| Portability | MPTCs |
|---|---|
| Stability | provisional |
| Maintainer | Edward Kmett <ekmett@gmail.com> |
| Safe Haskell | None |
Control.Monad.Trans.Either
Description
This module provides a minimalist Either monad transformer.
- newtype EitherT e m a = EitherT {
- runEitherT :: m (Either e a)
- eitherT :: Monad m => (a -> m c) -> (b -> m c) -> EitherT a m b -> m c
- bimapEitherT :: Functor m => (e -> f) -> (a -> b) -> EitherT e m a -> EitherT f m b
- mapEitherT :: (m (Either e a) -> n (Either e' b)) -> EitherT e m a -> EitherT e' n b
- hoistEither :: Monad m => Either e a -> EitherT e m a
- left :: Monad m => e -> EitherT e m a
- right :: Monad m => a -> EitherT e m a
Documentation
newtype EitherT e m a
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.
Constructors
| EitherT | |
Fields
| |
Instances
| 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) | |
| MonadTrans (EitherT e) | |
| 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) | |
| (Monad m, Monoid e) => Alternative (EitherT e m) | |
| 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) |
eitherT :: Monad m => (a -> m c) -> (b -> m c) -> EitherT a m b -> m c
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
Map over both failure and success.
mapEitherT :: (m (Either e a) -> n (Either e' b)) -> EitherT e m a -> EitherT e' n b
Map the unwrapped computation using the given function.
runEitherT(mapEitherTf m) = f (runEitherTm)
left :: Monad m => e -> EitherT e m a
Analogous to Left. Equivalent to throwError.