Control.Monad.Exception.Class

Defines MonadThrow and MonadCatch type classes and suitable instances for IO and Either.

Documentation

module Control.Monad

module Control.Monad.Exception.Throws

class Monad m => MonadThrow e m where

Methods

throw :: e -> m a

throwWithSrcLoc :: String -> e -> m a

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 where

Methods

catch :: m a -> (e -> m' a) -> m' a

catchWithSrcLoc :: m a -> ([String] -> e -> m' a) -> m' a

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

WrapException adds a phantom type parameter l to SomeException

Constructors

WrapException

Fields

unwrapException :: SomeException

Instances

Typeable1 WrapException

Show (WrapException l)

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

Methods

toException :: e -> SomeException

fromException :: SomeException -> Maybe e

Instances

Exception PatternMatchFail

Exception RecSelError

Exception RecConError

Exception RecUpdError

Exception NoMethodError

Exception NonTermination

Exception NestedAtomically

Exception BlockedIndefinitelyOnMVar

Exception BlockedIndefinitelyOnSTM

Exception Deadlock

Exception AssertionFailed

Exception AsyncException

Exception ArrayException

Exception ExitCode

Exception IOException

Exception SomeException

Exception ErrorCall

Exception ArithException

Exception MonadZeroException

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)

SomeException is at the top of the exception hierarchy . Capturing SomeException captures every possible exception

showExceptionWithTrace :: Exception e => [String] -> e -> String

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' a