Control.Monad.Trans.Except

Copyright (C) 2013 Ross Paterson
License BSD-style (see the file LICENSE)
Maintainer [email protected]
Stability experimental
Portability portable
Safe Haskell Safe
Language Haskell98

Description

This monad transformer extends a monad with the ability to throw exceptions.

A sequence of actions terminates normally, producing a value, only if none of the actions in the sequence throws an exception. If one throws an exception, the rest of the sequence is skipped and the composite action exits with that exception.

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

The Except monad

type Except e = ExceptT e Identity Source

The parameterizable exception monad.

Computations are either exceptions or normal values.

The return function returns a normal value, while >>= exits on the first exception. For a variant that continues after an error and collects all the errors, see Errors.

except :: Monad m => Either e a -> ExceptT e m a Source

Constructor for computations in the exception monad. (The inverse of runExcept).

runExcept :: Except e a -> Either e a Source

Extractor for computations in the exception monad. (The inverse of except).

mapExcept :: (Either e a -> Either e' b) -> Except e a -> Except e' b Source

Map the unwrapped computation using the given function.

withExcept :: (e -> e') -> Except e a -> Except e' a Source

Transform any exceptions thrown by the computation using the given function (a specialization of withExceptT).

The ExceptT monad transformer

newtype ExceptT e m a Source

A monad transformer that adds exceptions to other monads.

ExceptT constructs a monad parameterized over two things:

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

The return function yields a computation that produces the given value, while >>= sequences two subcomputations, exiting on the first exception.

Constructors

ExceptT (m (Either e a))
Instances
Instances details
MonadTrans (ExceptT e)
Instance details

Defined in Control.Monad.Trans.Except

Methods

lift :: Monad m => m a -> ExceptT e m a Source

Monad m => Monad (ExceptT e m)
Instance details

Defined in Control.Monad.Trans.Except

Methods

(>>=) :: ExceptT e m a -> (a -> ExceptT e m b) -> ExceptT e m b Source

(>>) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m b Source

return :: a -> ExceptT e m a Source

Functor m => Functor (ExceptT e m)
Instance details

Defined in Control.Monad.Trans.Except

Methods

fmap :: (a -> b) -> ExceptT e m a -> ExceptT e m b Source

(<$) :: a -> ExceptT e m b -> ExceptT e m a Source

MonadFix m => MonadFix (ExceptT e m)
Instance details

Defined in Control.Monad.Trans.Except

Methods

mfix :: (a -> ExceptT e m a) -> ExceptT e m a Source

MonadFail m => MonadFail (ExceptT e m)
Instance details

Defined in Control.Monad.Trans.Except

Methods

fail :: String -> ExceptT e m a Source

(Functor m, Monad m) => Applicative (ExceptT e m)
Instance details

Defined in Control.Monad.Trans.Except

Methods

pure :: a -> ExceptT e m a Source

(<*>) :: ExceptT e m (a -> b) -> ExceptT e m a -> ExceptT e m b Source

liftA2 :: (a -> b -> c) -> ExceptT e m a -> ExceptT e m b -> ExceptT e m c Source

(*>) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m b Source

(<*) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m a Source

Foldable f => Foldable (ExceptT e f)
Instance details

Defined in Control.Monad.Trans.Except

Methods

fold :: Monoid m => ExceptT e f m -> m Source

foldMap :: Monoid m => (a -> m) -> ExceptT e f a -> m Source

foldMap' :: Monoid m => (a -> m) -> ExceptT e f a -> m Source

foldr :: (a -> b -> b) -> b -> ExceptT e f a -> b Source

foldr' :: (a -> b -> b) -> b -> ExceptT e f a -> b Source

foldl :: (b -> a -> b) -> b -> ExceptT e f a -> b Source

foldl' :: (b -> a -> b) -> b -> ExceptT e f a -> b Source

foldr1 :: (a -> a -> a) -> ExceptT e f a -> a Source

foldl1 :: (a -> a -> a) -> ExceptT e f a -> a Source

toList :: ExceptT e f a -> [a] Source

null :: ExceptT e f a -> Bool Source

length :: ExceptT e f a -> Int Source

elem :: Eq a => a -> ExceptT e f a -> Bool Source

maximum :: Ord a => ExceptT e f a -> a Source

minimum :: Ord a => ExceptT e f a -> a Source

sum :: Num a => ExceptT e f a -> a Source

product :: Num a => ExceptT e f a -> a Source

Traversable f => Traversable (ExceptT e f)
Instance details

Defined in Control.Monad.Trans.Except

Methods

traverse :: Applicative f0 => (a -> f0 b) -> ExceptT e f a -> f0 (ExceptT e f b) Source

