Control.Monad.Catch

exceptions-0.10.0: Extensible optionally-pure exceptions

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
Safe Haskell | Trustworthy
Language | Haskell98

Control.Monad.Catch

Contents

- Typeclass
- Utilities
- Re-exports from Control.Exception

Description

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.

Synopsis

- class Monad m => MonadThrow m where
  - throwM :: Exception e => e -> m a
- class MonadThrow m => MonadCatch m where
  - catch :: Exception e => m a -> (e -> m a) -> m a
- class MonadCatch m => MonadMask m where
  - mask :: ((forall a. m a -> m a) -> m b) -> m b
  - uninterruptibleMask :: ((forall a. m a -> m a) -> m b) -> m b
  - generalBracket :: m a -> (a -> ExitCase b -> m c) -> (a -> m b) -> m (b, c)
- data ExitCase a
  - = ExitCaseSuccess a
  - | ExitCaseException SomeException
  - | ExitCaseAbort
- mask_ :: MonadMask m => m a -> m a
- uninterruptibleMask_ :: MonadMask m => m a -> m a
- catchAll :: MonadCatch m => m a -> (SomeException -> m a) -> m a
- catchIOError :: MonadCatch m => m a -> (IOError -> m a) -> m a
- catchJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> (b -> m a) -> m a
- catchIf :: (MonadCatch m, Exception e) => (e -> Bool) -> m a -> (e -> m a) -> m a
- data Handler m a = Exception e => Handler (e -> m a)
- catches :: (Foldable f, MonadCatch m) => m a -> f (Handler m a) -> m a
- handle :: (MonadCatch m, Exception e) => (e -> m a) -> m a -> m a
- handleAll :: MonadCatch m => (SomeException -> m a) -> m a -> m a
- handleIOError :: MonadCatch m => (IOError -> m a) -> m a -> m a
- handleJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> (b -> m a) -> m a -> m a
- handleIf :: (MonadCatch m, Exception e) => (e -> Bool) -> (e -> m a) -> m a -> m a
- try :: (MonadCatch m, Exception e) => m a -> m (Either e a)
- tryJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> m (Either b a)
- onException :: MonadCatch m => m a -> m b -> m a
- onError :: MonadMask m => m a -> m b -> m a
- bracket :: MonadMask m => m a -> (a -> m c) -> (a -> m b) -> m b
- bracket_ :: MonadMask m => m a -> m c -> m b -> m b
- finally :: MonadMask m => m a -> m b -> m a
- bracketOnError :: MonadMask m => m a -> (a -> m c) -> (a -> m b) -> m b
- class (Typeable e, Show e) => Exception e where
  - toException :: e -> SomeException
  - fromException :: SomeException -> Maybe e
  - displayException :: e -> String
- data SomeException where
  - SomeException :: forall e. Exception e => e -> SomeException

# Typeclass

The mtl style typeclass

class Monad m => MonadThrow m where #

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.

Methods

throwM :: Exception e => e -> m a #

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 `throwIO`.

Should satisfy the law:

    throwM e >> f = throwM e

Instances

MonadThrow [] # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> [a] #
MonadThrow Maybe # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> Maybe a #
MonadThrow IO # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> IO a #
MonadThrow Q # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> Q a #
MonadThrow STM # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> STM a #
e ~ SomeException => MonadThrow (Either e) # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e0 => e0 -> Either e a #
MonadThrow m => MonadThrow (ListT m) # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> ListT m a #
MonadThrow m => MonadThrow (MaybeT m) # | Throws exceptions into the base monad.
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> MaybeT m a #
Monad m => MonadThrow (CatchT m) # |
Instance details Defined in Control.Monad.Catch.Pure Methods throwM :: Exception e => e -> CatchT m a #
MonadThrow m => MonadThrow (IdentityT m) # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> IdentityT m a #
(Error e, MonadThrow m) => MonadThrow (ErrorT e m) # | Throws exceptions into the base monad.
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e0 => e0 -> ErrorT e m a #
MonadThrow m => MonadThrow (ExceptT e m) # | Throws exceptions into the base monad.
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e0 => e0 -> ExceptT e m a #
MonadThrow m => MonadThrow (ReaderT r m) # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> ReaderT r m a #
MonadThrow m => MonadThrow (StateT s m) # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> StateT s m a #
MonadThrow m => MonadThrow (StateT s m) # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> StateT s m a #
(MonadThrow m, Monoid w) => MonadThrow (WriterT w m) # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> WriterT w m a #
(MonadThrow m, Monoid w) => MonadThrow (WriterT w m) # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> WriterT w m a #
MonadThrow m => MonadThrow (ContT r m) # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> ContT r m a #
(MonadThrow m, Monoid w) => MonadThrow (RWST r w s m) # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> RWST r w s m a #
(MonadThrow m, Monoid w) => MonadThrow (RWST r w s m) # |
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> RWST r w s m a #

