{-# LANGUAGE CPP #-} {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE GeneralizedNewtypeDeriving #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE TypeFamilies #-} #if defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ >= 702 {-# LANGUAGE Trustworthy #-} #endif #ifndef MIN_VERSION_transformers #define MIN_VERSION_transformers(x,y,z) 1 #endif #ifndef MIN_VERSION_mtl #define MIN_VERSION_mtl(x,y,z) 1 #endif -------------------------------------------------------------------- -- | -- Copyright : (C) Edward Kmett 2013-2014, (c) Google Inc. 2012 -- License : BSD-style (see the file LICENSE) -- Maintainer : Edward Kmett <ekmett@gmail.com> -- Stability : experimental -- Portability : non-portable -- -- This module supports monads that can throw extensible exceptions. The -- exceptions are the very same from "Control.Exception", and the operations -- offered very similar, but here they are not limited to 'IO'. -- -- This code is in the style of both transformers and mtl, and is compatible -- with them, though doesn't mimic the module structure or offer the complete -- range of features in those packages. -- -- This is very similar to 'ErrorT' and 'MonadError', but based on features of -- "Control.Exception". In particular, it handles the complex case of -- asynchronous exceptions by including 'mask' in the typeclass. Note that the -- extensible extensions feature relies the RankNTypes language extension. -------------------------------------------------------------------- module Control.Monad.Catch ( -- * Typeclass -- $mtl MonadThrow(..) , MonadCatch(..) , MonadMask(..) -- * Utilities -- $utilities , mask_ , uninterruptibleMask_ , catchAll , catchIOError , catchJust , catchIf , Handler(..), catches , handle , handleAll , handleIOError , handleJust , handleIf , try , tryJust , onException , bracket , bracket_ , finally , bracketOnError -- * Re-exports from Control.Exception , Exception(..) , SomeException(..) ) where #if defined(__GLASGOW_HASKELL__) && (__GLASGOW_HASKELL__ >= 706) import Prelude hiding (foldr) #else import Prelude hiding (catch, foldr) #endif import Control.Exception (Exception(..), SomeException(..)) import qualified Control.Exception as ControlException import qualified Control.Monad.Trans.RWS.Lazy as LazyRWS import qualified Control.Monad.Trans.RWS.Strict as StrictRWS import qualified Control.Monad.Trans.State.Lazy as LazyS import qualified Control.Monad.Trans.State.Strict as StrictS import qualified Control.Monad.Trans.Writer.Lazy as LazyW import qualified Control.Monad.Trans.Writer.Strict as StrictW import Control.Monad.Trans.List (ListT(..), runListT) import Control.Monad.Trans.Maybe (MaybeT(..), runMaybeT) import Control.Monad.Trans.Error (ErrorT(..), Error, runErrorT) #if MIN_VERSION_transformers(0,4,0) import Control.Monad.Trans.Except (ExceptT(..), runExceptT) #endif import Control.Monad.Trans.Cont (ContT) import Control.Monad.Trans.Identity import Control.Monad.Reader as Reader import Control.Monad.Trans.Reader (runReaderT) import Control.Monad.RWS import Data.Foldable ------------------------------------------------------------------------------ -- $mtl -- The mtl style typeclass ------------------------------------------------------------------------------ -- | A class for monads in which exceptions may be thrown. -- -- Instances should obey the following law: -- -- > throwM e >> x = throwM e -- -- In other words, throwing an exception short-circuits the rest of the monadic -- computation. class Monad m => MonadThrow m where -- | Throw an exception. Note that this throws when this action is run in -- the monad @m@, not when it is applied. It is a generalization of -- "Control.Exception"'s 'ControlException.throwIO'. -- -- Should satisfy the law: -- -- > throwM e >> f = throwM e throwM :: Exception e => e -> m a -- | A class for monads which allow exceptions to be caught, in particular -- exceptions which were thrown by 'throwM'. -- -- Instances should obey the following law: -- -- > catch (throwM e) f = f e -- -- Note that the ability to catch an exception does /not/ guarantee that we can -- deal with all possible exit points from a computation. Some monads, such as -- continuation-based stacks, allow for more than just a success/failure -- strategy, and therefore @catch@ /cannot/ be used by those monads to properly -- implement a function such as @finally@. For more information, see -- 'MonadMask'. class MonadThrow m => MonadCatch m where -- | Provide a handler for exceptions thrown during execution of the first -- action. Note that type of the type of the argument to the handler will -- constrain which exceptions are caught. See "Control.Exception"'s -- 'ControlException.catch'. catch :: Exception e => m a -> (e -> m a) -> m a -- | A class for monads which provide for the ability to account for all -- possible exit points from a computation, and to mask asynchronous -- exceptions. Continuation-based monads, and stacks such as @ErrorT e IO@ -- which provide for multiple failure modes, are invalid instances of this -- class. -- -- Note that this package /does/ provide a @MonadMask@ instance for @CatchT@. -- This instance is /only/ valid if the base monad provides no ability to -- provide multiple exit. For example, @IO@ or @Either@ would be invalid base -- monads, but @Reader@ or @State@ would be acceptable. -- -- Instances should ensure that, in the following code: -- -- > f `finally` g -- -- The action @g@ is called regardless of what occurs within @f@, including -- async exceptions. class MonadCatch m => MonadMask m where -- | Runs an action with asynchronous exceptions disabled. The action is -- provided a method for restoring the async. environment to what it was -- at the 'mask' call. See "Control.Exception"'s 'ControlException.mask'. mask :: ((forall a. m a -> m a) -> m b) -> m b -- | Like 'mask', but the masked computation is not interruptible (see -- "Control.Exception"'s 'ControlException.uninterruptibleMask'. WARNING: -- Only use if you need to mask exceptions around an interruptible operation -- AND you can guarantee the interruptible operation will only block for a -- short period of time. Otherwise you render the program/thread unresponsive -- and/or unkillable. uninterruptibleMask :: ((forall a. m a -> m a) -> m b) -> m b instance MonadThrow [] where throwM _ = [] instance MonadThrow Maybe where throwM _ = Nothing instance e ~ SomeException => MonadThrow (Either e) where throwM = Left . toException instance MonadThrow IO where throwM = ControlException.throwIO instance MonadCatch IO where catch = ControlException.catch instance MonadMask IO where mask = ControlException.mask uninterruptibleMask = ControlException.uninterruptibleMask instance MonadThrow m => MonadThrow (IdentityT m) where throwM e = lift $ throwM e instance MonadCatch m => MonadCatch (IdentityT m) where catch (IdentityT m) f = IdentityT (catch m (runIdentityT . f)) instance MonadMask m => MonadMask (IdentityT m) where mask a = IdentityT $ mask $ \u -> runIdentityT (a $ q u) where q u = IdentityT . u . runIdentityT uninterruptibleMask a = IdentityT $ uninterruptibleMask $ \u -> runIdentityT (a $ q u) where q u = IdentityT . u . runIdentityT instance MonadThrow m => MonadThrow (LazyS.StateT s m) where throwM e = lift $ throwM e instance MonadCatch m => MonadCatch (LazyS.StateT s m) where catch = LazyS.liftCatch catch instance MonadMask m => MonadMask (LazyS.StateT s m) where mask a = LazyS.StateT $ \s -> mask $ \u -> LazyS.runStateT (a $ q u) s where q u (LazyS.StateT b) = LazyS.StateT (u . b) uninterruptibleMask a = LazyS.StateT $ \s -> uninterruptibleMask $ \u -> LazyS.runStateT (a $ q u) s where q u (LazyS.StateT b) = LazyS.StateT (u . b) instance MonadThrow m => MonadThrow (StrictS.StateT s m) where throwM e = lift $ throwM e instance MonadCatch m => MonadCatch (StrictS.StateT s m) where catch = StrictS.liftCatch catch instance MonadMask m => MonadMask (StrictS.StateT s m) where mask a = StrictS.StateT $ \s -> mask $ \u -> StrictS.runStateT (a $ q u) s where q u (StrictS.StateT b) = StrictS.StateT (u . b) uninterruptibleMask a = StrictS.StateT $ \s -> uninterruptibleMask $ \u -> StrictS.runStateT (a $ q u) s where q u (StrictS.StateT b) = StrictS.StateT (u . b) instance MonadThrow m => MonadThrow (ReaderT r m) where throwM e = lift $ throwM e instance MonadCatch m => MonadCatch (ReaderT r m) where catch (ReaderT m) c = ReaderT $ \r -> m r `catch` \e -> runReaderT (c e) r instance MonadMask m => MonadMask (ReaderT r m) where mask a = ReaderT $ \e -> mask $ \u -> runReaderT (a $ q u) e where q u (ReaderT b) = ReaderT (u . b) uninterruptibleMask a = ReaderT $ \e -> uninterruptibleMask $ \u -> runReaderT (a $ q u) e where q u (ReaderT b) = ReaderT (u . b) instance (MonadThrow m, Monoid w) => MonadThrow (StrictW.WriterT w m) where throwM e = lift $ throwM e instance (MonadCatch m, Monoid w) => MonadCatch (StrictW.WriterT w m) where catch (StrictW.WriterT m) h = StrictW.WriterT $ m `catch ` \e -> StrictW.runWriterT (h e) instance (MonadMask m, Monoid w) => MonadMask (StrictW.WriterT w m) where mask a = StrictW.WriterT $ mask $ \u -> StrictW.runWriterT (a $ q u) where q u b = StrictW.WriterT $ u (StrictW.runWriterT b) uninterruptibleMask a = StrictW.WriterT $ uninterruptibleMask $ \u -> StrictW.runWriterT (a $ q u) where q u b = StrictW.WriterT $ u (StrictW.runWriterT b) instance (MonadThrow m, Monoid w) => MonadThrow (LazyW.WriterT w m) where throwM e = lift $ throwM e instance (MonadCatch m, Monoid w) => MonadCatch (LazyW.WriterT w m) where catch (LazyW.WriterT m) h = LazyW.WriterT $ m `catch ` \e -> LazyW.runWriterT (h e) instance (MonadMask m, Monoid w) => MonadMask (LazyW.WriterT w m) where mask a = LazyW.WriterT $ mask $ \u -> LazyW.runWriterT (a $ q u) where q u b = LazyW.WriterT $ u (LazyW.runWriterT b) uninterruptibleMask a = LazyW.WriterT $ uninterruptibleMask $ \u -> LazyW.runWriterT (a $ q u) where q u b = LazyW.WriterT $ u (LazyW.runWriterT b) instance (MonadThrow m, Monoid w) => MonadThrow (LazyRWS.RWST r w s m) where throwM e = lift $ throwM e instance (MonadCatch m, Monoid w) => MonadCatch (LazyRWS.RWST r w s m) where catch (LazyRWS.RWST m) h = LazyRWS.RWST $ \r s -> m r s `catch` \e -> LazyRWS.runRWST (h e) r s instance (MonadMask m, Monoid w) => MonadMask (LazyRWS.RWST r w s m) where mask a = LazyRWS.RWST $ \r s -> mask $ \u -> LazyRWS.runRWST (a $ q u) r s where q u (LazyRWS.RWST b) = LazyRWS.RWST $ \ r s -> u (b r s) uninterruptibleMask a = LazyRWS.RWST $ \r s -> uninterruptibleMask $ \u -> LazyRWS.runRWST (a $ q u) r s where q u (LazyRWS.RWST b) = LazyRWS.RWST $ \ r s -> u (b r s) instance (MonadThrow m, Monoid w) => MonadThrow (StrictRWS.RWST r w s m) where throwM e = lift $ throwM e instance (MonadCatch m, Monoid w) => MonadCatch (StrictRWS.RWST r w s m) where catch (StrictRWS.RWST m) h = StrictRWS.RWST $ \r s -> m r s `catch` \e -> StrictRWS.runRWST (h e) r s instance (MonadMask m, Monoid w) => MonadMask (StrictRWS.RWST r w s m) where mask a = StrictRWS.RWST $ \r s -> mask $ \u -> StrictRWS.runRWST (a $ q u) r s where q u (StrictRWS.RWST b) = StrictRWS.RWST $ \ r s -> u (b r s) uninterruptibleMask a = StrictRWS.RWST $ \r s -> uninterruptibleMask $ \u -> StrictRWS.runRWST (a $ q u) r s where q u (StrictRWS.RWST b) = StrictRWS.RWST $ \ r s -> u (b r s) -- Transformers which are only instances of MonadThrow and MonadCatch, not MonadMask instance MonadThrow m => MonadThrow (ListT m) where throwM = lift . throwM instance MonadCatch m => MonadCatch (ListT m) where catch (ListT m) f = ListT $ catch m (runListT . f) -- | Throws exceptions into the base monad. instance MonadThrow m => MonadThrow (MaybeT m) where throwM = lift . throwM -- | Catches exceptions from the base monad. instance MonadCatch m => MonadCatch (MaybeT m) where catch (MaybeT m) f = MaybeT $ catch m (runMaybeT . f) -- | Throws exceptions into the base monad. instance (Error e, MonadThrow m) => MonadThrow (ErrorT e m) where throwM = lift . throwM -- | Catches exceptions from the base monad. instance (Error e, MonadCatch m) => MonadCatch (ErrorT e m) where catch (ErrorT m) f = ErrorT $ catch m (runErrorT . f) #if MIN_VERSION_transformers(0,4,0) -- | Throws exceptions into the base monad. instance MonadThrow m => MonadThrow (ExceptT e m) where throwM = lift . throwM -- | Catches exceptions from the base monad. instance MonadCatch m => MonadCatch (ExceptT e m) where catch (ExceptT m) f = ExceptT $ catch m (runExceptT . f) #endif instance MonadThrow m => MonadThrow (ContT r m) where throwM = lift . throwM -- I don't believe any valid of MonadCatch exists for ContT. -- instance MonadCatch m => MonadCatch (ContT r m) where ------------------------------------------------------------------------------ -- $utilities -- These functions follow those from "Control.Exception", except that they are -- based on methods from the 'MonadCatch' typeclass. See -- "Control.Exception" for API usage. ------------------------------------------------------------------------------ -- | Like 'mask', but does not pass a @restore@ action to the argument. mask_ :: MonadMask m => m a -> m a mask_ io = mask $ \_ -> io -- | Like 'uninterruptibleMask', but does not pass a @restore@ action to the -- argument. uninterruptibleMask_ :: MonadMask m => m a -> m a uninterruptibleMask_ io = uninterruptibleMask $ \_ -> io -- | Catches all exceptions, and somewhat defeats the purpose of the extensible -- exception system. Use sparingly. catchAll :: MonadCatch m => m a -> (SomeException -> m a) -> m a catchAll = catch -- | Catch all 'IOError' (eqv. 