Portability | non-portable |
---|---|

Stability | experimental |

Maintainer | Edward Kmett <ekmett@gmail.com> |

Safe Haskell | Trustworthy |

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.

- class Monad m => MonadThrow m where
- class MonadThrow m => MonadCatch m where
- 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

- 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 = forall e . 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
- bracket :: MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c
- bracket_ :: MonadMask m => m a -> m b -> m c -> m c
- finally :: MonadMask m => m a -> m b -> m a
- bracketOnError :: MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c
- class (Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e

- data SomeException where
- SomeException :: Exception e => e -> SomeException

# Typeclass

The mtl style typeclass

class Monad m => MonadThrow m whereSource

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.

throwM :: Exception e => e -> m aSource

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

MonadThrow [] | |

MonadThrow IO | |

MonadThrow Maybe | |

~ * e SomeException => MonadThrow (Either e) | |

MonadThrow m => MonadThrow (ListT m) | |

MonadThrow m => MonadThrow (MaybeT m) | Throws exceptions into the base monad. |

MonadThrow m => MonadThrow (IdentityT m) | |

Monad m => MonadThrow (CatchT m) | |

MonadThrow m => MonadThrow (ContT r m) | |

(Error e, MonadThrow m) => MonadThrow (ErrorT e m) | Throws exceptions into the base monad. |

MonadThrow m => MonadThrow (ReaderT r m) | |

MonadThrow m => MonadThrow (StateT s m) | |

MonadThrow m => MonadThrow (StateT s m) | |

(MonadThrow m, Monoid w) => MonadThrow (WriterT w m) | |

(MonadThrow m, Monoid w) => MonadThrow (WriterT w m) | |

(MonadThrow m, Monoid w) => MonadThrow (RWST r w s m) | |

(MonadThrow m, Monoid w) => MonadThrow (RWST r w s m) |

class MonadThrow m => MonadCatch m whereSource

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`

.

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

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`

.

MonadCatch IO | |

MonadCatch m => MonadCatch (ListT m) | |

MonadCatch m => MonadCatch (MaybeT m) | Catches exceptions from the base monad. |

MonadCatch m => MonadCatch (IdentityT m) | |

Monad m => MonadCatch (CatchT m) | |

(Error e, MonadCatch m) => MonadCatch (ErrorT e m) | Catches exceptions from the base monad. |

MonadCatch m => MonadCatch (ReaderT r m) | |

MonadCatch m => MonadCatch (StateT s m) | |

MonadCatch m => MonadCatch (StateT s m) | |

(MonadCatch m, Monoid w) => MonadCatch (WriterT w m) | |

(MonadCatch m, Monoid w) => MonadCatch (WriterT w m) | |

(MonadCatch m, Monoid w) => MonadCatch (RWST r w s m) | |

(MonadCatch m, Monoid w) => MonadCatch (RWST r w s m) |

class MonadCatch m => MonadMask m whereSource

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.

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

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 bSource

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.

MonadMask IO | |

MonadMask m => MonadMask (IdentityT m) | |

Monad m => MonadMask (CatchT m) | Note: This instance is only valid if the underlying monad has a single exit point! |

MonadMask m => MonadMask (ReaderT r m) | |

MonadMask m => MonadMask (StateT s m) | |

MonadMask m => MonadMask (StateT s m) | |

(MonadMask m, Monoid w) => MonadMask (WriterT w m) | |

(MonadMask m, Monoid w) => MonadMask (WriterT w m) | |

(MonadMask m, Monoid w) => MonadMask (RWST r w s m) | |

(MonadMask m, Monoid w) => MonadMask (RWST r w s m) |

# 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 aSource

Like `mask`

, but does not pass a `restore`

action to the argument.

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

Like `uninterruptibleMask`

, but does not pass a `restore`

action to the
argument.

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

Catches all exceptions, and somewhat defeats the purpose of the extensible exception system. Use sparingly.

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

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 aSource

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 aSource

Catch exceptions only if they pass some predicate. Often useful with the
predicates for testing `IOError`

values in System.IO.Error.

Generalized version of `Handler`

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

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

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

Flipped `catch`

. See Control.Exception's `handle`

.

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

Flipped `catchAll`

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

Flipped `catchIOError`

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

Flipped `catchJust`

. See Control.Exception's `handleJust`

.

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

Flipped `catchIf`

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

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)Source

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 aSource

Run an action only if an exception is thrown in the main action. The exception is not caught, simply rethrown.

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

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 -> m b -> m c -> m cSource

Version of `bracket`

without any value being passed to the second and
third actions.

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

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

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

Like `bracket`

, but only performs the final action if there was an
exception raised 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, Typeable) 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 deriving Typeable 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 deriving Typeable 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 (Typeable, 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

toException :: e -> SomeException

fromException :: SomeException -> Maybe e

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`

.

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