class MonadThrow m => MonadCatch m where #

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`.

Methods

catch :: Exception e => m a -> (e -> m a) -> m a #

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 `catch`.

Instances

MonadCatch IO # |
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => IO a -> (e -> IO a) -> IO a #
MonadCatch STM # |
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => STM a -> (e -> STM a) -> STM a #
e ~ SomeException => MonadCatch (Either e) # | Since: 0.8.3
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e0 => Either e a -> (e0 -> Either e a) -> Either e a #
MonadCatch m => MonadCatch (ListT m) # |
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => ListT m a -> (e -> ListT m a) -> ListT m a #
MonadCatch m => MonadCatch (MaybeT m) # | Catches exceptions from the base monad.
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => MaybeT m a -> (e -> MaybeT m a) -> MaybeT m a #
Monad m => MonadCatch (CatchT m) # |
Instance details Defined in Control.Monad.Catch.Pure Methods catch :: Exception e => CatchT m a -> (e -> CatchT m a) -> CatchT m a #
MonadCatch m => MonadCatch (IdentityT m) # |
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => IdentityT m a -> (e -> IdentityT m a) -> IdentityT m a #
(Error e, MonadCatch m) => MonadCatch (ErrorT e m) # | Catches exceptions from the base monad.
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e0 => ErrorT e m a -> (e0 -> ErrorT e m a) -> ErrorT e m a #
MonadCatch m => MonadCatch (ExceptT e m) # | Catches exceptions from the base monad.
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e0 => ExceptT e m a -> (e0 -> ExceptT e m a) -> ExceptT e m a #
MonadCatch m => MonadCatch (ReaderT r m) # |
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => ReaderT r m a -> (e -> ReaderT r m a) -> ReaderT r m a #
MonadCatch m => MonadCatch (StateT s m) # |
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => StateT s m a -> (e -> StateT s m a) -> StateT s m a #
MonadCatch m => MonadCatch (StateT s m) # |
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => StateT s m a -> (e -> StateT s m a) -> StateT s m a #
(MonadCatch m, Monoid w) => MonadCatch (WriterT w m) # |
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a #
(MonadCatch m, Monoid w) => MonadCatch (WriterT w m) # |
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a #
(MonadCatch m, Monoid w) => MonadCatch (RWST r w s m) # |
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => RWST r w s m a -> (e -> RWST r w s m a) -> RWST r w s m a #
(MonadCatch m, Monoid w) => MonadCatch (RWST r w s m) # |
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => RWST r w s m a -> (e -> RWST r w s m a) -> RWST r w s m a #

class MonadCatch m => MonadMask m where #

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, 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.

Methods

mask :: ((forall a. m a -> m a) -> m b) -> m b #

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 `mask`.

uninterruptibleMask :: ((forall a. m a -> m a) -> m b) -> m b #

Like `mask`, but the masked computation is not interruptible (see Control.Exception's `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.

generalBracket #

Arguments

:: 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 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 Lefts. We want the error from
        -- release to win.
        c <- ec
        b <- eb
        return (b, c)

