| Safe Haskell | None |
|---|---|
| Language | Haskell98 |
System.Console.Haskeline.MonadException
Contents
Description
This module redefines some of the functions in Control.Exception to
work for more general monads built on top of IO.
- class MonadIO m => MonadException m where
- catch :: (MonadException m, Exception e) => m a -> (e -> m a) -> m a
- handle :: (MonadException m, Exception e) => (e -> m a) -> m a -> m a
- catches :: MonadException m => m a -> [Handler m a] -> m a
- data Handler m a = Exception e => Handler (e -> m a)
- finally :: MonadException m => m a -> m b -> m a
- throwIO :: (MonadIO m, Exception e) => e -> m a
- throwTo :: (MonadIO m, Exception e) => ThreadId -> e -> m ()
- bracket :: MonadException m => m a -> (a -> m b) -> (a -> m c) -> m c
- liftIOOp :: MonadException m => ((a -> IO (m b)) -> IO (m c)) -> (a -> m b) -> m c
- liftIOOp_ :: MonadException m => (IO (m a) -> IO (m a)) -> m a -> m a
- newtype RunIO m = RunIO (forall b. m b -> IO (m b))
- class (Typeable * e, Show e) => Exception e
- data SomeException :: * where
- data IOException :: *
The MonadException class
class MonadIO m => MonadException m where Source #
An instance of MonadException is generally made up of monad transformers
layered on top of the IO monad.
The controlIO method enables us to "lift" a function that manages IO actions (such
as bracket or catch) into a function that wraps arbitrary monadic actions.
Minimal complete definition
Instances
| MonadException IO Source # | |
| MonadException m => MonadException (MaybeT m) Source # | |
| MonadException m => MonadException (ListT m) Source # | |
| MonadException m => MonadException (InputT m) Source # | |
| (Monoid w, MonadException m) => MonadException (WriterT w m) Source # | |
| MonadException m => MonadException (StateT s m) Source # | |
| (MonadException m, Error e) => MonadException (ErrorT e m) Source # | |
| MonadException m => MonadException (ReaderT * r m) Source # | |
| (Monoid w, MonadException m) => MonadException (RWST r w s m) Source # | |
Generalizations of Control.Exception
catch :: (MonadException m, Exception e) => m a -> (e -> m a) -> m a Source #
handle :: (MonadException m, Exception e) => (e -> m a) -> m a -> m a Source #
catches :: MonadException m => m a -> [Handler m a] -> m a Source #
finally :: MonadException m => m a -> m b -> m a Source #
bracket :: MonadException m => m a -> (a -> m b) -> (a -> m c) -> m c Source #
Helpers for defining "wrapper" functions
liftIOOp :: MonadException m => ((a -> IO (m b)) -> IO (m c)) -> (a -> m b) -> m c Source #
Lift a IO operation
wrap :: (a -> IO b) -> IO b
to a more general monadic operation
liftIOOp wrap :: MonadException m => (a -> m b) -> m b
For example:
liftIOOp(withFilef m) :: MonadException m => (Handle -> m r) -> m rliftIOOpalloca:: (MonadException m, Storable a) => (Ptr a -> m b) -> m bliftIOOp(withForeignPtrfp) :: MonadException m => (Ptr a -> m b) -> m b
liftIOOp_ :: MonadException m => (IO (m a) -> IO (m a)) -> m a -> m a Source #
Lift an IO operation
wrap :: IO a -> IO a
to a more general monadic operation
liftIOOp_ wrap :: MonadException m => m a -> m a
Internal implementation
A RunIO function takes a monadic action m as input,
and outputs an IO action which performs the underlying impure part of m
and returns the 'pure' part of m.
Note that (RunIO return) is an incorrect implementation, since it does not
separate the pure and impure parts of the monadic action. This module defines
implementations for several common monad transformers.
Extensible Exceptions
class (Typeable * e, Show e) => Exception e #
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 MyExceptionThe 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 = frontendExceptionFromExceptionWe can now catch a MismatchedParentheses exception as
MismatchedParentheses, SomeFrontendException or
SomeCompilerException, but not other types, e.g. IOException:
*Main> throw MismatchedParenthesescatche -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatche -> putStrLn ("Caught " ++ show (e :: SomeFrontendException)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatche -> putStrLn ("Caught " ++ show (e :: SomeCompilerException)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatche -> putStrLn ("Caught " ++ show (e :: IOException)) *** Exception: MismatchedParentheses
Instances
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 :: SomeException |
Instances
data IOException :: * #
Exceptions that occur in the IO monad.
An IOException records a more specific error type, a descriptive
string and maybe the handle that was used when the error was
flagged.
Instances