Control.Monad.Exception.Class
Description
Defines MonadThrow and MonadCatch type classes and suitable instances for IO and Either.
- module Control.Monad
- module Control.Monad.Exception.Throws
- class Monad m => MonadThrow e m where
- throw :: e -> m a
- throwWithSrcLoc :: String -> 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
- catchWithSrcLoc :: m a -> ([String] -> e -> m' a) -> m' a
- newtype WrapException l = WrapException {}
- class (Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- data SomeException where
- SomeException :: Exception e => e -> SomeException
- showExceptionWithTrace :: Exception e => [String] -> e -> String
- wrapException :: (MonadThrow e' m', MonadCatch e m m') => m a -> (e -> e') -> m' a
Documentation
module Control.Monad
class Monad m => MonadThrow e m whereSource
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
catchWithSrcLoc :: m a -> ([String] -> e -> m' a) -> m' aSource
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') |
newtype WrapException l Source
WrapException adds a phantom type parameter l to SomeException
Constructors
| WrapException | |
Fields | |
Instances
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
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 :: Exception e => e -> SomeException |
Instances
| Show SomeException | |
| Typeable SomeException | |
| Exception SomeException | |
| Exception e => Throws e (Caught SomeException l) |
|
showExceptionWithTrace :: Exception e => [String] -> e -> StringSource
Given a list of source locations and an exception, showExceptionWithTrace produces output of the form
<exception details>
in <module a>(<file a.hs>): (12,6)
<module b>(<file b.hs>): (11,7)
...
wrapException :: (MonadThrow e' m', MonadCatch e m m') => m a -> (e -> e') -> m' aSource