sequenceA :: Applicative f0 => ExceptT e f (f0 a) -> f0 (ExceptT e f a) Source

mapM :: Monad m => (a -> m b) -> ExceptT e f a -> m (ExceptT e f b) Source

sequence :: Monad m => ExceptT e f (m a) -> m (ExceptT e f a) Source

Contravariant m => Contravariant (ExceptT e m)
Instance details

Defined in Control.Monad.Trans.Except

Methods

contramap :: (a -> b) -> ExceptT e m b -> ExceptT e m a Source

(>$) :: b -> ExceptT e m b -> ExceptT e m a Source

(Eq e, Eq1 m) => Eq1 (ExceptT e m)
Instance details

Defined in Control.Monad.Trans.Except

Methods

liftEq :: (a -> b -> Bool) -> ExceptT e m a -> ExceptT e m b -> Bool Source

(Ord e, Ord1 m) => Ord1 (ExceptT e m)
Instance details

Defined in Control.Monad.Trans.Except

Methods

liftCompare :: (a -> b -> Ordering) -> ExceptT e m a -> ExceptT e m b -> Ordering Source

(Read e, Read1 m) => Read1 (ExceptT e m)
Instance details

Defined in Control.Monad.Trans.Except

Methods

liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (ExceptT e m a) Source

liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [ExceptT e m a] Source

liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (ExceptT e m a) Source

liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [ExceptT e m a] Source

(Show e, Show1 m) => Show1 (ExceptT e m)
Instance details

Defined in Control.Monad.Trans.Except

Methods

liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> ExceptT e m a -> ShowS Source

liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [ExceptT e m a] -> ShowS Source

MonadZip m => MonadZip (ExceptT e m)
Instance details

Defined in Control.Monad.Trans.Except

Methods

mzip :: ExceptT e m a -> ExceptT e m b -> ExceptT e m (a, b) Source

mzipWith :: (a -> b -> c) -> ExceptT e m a -> ExceptT e m b -> ExceptT e m c Source

munzip :: ExceptT e m (a, b) -> (ExceptT e m a, ExceptT e m b) Source

MonadIO m => MonadIO (ExceptT e m)
Instance details

Defined in Control.Monad.Trans.Except

Methods

liftIO :: IO a -> ExceptT e m a Source

(Functor m, Monad m, Monoid e) => Alternative (ExceptT e m)
Instance details

Defined in Control.Monad.Trans.Except

Methods

empty :: ExceptT e m a Source

(<|>) :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a Source

some :: ExceptT e m a -> ExceptT e m [a] Source

many :: ExceptT e m a -> ExceptT e m [a] Source

(Monad m, Monoid e) => MonadPlus (ExceptT e m)
Instance details

Defined in Control.Monad.Trans.Except

Methods

mzero :: ExceptT e m a Source

mplus :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a Source

(Eq e, Eq1 m, Eq a) => Eq (ExceptT e m a)
Instance details

Defined in Control.Monad.Trans.Except

Methods

(==) :: ExceptT e m a -> ExceptT e m a -> Bool

(/=) :: ExceptT e m a -> ExceptT e m a -> Bool

(Ord e, Ord1 m, Ord a) => Ord (ExceptT e m a)
Instance details

Defined in Control.Monad.Trans.Except

Methods

compare :: ExceptT e m a -> ExceptT e m a -> Ordering

(<) :: ExceptT e m a -> ExceptT e m a -> Bool

(<=) :: ExceptT e m a -> ExceptT e m a -> Bool

(>) :: ExceptT e m a -> ExceptT e m a -> Bool

(>=) :: ExceptT e m a -> ExceptT e m a -> Bool

max :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a

min :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a

(Read e, Read1 m, Read a) => Read (ExceptT e m a)
Instance details

Defined in Control.Monad.Trans.Except

Methods

readsPrec :: Int -> ReadS (ExceptT e m a) Source

readList :: ReadS [ExceptT e m a] Source

readPrec :: ReadPrec (ExceptT e m a) Source

readListPrec :: ReadPrec [ExceptT e m a] Source

(Show e, Show1 m, Show a) => Show (ExceptT e m a)
Instance details

Defined in Control.Monad.Trans.Except

Methods

showsPrec :: Int -> ExceptT e m a -> ShowS Source

show :: ExceptT e m a -> String Source

showList :: [ExceptT e m a] -> ShowS Source

runExceptT :: ExceptT e m a -> m (Either e a) Source

The inverse of ExceptT.

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

Map the unwrapped computation using the given function.

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

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

Exception operations

throwE :: Monad m => e -> ExceptT e m a Source

Signal an exception value e.

catchE Source

Arguments

:: Monad m
=> ExceptT e m a

the inner computation
-> (e -> ExceptT e' m a)

a handler for exceptions in the inner computation
-> ExceptT e' m a

Handle an exception.

Lifting other operations

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

Lift a callCC operation to the new monad.

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

Lift a listen operation to the new monad.

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

Lift a pass operation to the new monad.

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.10.2/docs/html/libraries/transformers-0.5.6.2/Control-Monad-Trans-Except.html