{-# LANGUAGE RankNTypes, TypeFamilies, FlexibleContexts, ScopedTypeVariables #-} {-# OPTIONS_GHC -fno-warn-orphans #-} {-| This module provides an orphan 'MonadCatch' instance for 'Proxy' of the form: > instance (MonadCatch m, MonadIO m) => MonadCatch (Proxy a' a b' b m) where ... so you can throw and catch exceptions within pipes using all 'MonadCatch' operations. This module also provides generalized versions of some 'MonadCatch' operations so that you can also protect against premature termination of connected components. For example, if you protect a 'readFile' computation using 'bracket' from this module: > -- readFile.hs > import Pipes > import qualified Pipes.Prelude as P > import Pipes.Safe > import qualified System.IO as IO > import Prelude hiding (readFile) > > readFile :: FilePath -> Producer' String (SafeT IO) () > readFile file = bracket > (do h <- IO.openFile file IO.ReadMode > putStrLn $ "{" ++ file ++ " open}" > return h ) > (\h -> do > IO.hClose h > putStrLn $ "{" ++ file ++ " closed}" ) > P.fromHandle ... then this generalized 'bracket' will guard against both exceptions and premature termination of other pipes: >>> runSafeT $ runEffect $ readFile "readFile.hs" >-> P.take 4 >-> P.stdoutLn {readFile.hs open} -- readFile.hs import Pipes import qualified Pipes.Prelude as P import Pipes.Safe {readFile.hs closed} Note that the 'MonadCatch' instance for 'Proxy' provides weaker versions of 'mask' and 'uninterruptibleMask' that do not completely prevent asynchronous exceptions. Instead, they provide a weaker guarantee that asynchronous exceptions will only occur during 'Pipes.await's or 'Pipes.yield's and nowhere else. For example, if you write: > mask_ $ do > x <- await > lift $ print x > lift $ print x ... then you may receive an asynchronous exception during the 'Pipes.await', but you will not receive an asynchronous exception during or in between the two 'print' statements. This weaker guarantee suffices to provide asynchronous exception safety. -} module Pipes.Safe ( -- * SafeT SafeT , runSafeT , runSafeP -- * MonadSafe , ReleaseKey , MonadSafe(..) -- * Utilities -- $utilities , onException , finally , bracket , bracket_ , bracketOnError -- * Re-exports -- $reexports , module Control.Monad.Catch , module Control.Exception ) where import Control.Applicative (Applicative(pure, (<*>))) import Control.Exception(Exception(..), SomeException(..)) import qualified Control.Monad.Catch as C import Control.Monad.Catch ( MonadCatch(..) , mask_ , uninterruptibleMask_ , catchAll , catchIOError , catchJust , catchIf , Handler(..) , catches , handle , handleAll , handleIOError , handleJust , handleIf , try , tryJust , Exception(..) , SomeException ) import Control.Monad.IO.Class (MonadIO(liftIO)) import Control.Monad.Trans.Class (MonadTrans(lift)) import qualified Control.Monad.Catch.Pure as E import qualified Control.Monad.Trans.Identity as I import qualified Control.Monad.Trans.Reader as R import qualified Control.Monad.Trans.RWS.Lazy as RWS import qualified Control.Monad.Trans.RWS.Strict as RWS' import qualified Control.Monad.Trans.State.Lazy as S import qualified Control.Monad.Trans.State.Strict as S' import qualified Control.Monad.Trans.Writer.Lazy as W import qualified Control.Monad.Trans.Writer.Strict as W' import Data.IORef (IORef, newIORef, readIORef, writeIORef) import qualified Data.Map as M import Data.Monoid (Monoid) import Pipes (Proxy, Effect, Effect', runEffect) import Pipes.Internal (unsafeHoist, Proxy(..)) import Pipes.Lift (liftCatchError) data Restore m = Unmasked | Masked (forall x . m x -> m x) liftMask :: forall m a' a b' b r . (MonadIO m, MonadCatch m) => (forall s . ((forall x . m x -> m x) -> m s) -> m s) -> ((forall x . Proxy a' a b' b m x -> Proxy a' a b' b m x) -> Proxy a' a b' b m r) -> Proxy a' a b' b m r liftMask maskFunction k = do ioref <- liftIO (newIORef Unmasked) let unmask :: forall y . (Monad m) => Proxy a' a b' b m y -> Proxy a' a b' b m y unmask p = do mRestore <- liftIO (readIORef ioref) case mRestore of Unmasked -> p Masked restore -> do r <- unsafeHoist restore p lift $ restore $ return () return r loop p = case p of Request a' fa -> Request a' (loop . fa ) Respond b fb' -> Respond b (loop . fb') M m0 -> M $ maskFunction $ \restore -> do liftIO $ writeIORef ioref (Masked restore) let loop' m = do p' <- m case p' of M m' -> loop' m' _ -> return p' p' <- loop' m0 liftIO $ writeIORef ioref Unmasked return (loop p') Pure r -> Pure r loop (k unmask) instance (MonadCatch m, MonadIO m) => MonadCatch (Proxy a' a b' b m) where throwM = lift . throwM catch = liftCatchError C.catch mask = liftMask mask uninterruptibleMask = liftMask uninterruptibleMask data Finalizers m = Finalizers { _nextKey :: !Integer , _finalizers :: !