Control.Monad.Trans.Error

This monad transformer adds the ability to fail or throw exceptions to a monad.

A sequence of actions succeeds, producing a value, only if all the actions in the sequence are successful. If one fails with an error, the rest of the sequence is skipped and the composite action fails with that error.

If the value of the error is not required, the variant in Control.Monad.Trans.Maybe may be used instead.

The ErrorT monad transformer

class Error a where

An exception to be thrown.

Minimal complete definition: noMsg or strMsg.

Methods

noMsg :: a

Creates an exception without a message. The default implementation is strMsg "".

strMsg :: String -> a

Creates an exception with a message. The default implementation of strMsg s is noMsg.

Instances

Error IOException

ErrorList a => Error [a]

A string can be thrown as an error.

class ErrorList a where

Workaround so that we can have a Haskell 98 instance Error String.

Methods

listMsg :: String -> [a]

Instances

ErrorList Char

newtype ErrorT e m a

The error monad transformer. It can be used to add error handling to other monads.

The ErrorT Monad structure is parameterized over two things:

• e - The error type.
• m - The inner monad.

The return function yields a successful computation, while >>= sequences two subcomputations, failing on the first error.

Constructors

ErrorT

Fields

runErrorT :: m (Either e a)

Instances

Error e => MonadTrans (ErrorT e)

(Monad m, Error e) => Monad (ErrorT e m)

Functor m => Functor (ErrorT e m)

(MonadFix m, Error e) => MonadFix (ErrorT e m)

(Monad m, Error e) => MonadPlus (ErrorT e m)

(Functor m, Monad m) => Applicative (ErrorT e m)

(Functor m, Monad m, Error e) => Alternative (ErrorT e m)

(Error e, MonadIO m) => MonadIO (ErrorT e m)

mapErrorT :: (m (Either e a) -> n (Either e' b)) -> ErrorT e m a -> ErrorT e' n b

Map the unwrapped computation using the given function.

• runErrorT (mapErrorT f m) = f (runErrorT m)

Error operations

throwError :: (Monad m, Error e) => e -> ErrorT e m a

Signal an error value e.

• runErrorT (throwErrorT e) = return (Left e)

catchError

Arguments

:: (Monad m, Error e)

=> ErrorT e m a

the inner computation

-> (e -> ErrorT e m a)

a handler for errors in the inner computation

-> ErrorT e m a

Handle an error.

Lifting other operations

liftCallCC :: (((Either e a -> m (Either e b)) -> m (Either e a)) -> m (Either e a)) -> ((a -> ErrorT e m b) -> ErrorT e m a) -> ErrorT e m a

Lift a callCC operation to the new monad.

liftListen :: Monad m => (m (Either e a) -> m (Either e a, w)) -> ErrorT e m a -> ErrorT e m (a, w)

Lift a listen operation to the new monad.

liftPass :: Monad m => (m (Either e a, w -> w) -> m (Either e a)) -> ErrorT e m (a, w -> w) -> ErrorT e m a

Lift a pass operation to the new monad.

Examples

Wrapping an IO action that can throw an error e:

 type ErrorWithIO e a = ErrorT e IO a
 ==> ErrorT (IO (Either e a))

An IO monad wrapped in StateT inside of ErrorT:

 type ErrorAndStateWithIO e s a = ErrorT e (StateT s IO) a
 ==> ErrorT (StateT s IO (Either e a))
 ==> ErrorT (StateT (s -> IO (Either e a,s)))