control-monad-exception-0.3.1: Explicitly typed exceptionsSource codeContentsIndex
Control.Monad.Exception.Class
Synopsis
module Control.Monad
class Monad m => MonadThrow e m where
throw :: e -> m a
class (Monad m, Monad m') => MonadCatch e m m' | e m -> m', e m' -> m where
catch :: m a -> (e -> m' a) -> m' a
class (Private l, Exception e) => Throws e l
data Caught e l
newtype WrapException l = WrapException {
wrapException :: SomeException
}
class (Typeable e, Show e) => Exception e where
toException :: e -> SomeException
fromException :: SomeException -> Maybe e
data SomeException where
SomeException :: Exception e => e -> SomeException
class Exception e => UncaughtException e
data NoExceptions
data ParanoidMode
Documentation
module Control.Monad
class Monad m => MonadThrow e m whereSource
Methods
throw :: e -> m aSource
show/hide Instances
Exception e => MonadThrow e IO
MonadThrow e m => MonadThrow e (ListT m)
Error e => MonadThrow e (Either e)
MonadThrow e m => MonadThrow e (StateT s m)
(Monoid w, MonadThrow e m) => MonadThrow e (WriterT w m)
MonadThrow e m => MonadThrow e (ReaderT r m)
(Error e, Monad m) => MonadThrow e (ErrorT e m)
(Exception e, Throws e l, Monad m) => MonadThrow e (EMT l m)
(Monoid w, MonadThrow e m) => MonadThrow e (RWST r w s m)
class (Monad m, Monad m') => MonadCatch e m m' | e m -> m', e m' -> m whereSource
Methods
catch :: m a -> (e -> m' a) -> m' aSource
show/hide Instances
Exception e => MonadCatch e IO IO
MonadCatch e m m' => MonadCatch e (ListT m) (ListT m')
Error e => MonadCatch e (Either e) (Either e)
MonadCatch e m m' => MonadCatch e (StateT s m) (StateT s m')
(Monoid w, MonadCatch e m m') => MonadCatch e (WriterT w m) (WriterT w m')
MonadCatch e m m' => MonadCatch e (ReaderT r m) (ReaderT r m')
(Error e, Monad m) => MonadCatch e (ErrorT e m) (ErrorT e m)
(Exception e, Monad m) => MonadCatch e (EMT (Caught e l) m) (EMT l m)
(Monoid w, MonadCatch e m m') => MonadCatch e (RWST r w s m) (RWST r w s m')
class (Private l, Exception e) => Throws e l Source

Throws is the mechanism used to keep a type level list of exceptions.

Usually there is no need for the user of this library to add further instances to Throws except in one case: to encode subtyping. For instance if we have an IOException type and a FIleNotFoundException subcase, we need to add the instance:

 instance Throws FileNotFoundException (Caught IOException l)
show/hide Instances
data Caught e l Source
A type level witness of a exception handler.
show/hide Instances
Exception e => Throws e (Caught SomeException l)
Throws e l => Throws e (Caught e' l)
Exception e => Throws e (Caught e l)
(Exception e, Monad m) => MonadCatch e (EMT (Caught e l) m) (EMT l m)
Private (Caught e l)
newtype WrapException l Source
Constructors
WrapException
wrapException :: SomeException
show/hide Instances
class (Typeable e, Show e) => Exception e whereSource

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
Methods
toException :: e -> SomeExceptionSource
fromException :: SomeException -> Maybe eSource
show/hide Instances
data SomeException whereSource
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 :: Exception e => e -> SomeException
show/hide Instances
class Exception e => UncaughtException e Source

Uncaught Exceptions model runtime exceptions which are not checked.

In order to declare a runtime exception it must be made an instance of UncaughtException

data NoExceptions Source
show/hide Instances
data ParanoidMode Source
show/hide Instances
Private ParanoidMode
Produced by Haddock version 2.6.0