- module Control.Monad
- class Exception e => Throws e s
- 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
- data Caught e l
- class (Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- data SomeException
Documentation
module Control.Monad
class Monad m => MonadThrow e m whereSource
Exception e => MonadThrow e IO | |
MonadThrow e m => MonadThrow e (ListT m) | |
Error e => MonadThrow e (Either e) | |
(Exception e, Throws e l) => MonadThrow e (EM l) | |
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
Exception e => MonadCatch e IO IO | |
MonadCatch e m m' => MonadCatch e (ListT m) (ListT m') | |
Error e => MonadCatch e (Either e) (Either e) | |
Exception e => MonadCatch e (EM (Caught e l)) (EM l) | |
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 (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
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
.