Since: 0.9.0

Instances

MonadMask IO # |
Instance details Defined in Control.Monad.Catch Methods mask :: ((forall a. IO a -> IO a) -> IO b) -> IO b # uninterruptibleMask :: ((forall a. IO a -> IO a) -> IO b) -> IO b # generalBracket :: IO a -> (a -> ExitCase b -> IO c) -> (a -> IO b) -> IO (b, c) #
e ~ SomeException => MonadMask (Either e) # | Since: 0.8.3
Instance details Defined in Control.Monad.Catch Methods mask :: ((forall a. Either e a -> Either e a) -> Either e b) -> Either e b # uninterruptibleMask :: ((forall a. Either e a -> Either e a) -> Either e b) -> Either e b # generalBracket :: Either e a -> (a -> ExitCase b -> Either e c) -> (a -> Either e b) -> Either e (b, c) #
MonadMask m => MonadMask (MaybeT m) # | Since: 0.10.0
Instance details Defined in Control.Monad.Catch Methods mask :: ((forall a. MaybeT m a -> MaybeT m a) -> MaybeT m b) -> MaybeT m b # uninterruptibleMask :: ((forall a. MaybeT m a -> MaybeT m a) -> MaybeT m b) -> MaybeT m b # generalBracket :: MaybeT m a -> (a -> ExitCase b -> MaybeT m c) -> (a -> MaybeT m b) -> MaybeT m (b, c) #
Monad m => MonadMask (CatchT m) # | Note: This instance is only valid if the underlying monad has a single exit point! For example, `IO` or `Either` would be invalid base monads, but `Reader` or `State` would be acceptable.
Instance details Defined in Control.Monad.Catch.Pure Methods mask :: ((forall a. CatchT m a -> CatchT m a) -> CatchT m b) -> CatchT m b # uninterruptibleMask :: ((forall a. CatchT m a -> CatchT m a) -> CatchT m b) -> CatchT m b # generalBracket :: CatchT m a -> (a -> ExitCase b -> CatchT m c) -> (a -> CatchT m b) -> CatchT m (b, c) #
MonadMask m => MonadMask (IdentityT m) # |
Instance details Defined in Control.Monad.Catch Methods mask :: ((forall a. IdentityT m a -> IdentityT m a) -> IdentityT m b) -> IdentityT m b # uninterruptibleMask :: ((forall a. IdentityT m a -> IdentityT m a) -> IdentityT m b) -> IdentityT m b # generalBracket :: IdentityT m a -> (a -> ExitCase b -> IdentityT m c) -> (a -> IdentityT m b) -> IdentityT m (b, c) #
(Error e, MonadMask m) => MonadMask (ErrorT e m) # |
Instance details Defined in Control.Monad.Catch Methods mask :: ((forall a. ErrorT e m a -> ErrorT e m a) -> ErrorT e m b) -> ErrorT e m b # uninterruptibleMask :: ((forall a. ErrorT e m a -> ErrorT e m a) -> ErrorT e m b) -> ErrorT e m b # generalBracket :: ErrorT e m a -> (a -> ExitCase b -> ErrorT e m c) -> (a -> ErrorT e m b) -> ErrorT e m (b, c) #
MonadMask m => MonadMask (ExceptT e m) # | Since: 0.9.0
Instance details Defined in Control.Monad.Catch Methods mask :: ((forall a. ExceptT e m a -> ExceptT e m a) -> ExceptT e m b) -> ExceptT e m b # uninterruptibleMask :: ((forall a. ExceptT e m a -> ExceptT e m a) -> ExceptT e m b) -> ExceptT e m b # generalBracket :: ExceptT e m a -> (a -> ExitCase b -> ExceptT e m c) -> (a -> ExceptT e m b) -> ExceptT e m (b, c) #
