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
.
Synopsis
- 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 # | |
MonadException m => MonadException (MaybeT m) # | |
MonadException m => MonadException (ListT m) # | |
MonadException m => MonadException (InputT m) # | |
(Monoid w, MonadException m) => MonadException (WriterT w m) # | |
MonadException m => MonadException (StateT s m) # | |
MonadException m => MonadException (IdentityT m) # | |
(MonadException m, Error e) => MonadException (ErrorT e m) # | |
MonadException m => MonadException (ReaderT r m) # | |
(Monoid w, MonadException m) => MonadException (RWST r w s m) # | |
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
(withFile
f m) :: MonadException m => (Handle -> m r) -> m rliftIOOp
alloca
:: (MonadException m, Storable a) => (Ptr a -> m b) -> m bliftIOOp
(withForeignPtr
fp) :: 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 Source #
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
Instances
data SomeException :: * where Source #
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
Show SomeException | Since: 3.0 |
Exception SomeException | Since: 3.0 |
Methods toException :: SomeException -> SomeException Source # fromException :: SomeException -> Maybe SomeException Source # |
data IOException :: * Source #
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
Eq IOException | Since: 4.1.0.0 |
Show IOException | Since: 4.1.0.0 |
Exception IOException | Since: 4.1.0.0 |
Methods toException :: IOException -> SomeException Source # fromException :: SomeException -> Maybe IOException Source # displayException :: IOException -> String Source # | |
Error IOException | |