{-# 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 {-# OPTIONS_GHC -fno-warn-deprecations #-} -------------------------------------------------------------------- -- | -- Copyright : (C) Edward Kmett 2013-2015, (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 exceptions feature relies on the RankNTypes language extension. -------------------------------------------------------------------- module Control.Monad.Catch ( -- * Typeclass -- $mtl MonadThrow(..) , MonadCatch(..) , MonadMask(..) , ExitCase(..) -- * Utilities -- $utilities , mask_ , uninterruptibleMask_ , catchAll , catchIOError , catchJust , catchIf , Handler(..), catches , handle , handleAll , handleIOError , handleJust , handleIf , try , tryJust , onException , onError , bracket , bracket_ , finally , bracketOnError -- * Re-exports from Control.Exception , Exception(..) , SomeException(..) ) where import Control.Exception (Exception(..), SomeException(..)) import qualified Control.Exception as ControlException import qualified Control.Monad.STM as STM 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.ST (ST) import Control.Monad.STM (STM) import Control.Monad.Trans.List (ListT(..), runListT) import Control.Monad.Trans.Maybe (MaybeT(..), runMaybeT) import Control.Monad.Trans.Error (ErrorT(..), Error, runErrorT) import Control.Monad.Trans.Except (ExceptT(..), runExceptT) import Control.Monad.Trans.Cont (ContT) import Control.Monad.Trans.Identity import Control.Monad.Reader as Reader import Language.Haskell.TH.Syntax (Q) #if MIN_VERSION_base(4,4,0) import Control.Monad.ST.Unsafe (unsafeIOToST) #else import Control.Monad.ST (unsafeIOToST) #endif #if __GLASGOW_HASKELL__ < 706 import Prelude hiding (catch, foldr) import Data.Foldable import Data.Monoid #elif __GLASGOW_HASKELL__ < 710 import Prelude hiding (foldr) import Data.Foldable import Data.Monoid #endif #if __GLASGOW_HASKELL__ < 710 import Control.Applicative #endif ------------------------------------------------------------------------------ -- $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 are invalid -- instances of this class. -- -- Instances should ensure that, in the following code: -- -- > fg = f `finally` g -- -- The action @g@ is called regardless of what occurs within @f@, including -- async exceptions. Some monads allow @f@ to abort the computation via other -- effects than throwing an exception. For simplicity, we will consider aborting -- and throwing an exception to be two forms of "throwing an error". -- -- If @f@ and @g@ both throw an error, the error thrown by @fg@ depends on which -- errors we're talking about. In a monad transformer stack, the deeper layers -- override the effects of the inner layers; for example, @ExceptT e1 (Except -- e2) a@ represents a value of type @Either e2 (Either e1 a)@, so throwing both -- an @e1@ and an @e2@ will result in @Left e2@. If @f@ and @g@ both throw an -- error from the same layer, instances should ensure that the error from @g@ -- wins. -- -- Effects other than throwing an error are also overriden by the deeper layers. -- For example, @StateT s Maybe a@ represents a value of type @s -> Maybe (a, -- s)@, so if an error thrown from @f@ causes this function to return @Nothing@, -- any changes to the state which @f@ also performed will be erased. As a -- result, @g@ will see the state as it was before @f@. Once @g@ completes, -- @f@'s error will be rethrown, so @g@' state changes will be erased as well. -- This is the normal interaction between effects in a monad transformer stack. -- -- By contrast, <https://hackage.haskell.org/package/lifted-base lifted-base>'s -- version of 'finally' always discards all of @g@'s non-IO effects, and @g@ -- never sees any of @f@'s non-IO effects, regardless of the layer ordering and -- regardless of whether @f@ throws an error. This is not the result of -- interacting effects, but a consequence of @MonadBaseControl@'s approach. 