MonadMask m => MonadMask (ReaderT r m) # |
Instance details Defined in Control.Monad.Catch Methods mask :: ((forall a. ReaderT r m a -> ReaderT r m a) -> ReaderT r m b) -> ReaderT r m b # uninterruptibleMask :: ((forall a. ReaderT r m a -> ReaderT r m a) -> ReaderT r m b) -> ReaderT r m b # generalBracket :: ReaderT r m a -> (a -> ExitCase b -> ReaderT r m c) -> (a -> ReaderT r m b) -> ReaderT r m (b, c) #
MonadMask m => MonadMask (StateT s m) # |
Instance details Defined in Control.Monad.Catch Methods mask :: ((forall a. StateT s m a -> StateT s m a) -> StateT s m b) -> StateT s m b # uninterruptibleMask :: ((forall a. StateT s m a -> StateT s m a) -> StateT s m b) -> StateT s m b # generalBracket :: StateT s m a -> (a -> ExitCase b -> StateT s m c) -> (a -> StateT s m b) -> StateT s m (b, c) #
MonadMask m => MonadMask (StateT s m) # |
Instance details Defined in Control.Monad.Catch Methods mask :: ((forall a. StateT s m a -> StateT s m a) -> StateT s m b) -> StateT s m b # uninterruptibleMask :: ((forall a. StateT s m a -> StateT s m a) -> StateT s m b) -> StateT s m b # generalBracket :: StateT s m a -> (a -> ExitCase b -> StateT s m c) -> (a -> StateT s m b) -> StateT s m (b, c) #
(MonadMask m, Monoid w) => MonadMask (WriterT w m) # |
Instance details Defined in Control.Monad.Catch Methods mask :: ((forall a. WriterT w m a -> WriterT w m a) -> WriterT w m b) -> WriterT w m b # uninterruptibleMask :: ((forall a. WriterT w m a -> WriterT w m a) -> WriterT w m b) -> WriterT w m b # generalBracket :: WriterT w m a -> (a -> ExitCase b -> WriterT w m c) -> (a -> WriterT w m b) -> WriterT w m (b, c) #
(MonadMask m, Monoid w) => MonadMask (WriterT w m) # |
Instance details Defined in Control.Monad.Catch Methods mask :: ((forall a. WriterT w m a -> WriterT w m a) -> WriterT w m b) -> WriterT w m b # uninterruptibleMask :: ((forall a. WriterT w m a -> WriterT w m a) -> WriterT w m b) -> WriterT w m b # generalBracket :: WriterT w m a -> (a -> ExitCase b -> WriterT w m c) -> (a -> WriterT w m b) -> WriterT w m (b, c) #
(MonadMask m, Monoid w) => MonadMask (RWST r w s m) # |
Instance details Defined in Control.Monad.Catch Methods mask :: ((forall a. RWST r w s m a -> RWST r w s m a) -> RWST r w s m b) -> RWST r w s m b # uninterruptibleMask :: ((forall a. RWST r w s m a -> RWST r w s m a) -> RWST r w s m b) -> RWST r w s m b # generalBracket :: RWST r w s m a -> (a -> ExitCase b -> RWST r w s m c) -> (a -> RWST r w s m b) -> RWST r w s m (b, c) #
(MonadMask m, Monoid w) => MonadMask (RWST r w s m) # |
Instance details Defined in Control.Monad.Catch Methods mask :: ((forall a. RWST r w s m a -> RWST r w s m a) -> RWST r w s m b) -> RWST r w s m b # uninterruptibleMask :: ((forall a. RWST r w s m a -> RWST r w s m a) -> RWST r w s m b) -> RWST r w s m b # generalBracket :: RWST r w s m a -> (a -> ExitCase b -> RWST r w s m c) -> (a -> RWST r w s m b) -> RWST r w s m (b, c) #

