = liftIO (atomicModifyIORef var upd)

    -- | Strict variant of 'atomicUpdMutVar'.
    atomicUpdMutVar' :: IORef a -> (a -> (a, b)) -> IOEnv env b
    atomicUpdMutVar' var upd = do
      r <- atomicUpdMutVar var upd
      _ <- liftIO . evaluate =<< readMutVar var
      return r

    ----------------------------------------------------------------------
    -- Accessing the environment
    ----------------------------------------------------------------------

    getEnv :: IOEnv env env
    {-# INLINE getEnv #-}
    getEnv = IOEnv (\ env -> return env)

    -- | Perform a computation with a different environment
    setEnv :: env' -> IOEnv env' a -> IOEnv env a
    {-# INLINE setEnv #-}
    setEnv new_env (IOEnv m) = IOEnv (\ _ -> m new_env)

    -- | Perform a computation with an altered environment
    updEnv :: (env -> env') -> IOEnv env' a -> IOEnv env a
    {-# INLINE updEnv #-}
    updEnv upd (IOEnv m) = IOEnv (\ env -> m (upd env))

    ----------------------------------------------------------------------
    -- Standard combinators, but specialised for this monad
    -- (for efficiency)
    ----------------------------------------------------------------------

    -- {-# SPECIALIZE mapM          :: (a -> IOEnv env b) -> [a] -> IOEnv env [b] #-}
    -- {-# SPECIALIZE mapM_         :: (a -> IOEnv env b) -> [a] -> IOEnv env () #-}
    -- {-# SPECIALIZE mapSndM       :: (b -> IOEnv env c) -> [(a,b)] -> IOEnv env [(a,c)] #-}
    -- {-# SPECIALIZE sequence      :: [IOEnv env a] -> IOEnv env [a] #-}
    -- {-# SPECIALIZE sequence_     :: [IOEnv env a] -> IOEnv env () #-}
    -- {-# SPECIALIZE foldlM        :: (a -> b -> IOEnv env a)  -> a -> [b] -> IOEnv env a #-}
    -- {-# SPECIALIZE foldrM        :: (b -> a -> IOEnv env a)  -> a -> [b] -> IOEnv env a #-}
    -- {-# SPECIALIZE mapAndUnzipM  :: (a -> IOEnv env (b,c))   -> [a] -> IOEnv env ([b],[c]) #-}
    -- {-# SPECIALIZE mapAndUnzip3M :: (a -> IOEnv env (b,c,d)) -> [a] -> IOEnv env ([b],[c],[d]) #-}
    -- {-# SPECIALIZE zipWithM      :: (a -> b -> IOEnv env c) -> [a] -> [b] -> IOEnv env [c] #-}
    -- {-# SPECIALIZE zipWithM_     :: (a -> b -> IOEnv env c) -> [a] -> [b] -> IOEnv env () #-}
    -- {-# SPECIALIZE anyM          :: (a -> IOEnv env Bool) -> [a] -> IOEnv env Bool #-}
    -- {-# SPECIALIZE when          :: Bool -> IOEnv env a -> IOEnv env () #-}
    -- {-# SPECIALIZE unless        :: Bool -> IOEnv env a -> IOEnv env () #-}