DeepControl.Monad.Except

This module is just a concise mimic for Except Monad in mtl(monad-transformer-library). The qualifier "concise" means that this module doesn't make no attempt to transform functions of any kind of Monad automatically. So when making some new data type of ExceptT, you have to manually define involved Monad instances, for example MonadReader, MonadWriter or MonadState, by making use of the transformation functions such as trans, trans2, etc. Admittedly it is tedious though, you can deeply understand monad-transformation mechanism instead.

Classes

class Error a where

Minimal complete definition

Nothing

Methods

noMsg :: a

strMsg :: String -> a

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 (MaybeT m)

MonadError e m => MonadError e (ListT m)

MonadError e m => MonadError e (IdentityT m)

MonadError e (Either e)

MonadError e (Except e)

(Monad m, Error e) => MonadError e (ExceptT e m)

(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)

(Monoid w, MonadError e m) => MonadError e (RWST r w s m)

(Monoid w, MonadError e m) => MonadError e (RWST r w s m)

Level-0

newtype Except e a

Constructors

Except

Fields

runExcept :: Either e a

Instances

MonadError e (Except e)

Monad (Except e)

Functor (Except e)

Applicative (Except e)

Commutative (Except e)

(Show e, Show a) => Show (Except e a)

mapExcept :: (Either e a -> Either e' b) -> Except e a -> Except e' b

withExcept :: (e -> e') -> Except e a -> Except e' a

Level-1

newtype ExceptT e m a

Constructors

ExceptT

Fields

runExceptT :: m (Either e a)

Instances

(Monad m, Error e) => MonadError e (ExceptT e m)

MonadTrans (ExceptT e)

Monad m => Monad (ExceptT e m)

Functor m => Functor (ExceptT e m)

Monad m => Applicative (ExceptT e m)

MonadIO m => MonadIO (ExceptT e m)

mapExceptT :: (m (Either e a) -> n (Either e' b)) -> ExceptT e m a -> ExceptT e' n b

withExceptT :: Functor m => (e -> e') -> ExceptT e m a -> ExceptT e' m a