data ExitCase a #

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 `throwE` to abort with a value of type `e` (`ExitCaseAbort`).

Constructors

ExitCaseSuccess a |
ExitCaseException SomeException |
ExitCaseAbort |

Instances

Show a => Show (ExitCase a) # |
Instance details Defined in Control.Monad.Catch Methods showsPrec :: Int -> ExitCase a -> ShowS # show :: ExitCase a -> String # showList :: [ExitCase a] -> ShowS #

# 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.

mask_ :: MonadMask m => m a -> m a #

Like `mask`, but does not pass a `restore` action to the argument.

uninterruptibleMask_ :: MonadMask m => m a -> m a #

Like `uninterruptibleMask`, but does not pass a `restore` action to the argument.

catchAll :: MonadCatch m => m a -> (SomeException -> m a) -> m a #

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.

catchIOError :: MonadCatch m => m a -> (IOError -> m a) -> m a #

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.

catchJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> (b -> m a) -> m a #

A more generalized way of determining which exceptions to catch at run time.

catchIf :: (MonadCatch m, Exception e) => (e -> Bool) -> m a -> (e -> m a) -> m a #

Catch exceptions only if they pass some predicate. Often useful with the predicates for testing `IOError` values in System.IO.Error.

data Handler m a #

Generalized version of `Handler`

Constructors

Exception e => Handler (e -> m a) |

Instances

Monad m => Functor (Handler m) # |
Instance details Defined in Control.Monad.Catch Methods fmap :: (a -> b) -> Handler m a -> Handler m b # (<$) :: a -> Handler m b -> Handler m a #

catches :: (Foldable f, MonadCatch m) => m a -> f (Handler m a) -> m a #

Catches different sorts of exceptions. See Control.Exception's `catches`

handle :: (MonadCatch m, Exception e) => (e -> m a) -> m a -> m a #

Flipped `catch`. See Control.Exception's `handle`.

handleAll :: MonadCatch m => (SomeException -> m a) -> m a -> m a #

Flipped `catchAll`

handleIOError :: MonadCatch m => (IOError -> m a) -> m a -> m a #

Flipped `catchIOError`

handleJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> (b -> m a) -> m a -> m a #

Flipped `catchJust`. See Control.Exception's `handleJust`.

handleIf :: (MonadCatch m, Exception e) => (e -> Bool) -> (e -> m a) -> m a -> m a #

Flipped `catchIf`

try :: (MonadCatch m, Exception e) => m a -> m (Either e a) #

Similar to `catch`, but returns an `Either` result. See Control.Exception's `try`.

tryJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> m (Either b a) #

A variant of `try` that takes an exception predicate to select which exceptions are caught. See Control.Exception's `tryJust`

onException :: MonadCatch m => m a -> m b -> m a #

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`.

onError :: MonadMask m => m a -> m b -> m a #

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

bracket :: MonadMask m => m a -> (a -> m c) -> (a -> m b) -> m b #

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 -> m c -> m b -> m b #

Version of `bracket` without any value being passed to the second and third actions.

finally :: MonadMask m => m a -> m b -> m a #

Perform an action with a finalizer action that is run, even if an error occurs.

bracketOnError :: MonadMask m => m a -> (a -> m c) -> (a -> m b) -> m b #

Like `bracket`, but only performs the final action if an error is thrown by the in-between computation.

# Re-exports from Control.Exception

class (Typeable e, Show e) => Exception e where #

Any type that you wish to throw or catch as an exception must be an instance of the `Exception` class. The simplest case is a new exception type directly below the root:

    data MyException = ThisException | ThatException
        deriving Show

    instance Exception MyException

The default method definitions in the `Exception` class do what we need in this case. You can now throw and catch `ThisException` and `ThatException` as exceptions:

    *Main> throw ThisException `catch` \e -> putStrLn ("Caught " ++ show (e :: MyException))
    Caught ThisException

In more complicated examples, you may wish to define a whole hierarchy of exceptions:

    ---------------------------------------------------------------------
    -- Make the root exception type for all the exceptions in a compiler

    data SomeCompilerException = forall e . Exception e => SomeCompilerException e

    instance Show SomeCompilerException where
        show (SomeCompilerException e) = show e

    instance Exception SomeCompilerException

    compilerExceptionToException :: Exception e => e -> SomeException
    compilerExceptionToException = toException . SomeCompilerException

    compilerExceptionFromException :: Exception e => SomeException -> Maybe e
    compilerExceptionFromException x = do
        SomeCompilerException a <- fromException x
        cast a

    ---------------------------------------------------------------------
    -- Make a subhierarchy for exceptions in the frontend of the compiler

    data SomeFrontendException = forall e . Exception e => SomeFrontendException e

    instance Show SomeFrontendException where
        show (SomeFrontendException e) = show e

    instance Exception SomeFrontendException where
        toException = compilerExceptionToException
        fromException = compilerExceptionFromException

    frontendExceptionToException :: Exception e => e -> SomeException
    frontendExceptionToException = toException . SomeFrontendException

    frontendExceptionFromException :: Exception e => SomeException -> Maybe e
    frontendExceptionFromException x = do
        SomeFrontendException a <- fromException x
        cast a

    ---------------------------------------------------------------------
    -- Make an exception type for a particular frontend compiler exception

    data MismatchedParentheses = MismatchedParentheses
        deriving Show

    instance Exception MismatchedParentheses where
        toException   = frontendExceptionToException
        fromException = frontendExceptionFromException

