-- -- (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 () #-}