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 -- | A generalized version of 'bracket' which uses 'ExitCase' to distinguish -- the different exit cases, and returns the values of both the 'use' and -- 'release' actions. In practice, this extra information is rarely needed, -- so it is often more convenient to use one of the simpler functions which -- are defined in terms of this one, such as 'bracket', 'finally', 'onError', -- and 'bracketOnError'. -- -- This function exists because in order to thread their effects through the -- execution of 'bracket', monad transformers need values to be threaded from -- 'use' to 'release' and from 'release' to the output value. -- -- /NOTE/ This method was added in version 0.9.0 of this -- library. Previously, implementation of functions like 'bracket' -- and 'finally' in this module were based on the 'mask' and -- 'uninterruptibleMask' functions only, disallowing some classes of -- tranformers from having @MonadMask@ instances (notably -- multi-exit-point transformers like 'ExceptT'). If you are a -- library author, you'll now need to provide an implementation for -- this method. The @StateT@ implementation demonstrates most of the -- subtleties: -- -- @ -- generalBracket acquire release use = StateT $ \s0 -> do -- ((b, _s2), (c, s3)) <- generalBracket -- (runStateT acquire s0) -- (\(resource, s1) exitCase -> case exitCase of -- ExitCaseSuccess (b, s2) -> runStateT (release resource (ExitCaseSuccess b)) s2 -- -- -- In the two other cases, the base monad overrides @use@'s state -- -- changes and the state reverts to @s1@. -- ExitCaseException e -> runStateT (release resource (ExitCaseException e)) s1 -- ExitCaseAbort -> runStateT (release resource ExitCaseAbort) s1 -- ) -- (\(resource, s1) -> runStateT (use resource) s1) -- return ((b, c), s3) -- @ -- -- The @StateT s m@ implementation of @generalBracket@ delegates to the @m@ -- implementation of @generalBracket@. The @acquire@, @use@, and @release@ -- arguments given to @StateT@'s implementation produce actions of type -- @StateT s m a@, @StateT s m b@, and @StateT s m c@. In order to run those -- actions in the base monad, we need to call @runStateT@, from which we -- obtain actions of type @m (a, s)@, @m (b, s)@, and @m (c, s)@. Since each -- action produces the next state, it is important to feed the state produced -- by the previous action to the next action. -- -- In the 'ExitCaseSuccess' case, the state starts at @s0@, flows through -- @acquire@ to become @s1@, flows through @use@ to become @s2@, and finally -- flows through @release@ to become @s3@. In the other two cases, @release@ -- does not receive the value @s2@, so its action cannot see the state changes -- performed by @use@. This is fine, because in those two cases, an error was -- thrown in the base monad, so as per the usual interaction between effects -- in a monad transformer stack, those state changes get reverted. So we start -- from @s1@ instead. -- -- Finally, the @m@ implementation of @generalBracket@ returns the pairs -- @(b, s)@ and @(c, s)@. For monad transformers other than @StateT@, this -- will be some other type representing the effects and values performed and -- returned by the @use@ and @release@ actions. The effect part of the @use@ -- result, in this case @_s2@, usually needs to be discarded, since those -- effects have already been incorporated in the @release@ action. -- -- The only effect which is intentionally not incorporated in the @release@ -- action is the effect of throwing an error. In that case, the error must be -- re-thrown. One subtlety which is easy to miss is that in the case in which -- @use@ and @release@ both throw an error, the error from @release@ should -- take priority. Here is an implementation for @ExceptT@ which demonstrates -- how to do this. -- -- @ -- generalBracket acquire release use = ExceptT $ do -- (eb, ec) <- generalBracket -- (runExceptT acquire) -- (\eresource exitCase -> case eresource of -- Left e -> return (Left e) -- nothing to release, acquire didn't succeed -- Right resource -> case exitCase of -- ExitCaseSuccess (Right b) -> runExceptT (release resource (ExitCaseSuccess b)) -- ExitCaseException e -> runExceptT (release resource (ExitCaseException e)) -- _ -> runExceptT (release resource ExitCaseAbort)) -- (either (return . Left) (runExceptT . use)) -- return $ do -- -- The order