utils/IOEnv.hs

    --
    -- (c) The University of Glasgow 2002-2006
    --
    -- The IO Monad with an environment
    --
    -- The environment is passed around as a Reader monad but
    -- as its in the IO monad, mutable references can be used
    -- for updating state.
    --

    module IOEnv (
            IOEnv, -- Instance of Monad

            -- Monad utilities
            module MonadUtils,

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

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

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

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

    import GhcPrelude

    import DynFlags
    import Exception
    import Module
    import Panic

    import Data.IORef       ( IORef, newIORef, readIORef, writeIORef, modifyIORef,
                              atomicModifyIORef, atomicModifyIORef' )
    import System.IO.Unsafe ( unsafeInterleaveIO )
    import System.IO        ( fixIO )
    import Control.Monad
    import qualified Control.Monad.Fail as MonadFail
    import MonadUtils
    import Control.Applicative (Alternative(..))

    ----------------------------------------------------------------------
    -- 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
        (>>)   = (*>)
        fail   = MonadFail.fail

    instance MonadFail.MonadFail (IOEnv m) where
        fail _ = failM -- Ignore the string

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

    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

    instance Show IOEnvFailure where
        show IOEnvFailure = "IOEnv failure"

    instance Exception IOEnvFailure

    instance ExceptionMonad (IOEnv a) where
      gcatch act handle =
          IOEnv $ \s -> unIOEnv act s `gcatch` \e -> unIOEnv (handle e) s
      gmask f =
          IOEnv $ \s -> gmask $ \io_restore ->
                                 let
                                    g_restore (IOEnv m) = IOEnv $ \s -> io_restore (m s)
                                 in
                                    unIOEnv (f g_restore) s

    instance ContainsDynFlags env => HasDynFlags (IOEnv env) where
        getDynFlags = do env <- getEnv
                         return $ extractDynFlags env

    instance ContainsModule env => HasModule (IOEnv env) where
        getModule = do env <- getEnv
                       return $ extractModule env

    ----------------------------------------------------------------------
    -- Fundamental combinators specific to the monad
    ----------------------------------------------------------------------

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

    ---------------------------
    {-# NOINLINE fixM #-}
      -- Aargh!  Not inlining fixM alleviates a space leak problem.
      -- Normally fixM is used with a lazy tuple match: if the optimiser is
      -- shown the definition of fixM, 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))

    uninterruptibleMaskM_ :: IOEnv env a -> IOEnv env a
    uninterruptibleMaskM_ (IOEnv m) = IOEnv (\ env -> uninterruptibleMask_ (m env))

    ----------------------------------------------------------------------
    -- Alternative/MonadPlus
    ----------------------------------------------------------------------

    instance Alternative (IOEnv env) where
        empty   = IOEnv (const empty)
        m <|> n = IOEnv (\env -> unIOEnv m env <|> unIOEnv n env)

    instance MonadPlus (IOEnv 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 = liftIO (atomicModifyIORef' var upd)

    ----------------------------------------------------------------------
    -- 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))