transformers- Concrete functor and monad transformers

Copyright(c) Michael Weber <> 2001, (c) Jeff Newbern 2003-2006, (c) Andriy Palamarchuk 2006
LicenseBSD-style (see the file LICENSE)
Safe HaskellSafe




Deprecated: Use Control.Monad.Trans.Except instead

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.

Note: This module will be removed in a future release. Instead, use Control.Monad.Trans.Except, which does not restrict the exception type, and also includes a base exception monad.


The ErrorT monad transformer

class Error a where Source

An exception to be thrown.

Minimal complete definition: noMsg or strMsg.

Minimal complete definition



noMsg :: a Source

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

strMsg :: String -> a Source

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


Error IOException 
ErrorList a => Error [a]

A string can be thrown as an error.

class ErrorList a where Source

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


listMsg :: String -> [a] Source


newtype ErrorT e m a Source

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.




runErrorT :: m (Either e a)


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) 
(Functor m, Monad m) => Applicative (ErrorT e m) 
Foldable f => Foldable (ErrorT e f) 
Traversable f => Traversable (ErrorT e f) 
(Functor m, Monad m, Error e) => Alternative (ErrorT e m) 
(Monad m, Error e) => MonadPlus (ErrorT e m) 
(Error e, MonadIO m) => MonadIO (ErrorT e m) 
(Show e, Show1 m) => Show1 (ErrorT e m) 
(Read e, Read1 m) => Read1 (ErrorT e m) 
(Ord e, Ord1 m) => Ord1 (ErrorT e m) 
(Eq e, Eq1 m) => Eq1 (ErrorT e m) 
(Eq e, Eq1 m, Eq a) => Eq (ErrorT e m a) 
(Ord e, Ord1 m, Ord a) => Ord (ErrorT e m a) 
(Read e, Read1 m, Read a) => Read (ErrorT e m a) 
(Show e, Show1 m, Show a) => Show (ErrorT e m a) 

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

Map the unwrapped computation using the given function.

Error operations

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

Signal an error value e.

catchError Source


:: (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 :: CallCC m (Either e a) (Either e b) -> CallCC (ErrorT e m) a b Source

Lift a callCC operation to the new monad.

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

Lift a listen operation to the new monad.

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

Lift a pass operation to the new monad.


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)))