in which we perform those two 'Either' effects determines -- -- which error will win if they are both 'Left's. We want the error from -- -- 'release' to win. -- c <- ec -- b <- eb -- return (b, c) -- @ -- -- @since 0.9.0 generalBracket :: m a -- ^ acquire some resource -> (a -> ExitCase b -> m c) -- ^ release the resource, observing the outcome of the inner action -> (a -> m b) -- ^ inner action to perform with the resource -> m (b, c) -- | A 'MonadMask' computation may either succeed with a value, abort with an -- exception, or abort for some other reason. For example, in @ExceptT e IO@ -- you can use 'throwM' to abort with an exception ('ExitCaseException') or -- 'Control.Monad.Trans.Except.throwE' to abort with a value of type 'e' -- ('ExitCaseAbort'). data ExitCase a = ExitCaseSuccess a | ExitCaseException SomeException | ExitCaseAbort deriving Show instance MonadThrow [] where throwM _ = [] instance MonadThrow Maybe where throwM _ = Nothing instance MonadThrow Q where throwM = fail . show instance MonadThrow IO where throwM = ControlException.throwIO instance MonadCatch IO where catch = ControlException.catch instance MonadMask IO where mask = ControlException.mask uninterruptibleMask = ControlException.uninterruptibleMask generalBracket acquire release use = mask $ \unmasked -> do resource <- acquire b <- unmasked (use resource) `catch` \e -> do _ <- release resource (ExitCaseException e) throwM e c <- release resource (ExitCaseSuccess b) return (b, c) instance MonadThrow (ST s) where throwM = unsafeIOToST . ControlException.throwIO instance MonadThrow STM where throwM = STM.throwSTM instance MonadCatch STM where catch = STM.catchSTM instance e ~ SomeException => MonadThrow (Either e) where throwM = Left . toException -- | @since 0.8.3 instance e ~ SomeException => MonadCatch (Either e) where catch (Left e) f = case fromException e of Nothing -> Left e Just e' -> f e' catch x@(Right _) _ = x -- | @since 0.8.3 instance e ~ SomeException => MonadMask (Either e) where mask f = f id uninterruptibleMask f = f id generalBracket acquire release use = case acquire of Left e -> Left e Right resource -> case use resource of Left e -> release resource (ExitCaseException e) >> Left e Right b -> do c <- release resource (ExitCaseSuccess b) return (b, c) 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 :: (m a -> m a) -> IdentityT m a -> IdentityT m a q u = IdentityT . u . runIdentityT uninterruptibleMask a = IdentityT $ uninterruptibleMask $ \u -> runIdentityT (a $ q u) where q :: (m a -> m a) -> IdentityT m a -> IdentityT m a q u = IdentityT . u . runIdentityT generalBracket acquire release use = IdentityT $ generalBracket (runIdentityT acquire) (\resource exitCase -> runIdentityT (release resource exitCase)) (\resource -> runIdentityT (use resource)) 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 :: (m (a, s) -> m (a, s)) -> LazyS.StateT s m a -> LazyS.StateT s m a q u (LazyS.StateT b) = LazyS.StateT (u . b) uninterruptibleMask a = LazyS.StateT $ \s -> uninterruptibleMask $ \u -> LazyS.runStateT (a $ q u) s where q :: (m (a, s) -> m (a, s)) -> LazyS.StateT s m a -> LazyS.StateT s m a q u (LazyS.StateT b) = LazyS.StateT (u . b) generalBracket acquire release use = LazyS.StateT $ \s0 -> do -- This implementation is given as an example in the documentation of -- 'generalBracket', so when changing it, remember to update the -- documentation's copy as well ((b, _s2), (c, s3)) <- generalBracket (LazyS.runStateT acquire s0) (\(resource, s1) exitCase -> case exitCase of ExitCaseSuccess (b, s2) -> LazyS.runStateT (release resource (ExitCaseSuccess b)) s2 -- In the two other cases, the base monad overrides @use@'s state -- changes and the state reverts to @s1@. ExitCaseException e -> LazyS.runStateT (release resource (ExitCaseException e)) s1 ExitCaseAbort -> LazyS.runStateT (release resource ExitCaseAbort) s1) (\(resource, s1) -> LazyS.runStateT (use resource) s1) return ((b, c), s3) 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 :: (m (a, s) -> m (a, s)) -> StrictS.StateT s m a -> StrictS.StateT s m a q u (StrictS.StateT b) = StrictS.StateT (u . b) uninterruptibleMask