'IOException') exceptions. Still somewhat too -- general, but better than using 'catchAll'. See 'catchIf' for an easy way -- of catching specific 'IOError's based on the predicates in "System.IO.Error". catchIOError :: MonadCatch m => m a -> (IOError -> m a) -> m a catchIOError = catch -- | Catch exceptions only if they pass some predicate. Often useful with the -- predicates for testing 'IOError' values in "System.IO.Error". catchIf :: (MonadCatch m, Exception e) => (e -> Bool) -> m a -> (e -> m a) -> m a catchIf f a b = a `catch` \e -> if f e then b e else throwM e -- | A more generalized way of determining which exceptions to catch at -- run time. catchJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> (b -> m a) -> m a catchJust f a b = a `catch` \e -> maybe (throwM e) b $ f e -- | Flipped 'catch'. See "Control.Exception"'s 'ControlException.handle'. handle :: (MonadCatch m, Exception e) => (e -> m a) -> m a -> m a handle = flip catch {-# INLINE handle #-} -- | Flipped 'catchIOError' handleIOError :: MonadCatch m => (IOError -> m a) -> m a -> m a handleIOError = handle -- | Flipped 'catchAll' handleAll :: MonadCatch m => (SomeException -> m a) -> m a -> m a handleAll = handle -- | Flipped 'catchIf' handleIf :: (MonadCatch m, Exception e) => (e -> Bool) -> (e -> m a) -> m a -> m a handleIf f = flip (catchIf f) -- | Flipped 'catchJust'. See "Control.Exception"'s 'ControlException.handleJust'. handleJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> (b -> m a) -> m a -> m a handleJust f = flip (catchJust f) {-# INLINE handleJust #-} -- | Similar to 'catch', but returns an 'Either' result. See "Control.Exception"'s -- 'Control.Exception.try'. try :: (MonadCatch m, Exception e) => m a -> m (Either e a) try a = catch (Right `liftM` a) (return . Left) -- | A variant of 'try' that takes an exception predicate to select -- which exceptions are caught. See "Control.Exception"'s 'ControlException.tryJust' tryJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> m (Either b a) tryJust f a = catch (Right `liftM` a) (\e -> maybe (throwM e) (return . Left) (f e)) -- | Generalized version of 'ControlException.Handler' data Handler m a = forall e . ControlException.Exception e => Handler (e -> m a) instance Monad m => Functor (Handler m) where fmap f (Handler h) = Handler (liftM f . h) -- | Catches different sorts of exceptions. See "Control.Exception"'s 'ControlException.catches' catches :: (Foldable f, MonadCatch m) => m a -> f (Handler m a) -> m a catches a hs = a `catch` handler where handler e = foldr probe (throwM e) hs where probe (Handler h) xs = maybe xs h (ControlException.fromException e) -- | Run an action only if an exception is thrown in the main action. The -- exception is not caught, simply rethrown. onException :: MonadCatch m => m a -> m b -> m a onException action handler = action `catchAll` \e -> handler >> throwM e -- | Generalized abstracted pattern of safe resource acquisition and release -- in the face of exceptions. The first action \"acquires\" some value, which -- is \"released\" by the second action at the end. The third action \"uses\" -- the value and its result is the result of the 'bracket'. -- -- If an exception occurs during the use, the release still happens before the -- exception is rethrown. bracket :: MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c bracket acquire release use = mask $ \unmasked -> do resource <- acquire result <- unmasked (use resource) `onException` release resource _ <- release resource return result -- | Version of 'bracket' without any value being passed to the second and -- third actions. bracket_ :: MonadMask m => m a -> m b -> m c -> m c bracket_ before after action = bracket before (const after) (const action) -- | Perform an action with a finalizer action that is run, even if an -- exception occurs. finally :: MonadMask m => m a -> m b -> m a finally action finalizer = bracket_ (return ()) finalizer action -- | Like 'bracket', but only performs the final action if there was an -- exception raised by the in-between computation. bracketOnError :: MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c bracketOnError acquire release use = mask $ \unmasked -> do resource <- acquire unmasked (use resource) `onException` release resource