We can now catch a `MismatchedParentheses` exception as `MismatchedParentheses`, `SomeFrontendException` or `SomeCompilerException`, but not other types, e.g. `IOException`:

    *Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses))
    Caught MismatchedParentheses
    *Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: SomeFrontendException))
    Caught MismatchedParentheses
    *Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: SomeCompilerException))
    Caught MismatchedParentheses
    *Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: IOException))
    *** Exception: MismatchedParentheses

Minimal complete definition

Nothing

Methods

toException :: e -> SomeException #

fromException :: SomeException -> Maybe e #

displayException :: e -> String #

Render this exception value in a human-friendly manner.

Default implementation: ``show``.

Since: base-4.8.0.0

Instances

Exception BlockedIndefinitelyOnMVar | Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: BlockedIndefinitelyOnMVar -> SomeException # fromException :: SomeException -> Maybe BlockedIndefinitelyOnMVar # displayException :: BlockedIndefinitelyOnMVar -> String #
Exception BlockedIndefinitelyOnSTM | Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: BlockedIndefinitelyOnSTM -> SomeException # fromException :: SomeException -> Maybe BlockedIndefinitelyOnSTM # displayException :: BlockedIndefinitelyOnSTM -> String #
Exception Deadlock | Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: Deadlock -> SomeException # fromException :: SomeException -> Maybe Deadlock # displayException :: Deadlock -> String #
Exception AllocationLimitExceeded | Since: base-4.8.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: AllocationLimitExceeded -> SomeException # fromException :: SomeException -> Maybe AllocationLimitExceeded # displayException :: AllocationLimitExceeded -> String #
Exception CompactionFailed | Since: base-4.10.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: CompactionFailed -> SomeException # fromException :: SomeException -> Maybe CompactionFailed # displayException :: CompactionFailed -> String #
Exception AssertionFailed | Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: AssertionFailed -> SomeException # fromException :: SomeException -> Maybe AssertionFailed # displayException :: AssertionFailed -> String #
Exception SomeAsyncException | Since: base-4.7.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: SomeAsyncException -> SomeException # fromException :: SomeException -> Maybe SomeAsyncException # displayException :: SomeAsyncException -> String #
Exception AsyncException | Since: base-4.7.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: AsyncException -> SomeException # fromException :: SomeException -> Maybe AsyncException # displayException :: AsyncException -> String #
Exception ArrayException | Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: ArrayException -> SomeException # fromException :: SomeException -> Maybe ArrayException # displayException :: ArrayException -> String #
Exception FixIOException | Since: base-4.11.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: FixIOException -> SomeException # fromException :: SomeException -> Maybe FixIOException # displayException :: FixIOException -> String #
Exception ExitCode | Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: ExitCode -> SomeException # fromException :: SomeException -> Maybe ExitCode # displayException :: ExitCode -> String #
Exception IOException | Since: base-4.1.0.0
Instance details Defined in GHC.IO.Exception Methods toException :: IOException -> SomeException # fromException :: SomeException -> Maybe IOException # displayException :: IOException -> String #
Exception ArithException | Since: base-4.0.0.0
Instance details Defined in GHC.Exception.Type Methods toException :: ArithException -> SomeException # fromException :: SomeException -> Maybe ArithException # displayException :: ArithException -> String #
Exception SomeException | Since: base-3.0
Instance details Defined in GHC.Exception.Type Methods toException :: SomeException -> SomeException # fromException :: SomeException -> Maybe SomeException # displayException :: SomeException -> String #

data SomeException where #

The `SomeException` type is the root of the exception type hierarchy. When an exception of type `e` is thrown, behind the scenes it is encapsulated in a `SomeException`.

Constructors

SomeException :: forall e. Exception e => e -> SomeException |

Instances

Show SomeException | Since: base-3.0
Instance details Defined in GHC.Exception.Type Methods showsPrec :: Int -> SomeException -> ShowS # show :: SomeException -> String # showList :: [SomeException] -> ShowS #
Exception SomeException | Since: base-3.0
Instance details Defined in GHC.Exception.Type Methods toException :: SomeException -> SomeException # fromException :: SomeException -> Maybe SomeException # displayException :: SomeException -> String #