a = StrictS.StateT $ \s -> uninterruptibleMask $ \u -> StrictS.runStateT (a $ q u) s where q :: (m (a, s) -> m (a, s)) -> StrictS.StateT s m a -> StrictS.StateT s m a q u (StrictS.StateT b) = StrictS.StateT (u . b) generalBracket acquire release use = StrictS.StateT $ \s0 -> do ((b, _s2), (c, s3)) <- generalBracket (StrictS.runStateT acquire s0) (\(resource, s1) exitCase -> case exitCase of ExitCaseSuccess (b, s2) -> StrictS.runStateT (release resource (ExitCaseSuccess b)) s2 -- In the two other cases, the base monad overrides @use@'s state -- changes and the state reverts to @s1@. ExitCaseException e -> StrictS.runStateT (release resource (ExitCaseException e)) s1 ExitCaseAbort -> StrictS.runStateT (release resource ExitCaseAbort) s1) (\(resource, s1) -> StrictS.runStateT (use resource) s1) return ((b, c), s3) 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 :: (m a -> m a) -> ReaderT e m a -> ReaderT e m a q u (ReaderT b) = ReaderT (u . b) uninterruptibleMask a = ReaderT $ \e -> uninterruptibleMask $ \u -> runReaderT (a $ q u) e where q :: (m a -> m a) -> ReaderT e m a -> ReaderT e m a q u (ReaderT b) = ReaderT (u . b) generalBracket acquire release use = ReaderT $ \r -> generalBracket (runReaderT acquire r) (\resource exitCase -> runReaderT (release resource exitCase) r) (\resource -> runReaderT (use resource) r) 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 :: (m (a, w) -> m (a, w)) -> StrictW.WriterT w m a -> StrictW.WriterT w m a q u b = StrictW.WriterT $ u (StrictW.runWriterT b) uninterruptibleMask a = StrictW.WriterT $ uninterruptibleMask $ \u -> StrictW.runWriterT (a $ q u) where q :: (m (a, w) -> m (a, w)) -> StrictW.WriterT w m a -> StrictW.WriterT w m a q u b = StrictW.WriterT $ u (StrictW.runWriterT b) generalBracket acquire release use = StrictW.WriterT $ do ((b, _w12), (c, w123)) <- generalBracket (StrictW.runWriterT acquire) (\(resource, w1) exitCase -> case exitCase of ExitCaseSuccess (b, w12) -> do (c, w3) <- StrictW.runWriterT (release resource (ExitCaseSuccess b)) return (c, mappend w12 w3) -- In the two other cases, the base monad overrides @use@'s state -- changes and the state reverts to @w1@. ExitCaseException e -> do (c, w3) <- StrictW.runWriterT (release resource (ExitCaseException e)) return (c, mappend w1 w3) ExitCaseAbort -> do (c, w3) <- StrictW.runWriterT (release resource ExitCaseAbort) return (c, mappend w1 w3)) (\(resource, w1) -> do (a, w2) <- StrictW.runWriterT (use resource) return (a, mappend w1 w2)) return ((b, c), w123) 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 :: (m (a, w) -> m (a, w)) -> LazyW.WriterT w m a -> LazyW.WriterT w m a q u b = LazyW.WriterT $ u (LazyW.runWriterT b) uninterruptibleMask a = LazyW.WriterT $ uninterruptibleMask $ \u -> LazyW.runWriterT (a $ q u) where q :: (m (a, w) -> m (a, w)) -> LazyW.WriterT w m a -> LazyW.WriterT w m a q u b = LazyW.WriterT $ u (LazyW.runWriterT b) generalBracket acquire release use = LazyW.WriterT $ do ((b, _w12), (c, w123)) <- generalBracket (LazyW.runWriterT acquire) (\(resource, w1) exitCase -> case exitCase of ExitCaseSuccess (b, w12) -> do (c, w3) <- LazyW.runWriterT (release resource (ExitCaseSuccess b)) return (c, mappend w12 w3) -- In the two other cases, the base monad overrides @use@'s state -- changes and the state reverts to @w1@. ExitCaseException e -> do (c, w3) <- LazyW.runWriterT (release resource (ExitCaseException e)) return (c, mappend w1 w3) ExitCaseAbort -> do (c, w3) <- LazyW.runWriterT (release resource ExitCaseAbort) return (c, mappend w1 w3)) (\(resource, w1) -> do (a, w2) <- LazyW.runWriterT (use resource) return (a, mappend w1 w2)) return ((b, c), w123) 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 :: (m (a, s, w) -> m (a, s, w)) -> LazyRWS.RWST r w s m a -> LazyRWS.RWST r w s m a 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 :: (m (a, s, w) -> m (a, s, w)) -> LazyRWS.RWST r w s m a -> LazyRWS.RWST r w s m a q u (LazyRWS.RWST b) = LazyRWS.RWST $ \ r s -> u (b r s) generalBracket acquire release use = LazyRWS.RWST $ \r s0 -> do ((b, _s2, _w12), (c, s3, w123)) <- generalBracket (LazyRWS.runRWST acquire r s0) (\(resource, s1, w1) exitCase -> case exitCase of ExitCaseSuccess (b, s2, w12) -> do (c, s3, w3) <- LazyRWS.runRWST (release resource (ExitCaseSuccess b)) r s2 return (c, s3, mappend w12 w3) -- In the two other cases, the base monad overrides @use@'s state -- changes and the state reverts to @s1@ and @w1@. ExitCaseException e -> do (c, s3, w3) <- LazyRWS.runRWST (release resource (ExitCaseException e)) r s1 return (c, s3, mappend w1 w3) ExitCaseAbort -> do (c, s3, w3) <- LazyRWS.runRWST (release resource ExitCaseAbort) r s1 return (c, s3, mappend w1 w3)) (\(resource, s1, w1) -> do (a, s2, w2) <- LazyRWS.runRWST (use resource) r s1 return (a, s2, mappend w1 w2)) return ((b, c), s3, w123) 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 :: (m (a, s, w) -> m (a, s, w)) -> StrictRWS.RWST r w s m a -> StrictRWS.RWST r w s m a 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 :: (m (a, s, w) -> m (a, s, w)) -> StrictRWS.RWST r w s m a -> StrictRWS.RWST r w s m a q u (StrictRWS.RWST b) = StrictRWS.RWST $ \ r s -> u (b r s) generalBracket acquire release use = StrictRWS.RWST $ \r s0 -> do ((b, _s2, _w12), (c, s3, w123)) <- generalBracket (StrictRWS.runRWST acquire r s0) (\(resource, s1, w1) exitCase -> case exitCase of ExitCaseSuccess (b, s2, w12) -> do (c, s3, w3) <- StrictRWS.runRWST (release resource (ExitCaseSuccess b)) r s2 return (c, s3, mappend w12 w3) -- In the two other cases, the base monad overrides @use@'s state -- changes and the state reverts to @s1@ and @w1@. ExitCaseException e -> do (c, s3, w3) <- StrictRWS.runRWST (release resource (ExitCaseException e)) r s1 return (c, s3, mappend w1 w3) ExitCaseAbort -> do (c, s3, w3) <- StrictRWS.runRWST (release resource ExitCaseAbort) r s1 return (c, s3, mappend w1 w3)) (\(resource, s1, w1) -> do (a, s2, w2) <- StrictRWS.runRWST (use resource) r s1 return (a, s2, mappend w1 w2)) return ((b, c), s3, w123) -- 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) -- | @since 0.10.0 instance MonadMask m => MonadMask (MaybeT m) where mask f = MaybeT $ mask $ \u -> runMaybeT $ f (q u) where q :: (m (Maybe a) -> m (Maybe a)) -> MaybeT m a -> MaybeT m a q u (MaybeT b) = MaybeT (u b) uninterruptibleMask f = MaybeT $ uninterruptibleMask $ \u -> runMaybeT $ f (q u) where q :: (m (Maybe a) -> m (Maybe a)) -> MaybeT m a -> MaybeT m a q u (MaybeT b) = MaybeT (u b) generalBracket acquire release use = MaybeT $ do (eb, ec) <- generalBracket (runMaybeT acquire) (\resourceMay exitCase -> case resourceMay of Nothing -> return Nothing -- nothing to release, acquire didn't succeed Just resource -> case exitCase of ExitCaseSuccess (Just b) -> runMaybeT (release resource (ExitCaseSuccess b)) ExitCaseException e -> runMaybeT (release resource (ExitCaseException e)) _ -> runMaybeT (release resource ExitCaseAbort)) (\resourceMay -> case resourceMay of Nothing -> return Nothing Just resource -> runMaybeT (use resource)) -- The order in which we perform those two 'Maybe' effects doesn't matter, -- since the error message is the same regardless. return ((,) <$> eb <*> ec) -- | 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) instance (Error e, MonadMask m) => MonadMask (ErrorT e m) where mask f = ErrorT $ mask $ \u -> runErrorT $ f (q u) where q :: (m (Either e a) -> m (Either e a)) -> ErrorT e m a -> ErrorT e m a q u (ErrorT b) = ErrorT (u b) uninterruptibleMask f = ErrorT $ uninterruptibleMask $ \u -> runErrorT $ f (q u) where q :: (m (Either e a) -> m (Either e a)) -> ErrorT e m a -> ErrorT e m a q u (ErrorT b) = ErrorT (u b) generalBracket acquire release use = ErrorT $ do (eb, ec) <- generalBracket (runErrorT acquire) (\eresource exitCase -> case eresource of Left e -> return (Left e) -- nothing to release, acquire didn't succeed Right resource -> case exitCase of ExitCaseSuccess (Right b) -> runErrorT (release resource (ExitCaseSuccess b)) ExitCaseException e -> runErrorT (release resource (ExitCaseException e)) _ -> runErrorT (release resource ExitCaseAbort)) (either (return . Left) (runErrorT . use)) return $ do -- The order in which we perform those two 'Either' effects determines -- which error will win if they are both 'Left's. We want the error