either-4.4.1: An either monad transformer

Copyright(C) 2008-2014 Edward Kmett
LicenseBSD-style (see the file LICENSE)
MaintainerEdward Kmett <ekmett@gmail.com>
Safe HaskellTrustworthy



This module provides a minimalist Either monad transformer.



newtype EitherT e m a Source

EitherT is a version of ErrorT that does not require a spurious Error instance for the Left case.

Either is a perfectly usable Monad without such a constraint. ErrorT is not the generalization of the current Either monad, it is something else.

This is necessary for both theoretical and practical reasons. For instance an apomorphism is the generalized anamorphism for this Monad, but it cannot be written with ErrorT.

In addition to the combinators here, the errors package provides a large number of combinators for working with this type.




runEitherT :: m (Either e a)


(Functor f, MonadFree f m) => MonadFree f (EitherT e m) 
MonadBase b m => MonadBase b (EitherT e m) 
MonadBaseControl b m => MonadBaseControl b (EitherT e m) 
Monad m => MonadError e (EitherT e m) 
MonadReader r m => MonadReader r (EitherT e m) 
MonadState s m => MonadState s (EitherT e m) 
MonadWriter s m => MonadWriter s (EitherT e m) 
MFunctor (EitherT e) 
MonadTrans (EitherT e) 
MonadTransControl (EitherT e) 
(Monad m, Monoid e) => Alternative (EitherT e m) 
Monad m => Monad (EitherT e m) 
Monad m => Functor (EitherT e m) 
MonadFix m => MonadFix (EitherT e m) 
(Monad m, Monoid e) => MonadPlus (EitherT e m) 
Monad m => Applicative (EitherT e m) 
Foldable m => Foldable (EitherT e m) 
(Monad f, Traversable f) => Traversable (EitherT e f) 
MonadRandom m => MonadRandom (EitherT e m) 
MonadThrow m => MonadThrow (EitherT e m)

Throws exceptions into the base monad.

MonadCatch m => MonadCatch (EitherT e m)

Catches exceptions from the base monad.

MonadIO m => MonadIO (EitherT e m) 
MonadCont m => MonadCont (EitherT e m) 
(Monad m, Semigroup e) => Alt (EitherT e m) 
Monad m => Apply (EitherT e m) 
Monad m => Bind (EitherT e m) 
Eq (m (Either e a)) => Eq (EitherT e m a) 
Ord (m (Either e a)) => Ord (EitherT e m a) 
Read (m (Either e a)) => Read (EitherT e m a) 
Show (m (Either e a)) => Show (EitherT e m a) 
Monad m => Semigroup (EitherT e m a) 
type StT (EitherT e) a = Either e a 
type StM (EitherT e m) a = StM m (StT (EitherT e) a) 

eitherT :: Monad m => (a -> m c) -> (b -> m c) -> EitherT a m b -> m c Source

Given a pair of actions, one to perform in case of failure, and one to perform in case of success, run an EitherT and get back a monadic result.

bimapEitherT :: Functor m => (e -> f) -> (a -> b) -> EitherT e m a -> EitherT f m b Source

Map over both failure and success.

mapEitherT :: (m (Either e a) -> n (Either e' b)) -> EitherT e m a -> EitherT e' n b Source

Map the unwrapped computation using the given function.

runEitherT (mapEitherT f m) = f (runEitherT m)

hoistEither :: Monad m => Either e a -> EitherT e m a Source

Lift an Either into an EitherT

bracketEitherT :: Monad m => EitherT e m a -> (a -> EitherT e m b) -> (a -> EitherT e m c) -> EitherT e m c Source

Acquire a resource in EitherT and then perform an action with it, cleaning up afterwards regardless of error. Like bracket, but acting only in EitherT.

bracketEitherT_ :: Monad m => EitherT e m a -> EitherT e m b -> EitherT e m c -> EitherT e m c Source

Version of bracketEitherT which discards the result from the initial action.

left :: Monad m => e -> EitherT e m a Source

Analogous to Left. Equivalent to throwError.

right :: Monad m => a -> EitherT e m a Source

Analogous to Right. Equivalent to return.

swapEitherT :: Functor m => EitherT e m a -> EitherT a m e Source

Monad transformer version of swapEither.