Safe Haskell	Safe
Language	Haskell98

Network.Gitit.Compat.Except

Synopsis

Documentation

data ExceptT e m a :: * -> (* -> *) -> * -> *

A monad transformer that adds exceptions to other monads.

ExceptT constructs a monad parameterized over two things:

e - The exception type.
m - The inner monad.

The return function yields a computation that produces the given value, while >>= sequences two subcomputations, exiting on the first exception.

Instances

MonadReader r m => MonadReader r (ExceptT e m)
FilterMonad a m => FilterMonad a (ExceptT e m)
WebMonad a m => WebMonad a (ExceptT e m)
MonadBase b m => MonadBase b (ExceptT e m)
MonadBaseControl b m => MonadBaseControl b (ExceptT e m)
Monad m => MonadError e (ExceptT e m)
MonadState s m => MonadState s (ExceptT e m)
MonadTrans (ExceptT e)
MonadTransControl (ExceptT e)
Monad m => Monad (ExceptT e m)
Functor m => Functor (ExceptT e m)
MonadFix m => MonadFix (ExceptT e m)
(Functor m, Monad m) => Applicative (ExceptT e m)
Foldable f => Foldable (ExceptT e f)
Traversable f => Traversable (ExceptT e f)
(Functor m, Monad m, Monoid e) => Alternative (ExceptT e m)
(Monad m, Monoid e) => MonadPlus (ExceptT e m)
MonadIO m => MonadIO (ExceptT e m)
(Happstack m, Monoid e) => Happstack (ExceptT e m)
(Monad m, HasRqData m) => HasRqData (ExceptT e m)
ServerMonad m => ServerMonad (ExceptT e m)
PrimMonad m => PrimMonad (ExceptT e m)
MonadResource m => MonadResource (ExceptT e m)
(Eq e, Eq1 m) => Eq1 (ExceptT e m)
(Ord e, Ord1 m) => Ord1 (ExceptT e m)
(Read e, Read1 m) => Read1 (ExceptT e m)
(Show e, Show1 m) => Show1 (ExceptT e m)
(Eq e, Eq1 m, Eq a) => Eq (ExceptT e m a)
(Ord e, Ord1 m, Ord a) => Ord (ExceptT e m a)
(Read e, Read1 m, Read a) => Read (ExceptT e m a)
(Show e, Show1 m, Show a) => Show (ExceptT e m a)
type StT (ExceptT e) a = Either e a
type PrimState (ExceptT e m) = PrimState m
type StM (ExceptT e m) a = ComposeSt (ExceptT e) m a

type Except e = ExceptT e Identity

The parameterizable exception monad.

Computations are either exceptions or normal values.

The return function returns a normal value, while >>= exits on the first exception. For a variant that continies after an error and collects all the errors, see Errors.

class Error a where Source

Minimal complete definition

Nothing

Methods

noMsg :: a Source

strMsg :: String -> a Source

runExceptT :: ExceptT e m a -> m (Either e a)

The inverse of ExceptT.

runExcept :: Except e a -> Either e a

Extractor for computations in the exception monad. (The inverse of except).

class Monad m => MonadError e m | m -> e where

The strategy of combining computations that can throw exceptions by bypassing bound functions from the point an exception is thrown to the point that it is handled.

Is parameterized over the type of error information and the monad type constructor. It is common to use Either String as the monad type constructor for an error monad in which error descriptions take the form of strings. In that case and many other common cases the resulting monad is already defined as an instance of the MonadError class. You can also define your own error type and/or use a monad type constructor other than Either String or Either IOError. In these cases you will have to explicitly define instances of the Error and/or MonadError classes.

Methods

throwError :: e -> m a

Is used within a monadic computation to begin exception processing.

catchError :: m a -> (e -> m a) -> m a

A handler function to handle previous errors and return to normal execution. A common idiom is:

do { action1; action2; action3 } `catchError` handler

where the action functions can call throwError. Note that handler and the do-block must have the same return type.

Instances

MonadError IOException IO
MonadError e m => MonadError e (ResourceT m)
MonadError e m => MonadError e (WebT m)
(Monad m, MonadError e m) => MonadError e (ServerPartT m)
MonadError e m => MonadError e (MaybeT m)
MonadError e m => MonadError e (ListT m)
MonadError e m => MonadError e (IdentityT m)
MonadError e (Either e)
(Monoid w, MonadError e m) => MonadError e (WriterT w m)
(Monoid w, MonadError e m) => MonadError e (WriterT w m)
MonadError e m => MonadError e (StateT s m)
MonadError e m => MonadError e (StateT s m)
MonadError e m => MonadError e (ReaderT r m)
Monad m => MonadError e (ExceptT e m)
(Monad m, Error e) => MonadError e (ErrorT e m)
MonadError e m => MonadError e (ParsecT s u m)
(Monoid w, MonadError e m) => MonadError e (RWST r w s m)
(Monoid w, MonadError e m) => MonadError e (RWST r w s m)

throwError :: MonadError e m => forall a. e -> m a

Is used within a monadic computation to begin exception processing.

catchError :: MonadError e m => forall a. m a -> (e -> m a) -> m a

A handler function to handle previous errors and return to normal execution. A common idiom is:

do { action1; action2; action3 } `catchError` handler

where the action functions can call throwError. Note that handler and the do-block must have the same return type.