from -- 'release' to win. c <- ec b <- eb return (b, c) -- | 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) -- | @since 0.9.0 instance MonadMask m => MonadMask (ExceptT e m) where mask f = ExceptT $ mask $ \u -> runExceptT $ f (q u) where q :: (m (Either e a) -> m (Either e a)) -> ExceptT e m a -> ExceptT e m a q u (ExceptT b) = ExceptT (u b) uninterruptibleMask f = ExceptT $ uninterruptibleMask $ \u -> runExceptT $ f (q u) where q :: (m (Either e a) -> m (Either e a)) -> ExceptT e m a -> ExceptT e m a q u (ExceptT b) = ExceptT (u b) generalBracket acquire release use = ExceptT $ do -- This implementation is given as an example in the documentation of -- 'generalBracket', so when changing it, remember to update the -- documentation's copy as well (eb, ec) <- generalBracket (runExceptT acquire) (\eresource exitCase -> case eresource of Left e -> return (Left e) -- nothing to release, acquire didn't succeed Right resource -> case exitCase of ExitCaseSuccess (Right b) -> runExceptT (release resource (ExitCaseSuccess b)) ExitCaseException e -> runExceptT (release resource (ExitCaseException e)) _ -> runExceptT (release resource ExitCaseAbort)) (either (return . Left) (runExceptT . use)) return $ do -- The order in which we perform those two 'Either' effects determines -- which error will win if they are both 'Left's. We want the error from -- 'release' to win. c <- ec b <- eb return (b, c) 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. -- -- /NOTE/ This catches all /exceptions/, but if the monad supports other ways of -- aborting the computation, those other kinds of errors will not be caught. 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. -- -- /NOTE/ The action is only run if an /exception/ is thrown. If the monad -- supports other ways of aborting the computation, the action won't run if -- those other kinds of errors are thrown. See 'onError'. onException :: MonadCatch m => m a -> m b -> m a onException action handler = action `catchAll` \e -> handler >> throwM e -- | Run an action only if an error is thrown in the main action. Unlike -- 'onException', this works with every kind of error, not just exceptions. For -- example, if @f@ is an 'ExceptT' computation which aborts with a 'Left', the -- computation @onError f g@ will execute @g@, while @onException f g@ will not. -- -- This distinction is only meaningful for monads which have multiple exit -- points, such as 'Except' and 'MaybeT'. For monads that only have a single -- exit point, there is no difference between 'onException' and 'onError', -- except that 'onError' has a more constrained type. -- -- @since 0.10.0 onError :: MonadMask m => m a -> m b -> m a onError action handler = bracketOnError (return ()) (const handler) (const action) -- | Generalized abstracted pattern of safe resource acquisition and release -- in the face of errors. 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 error is thrown during the use, the release still happens before the -- error is rethrown. -- -- Note that this is essentially a type-specialized version of -- 'generalBracket'. This function has a more common signature (matching the -- signature from "Control.Exception"), and is often more convenient to use. By -- contrast, 'generalBracket' is more expressive, allowing us to implement -- other functions like 'bracketOnError'. bracket :: MonadMask m => m a -> (a -> m c) -> (a -> m b) -> m b bracket acquire release = liftM fst . generalBracket acquire (\a _exitCase -> release a) -- | Version of 'bracket' without any value being passed to the second and -- third actions. bracket_ :: MonadMask m => m a -> m c -> m b -> m b bracket_ before after action = bracket before (const after) (const action) -- | Perform an action with a finalizer action that is run, even if an -- error 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 an error is -- thrown by the in-between computation. bracketOnError :: MonadMask m => m a -> (a -> m c) -> (a -> m b) -> m b bracketOnError acquire release = liftM fst . generalBracket acquire (\a exitCase -> case exitCase of ExitCaseSuccess _ -> return () _ -> do _ <- release a return ())