(M.Map Integer (m ())) } {-| 'SafeT' is a monad transformer that extends the base monad with the ability to 'register' and 'release' finalizers. All unreleased finalizers are called at the end of the 'SafeT' block, even in the event of exceptions. -} newtype SafeT m r = SafeT { unSafeT :: R.ReaderT (IORef (Finalizers m)) m r } -- Deriving 'Functor' instance (Monad m) => Functor (SafeT m) where fmap f m = SafeT (do r <- unSafeT m return (f r) ) -- Deriving 'Applicative' instance (Monad m) => Applicative (SafeT m) where pure r = SafeT (return r) mf <*> mx = SafeT (do f <- unSafeT mf x <- unSafeT mx return (f x) ) -- Deriving 'Monad' instance (Monad m) => Monad (SafeT m) where return r = SafeT (return r) m >>= f = SafeT (do r <- unSafeT m unSafeT (f r) ) -- Deriving 'MonadIO' instance (MonadIO m) => MonadIO (SafeT m) where liftIO m = SafeT (liftIO m) -- Deriving 'MonadCatch' instance (MonadCatch m) => MonadCatch (SafeT m) where throwM e = SafeT (throwM e) m `catch` f = SafeT (unSafeT m `C.catch` \r -> unSafeT (f r)) mask k = SafeT (mask (\restore -> unSafeT (k (\ma -> SafeT (restore (unSafeT ma)))) )) uninterruptibleMask k = SafeT (uninterruptibleMask (\restore -> unSafeT (k (\ma -> SafeT (restore (unSafeT ma)))) )) instance MonadTrans SafeT where lift m = SafeT (lift m) {-| Run the 'SafeT' monad transformer, executing all unreleased finalizers at the end of the computation -} runSafeT :: (MonadCatch m, MonadIO m) => SafeT m r -> m r runSafeT m = C.bracket (liftIO $ newIORef $! Finalizers 0 M.empty) (\ioref -> do Finalizers _ fs <- liftIO (readIORef ioref) mapM snd (M.toDescList fs) ) (R.runReaderT (unSafeT m)) {-# INLINABLE runSafeT #-} {-| Run 'SafeT' in the base monad, executing all unreleased finalizers at the end of the computation Use 'runSafeP' to safely flush all unreleased finalizers and ensure prompt finalization without exiting the 'Proxy' monad. -} runSafeP :: (MonadCatch m, MonadIO m) => Effect (SafeT m) r -> Effect' m r runSafeP = lift . runSafeT . runEffect {-# INLINABLE runSafeP #-} -- | Token used to 'release' a previously 'register'ed finalizer newtype ReleaseKey = ReleaseKey { unlock :: Integer } {-| 'MonadSafe' lets you 'register' and 'release' finalizers that execute in a 'Base' monad -} class (MonadCatch m, MonadIO m, MonadIO (Base m)) => MonadSafe m where {-| The monad used to run resource management actions, corresponding to the monad directly beneath 'SafeT' -} type Base (m :: * -> *) :: * -> * -- | Lift an action from the 'Base' monad liftBase :: Base m r -> m r {-| 'register' a finalizer, ensuring that the finalizer gets called if the finalizer is not 'release'd before the end of the surrounding 'SafeT' block. -} register :: Base m () -> m ReleaseKey {-| 'release' a registered finalizer You can safely call 'release' more than once on the same 'ReleaseKey'. Every 'release' after the first one does nothing. -} release :: ReleaseKey -> m () instance (MonadIO m, MonadCatch m) => MonadSafe (SafeT m) where type Base (SafeT m) = m liftBase = lift register io = do ioref <- SafeT R.ask liftIO $ do Finalizers n fs <- readIORef ioref writeIORef ioref $! Finalizers (n + 1) (M.insert n io fs) return (ReleaseKey n) release key = do ioref <- SafeT R.ask liftIO $ do Finalizers n fs <- readIORef ioref writeIORef ioref $! Finalizers n (M.delete (unlock key) fs) instance (MonadSafe m) => MonadSafe (Proxy a' a b' b m) where type Base (Proxy a' a b' b m) = Base m liftBase = lift . liftBase register = lift . register release = lift . release instance (MonadSafe m) => MonadSafe (I.IdentityT m) where type Base (I.IdentityT m) = Base m liftBase = lift . liftBase register = lift . register release = lift . release instance (MonadSafe m) => MonadSafe (E.CatchT m) where type Base (E.CatchT m) = Base m liftBase = lift . liftBase register = lift . register release = lift . release instance (MonadSafe m) => MonadSafe (R.ReaderT i m) where type Base (R.ReaderT i m) = Base m liftBase = lift . liftBase register = lift . register release = lift . release instance (MonadSafe m) => MonadSafe (S.StateT s m) where type Base (S.StateT s m) = Base m liftBase = lift . liftBase register = lift . register release = lift . release instance (MonadSafe m) => MonadSafe (S'.StateT s m) where type Base (S'.StateT s m) = Base m liftBase = lift . liftBase register = lift . register release = lift . release instance (MonadSafe m, Monoid w) => MonadSafe (W.WriterT w m) where type Base (W.WriterT w m) = Base m liftBase = lift . liftBase register = lift . register release = lift . release instance (MonadSafe m, Monoid w) => MonadSafe (W'.WriterT w m) where type Base (W'.WriterT w m) = Base m liftBase = lift . liftBase register = lift . register release = lift . release instance (MonadSafe m, Monoid w) => MonadSafe (RWS.RWST i w s m) where type Base (RWS.RWST i w s m) = Base m liftBase = lift . liftBase register = lift . register release = lift . release instance (MonadSafe m, Monoid w) => MonadSafe (RWS'.RWST i w s m) where type Base (RWS'.RWST i w s m) = Base m liftBase = lift . liftBase register = lift . register release = lift . release {-| Analogous to 'C.onException' from @Control.Monad.Catch@, except this also protects against premature termination @(\`onException\` io)@ is a monad morphism. -} onException :: (MonadSafe m) => m a -> Base m b -> m a m1 `onException` io = do key <- register (io >> return ()) r <- m1 release key return r {-# INLINABLE onException #-} {- $utilities These utilities let you supply a finalizer that runs in the 'Base' monad (i.e. the monad directly beneath 'SafeT'). If you don't need to use the full power of the 'Base' monad and you only need to use to use 'IO', then just wrap the finalizer in 'liftIO', like this: > myAction `finally` (liftIO myFinalizer) This will lead to a simple inferred type with a single 'MonadSafe' constraint: > (MonadSafe m) => ... For examples of this, see the utilities in "Pipes.Safe.Prelude". If you omit the 'liftIO', the compiler will infer the following constraint instead: > (MonadSafe m, Base m ~ IO) => ... This means that this function would require 'IO' directly beneath the 'SafeT' monad transformer, which might not be what you want. -} {-| Analogous to 'C.finally' from @Control.Monad.Catch@, except this also protects against premature termination -} finally :: (MonadSafe m) => m a -> Base m b -> m a m1 `finally` after = bracket_ (return ()) after m1 {-# INLINABLE finally #-} {-| Analogous to 'C.bracket' from @Control.Monad.Catch@, except this also protects against premature termination -} bracket :: (MonadSafe m) => Base m a -> (a -> Base m b) -> (a -> m c) -> m c bracket before after action = mask $ \restore -> do h <- liftBase before r <- restore (action h) `onException` after h _ <- liftBase (after h) return r {-# INLINABLE bracket #-} {-| Analogous to 'C.bracket_' from @Control.Monad.Catch@, except this also protects against premature termination -} bracket_ :: (MonadSafe m) => Base m a -> Base m b -> m c -> m c bracket_ before after action = bracket before (\_ -> after) (\_ -> action) {-# INLINABLE bracket_ #-} {-| Analogous to 'C.bracketOnError' from @Control.Monad.Catch@, except this also protects against premature termination -} bracketOnError :: (MonadSafe m) => Base m a -> (a -> Base m b) -> (a -> m c) -> m c bracketOnError before after action = mask $ \restore -> do h <- liftBase before restore (action h) `onException` after h {-# INLINABLE bracketOnError #-} {- $reexports @Control.Monad.Catch@ re-exports all functions except for the ones that conflict with the generalized versions provided here (i.e. 'bracket', 'finally', etc.). @Control.Exception@ re-exports 'Exception' and 'SomeException'. -}