Control.Monad.Except.Trans

newtype ExceptT e m a

A monad transformer which adds exceptions to other monads, in the same way as Except. As before, e is the type of exceptions, and a is the type of successful results. The new type parameter m is the inner monad that computations run in.

Constructors

• ExceptT (m (Either e a))

Instances

• Newtype (ExceptT e m a) _
• (Functor m) => Functor (ExceptT e m)
• (Monad m) => Apply (ExceptT e m)
• (Monad m) => Applicative (ExceptT e m)
• (Monad m) => Bind (ExceptT e m)
• (Monad m) => Monad (ExceptT e m)
• (MonadRec m) => MonadRec (ExceptT e m)
• (Semigroup e, Monad m) => Alt (ExceptT e m)
• (Monoid e, Monad m) => Plus (ExceptT e m)
• (Monoid e, Monad m) => Alternative (ExceptT e m)
• (Monoid e, Monad m) => MonadPlus (ExceptT e m)
• (Monoid e, Monad m) => MonadZero (ExceptT e m)
• MonadTrans (ExceptT e)
• (MonadEff eff m) => MonadEff eff (ExceptT e m)
• (MonadCont m) => MonadCont (ExceptT e m)
• (Monad m) => MonadThrow e (ExceptT e m)
• (Monad m) => MonadError e (ExceptT e m)
• (MonadAsk r m) => MonadAsk r (ExceptT e m)
• (MonadReader r m) => MonadReader r (ExceptT e m)
• (MonadState s m) => MonadState s (ExceptT e m)
• (MonadTell w m) => MonadTell w (ExceptT e m)
• (MonadWriter w m) => MonadWriter w (ExceptT e m)

runExceptT :: forall a m e. ExceptT e m a -> m (Either e a)

The inverse of ExceptT. Run a computation in the ExceptT monad.

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

Transform any exceptions thrown by an ExceptT computation using the given function.

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

Transform the unwrapped computation using the given function.

except :: forall a m e. Applicative m => Either e a -> ExceptT e m a

Construct a computation in the ExceptT transformer from an Either value.

class (MonadThrow e m) <= MonadError e m | m -> e where

The MonadError type class represents those monads which support catching errors.

• catchError x f calls the error handler f if an error is thrown during the evaluation of x.

An implementation is provided for ExceptT, and for other monad transformers defined in this library.

Laws:

• Catch: catchError (throwError e) f = f e
• Pure: catchError (pure a) f = pure a

Members

• catchError :: forall a. m a -> (e -> m a) -> m a

Instances

• MonadError e (Either e)
• MonadError Unit Maybe

class (Monad m) <= MonadThrow e m | m -> e where

The MonadThrow type class represents those monads which support errors via throwError, where throwError e halts, yielding the error e.

An implementation is provided for ExceptT, and for other monad transformers defined in this library.

Laws:

• Left zero: throwError e >>= f = throwError e

Members

• throwError :: forall a. e -> m a

Instances

• MonadThrow e (Either e)
• MonadThrow Unit Maybe

class MonadTrans t  where

The MonadTrans type class represents monad transformers.

A monad transformer is a type constructor of kind (* -> *) -> * -> *, which takes a Monad as its first argument, and returns another Monad.

This allows us to add additional effects to an existing monad. By iterating this process, we create monad transformer stacks, which contain all of the effects required for a particular computation.

The laws state that lift is a Monad morphism.

Laws:

• lift (pure a) = pure a
• lift (do { x <- m ; y }) = do { x <- lift m ; lift y }

Members

• lift :: forall a m. Monad m => m a -> t m a