transformers-0.6.0.4: Concrete functor and monad transformers
Copyright(C) 2013 Ross Paterson
LicenseBSD-style (see the file LICENSE)
MaintainerR.Paterson@city.ac.uk
Stabilityexperimental
Portabilityportable
Safe HaskellSafe
LanguageHaskell2010

Control.Monad.Trans.Except

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.

Synopsis

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) Source # 
Instance details

Defined in Control.Monad.Trans.Except

Methods

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

Functor m => Generic1 (ExceptT e m :: Type -> Type) Source # 
Instance details

Defined in Control.Monad.Trans.Except

Associated Types

type Rep1 (ExceptT e m) :: k -> Type #

Methods

from1 :: forall (a :: k). ExceptT e m a -> Rep1 (ExceptT e m) a #

to1 :: forall (a :: k). Rep1 (ExceptT e m) a -> ExceptT e m a #

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

Defined in Control.Monad.Trans.Except

Methods

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

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

return :: a -> ExceptT e m a #

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

Defined in Control.Monad.Trans.Except

Methods

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

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

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

Defined in Control.Monad.Trans.Except

Methods

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

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

Defined in Control.Monad.Trans.Except

Methods

fail :: String -> ExceptT e m a #

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

Defined in Control.Monad.Trans.Except

Methods

pure :: a -> ExceptT e m a #

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

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

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

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

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

Defined in Control.Monad.Trans.Except

Methods

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

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

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

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

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

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

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

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

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

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

null :: ExceptT e f a -> Bool #

length :: ExceptT e f a -> Int #

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

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

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

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

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

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

Defined in Control.Monad.Trans.Except

Methods

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

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

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

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

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

Defined in Control.Monad.Trans.Except

Methods

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

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

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

Defined in Control.Monad.Trans.Except

Methods

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

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

Defined in Control.Monad.Trans.Except

Methods

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

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

Defined in Control.Monad.Trans.Except

Methods

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

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

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

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

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

Defined in Control.Monad.Trans.Except

Methods

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

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

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

Defined in Control.Monad.Trans.Except

Methods

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

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

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

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

Defined in Control.Monad.Trans.Except

Methods

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

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

Defined in Control.Monad.Trans.Except

Methods

empty :: ExceptT e m a #

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

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

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

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

Defined in Control.Monad.Trans.Except

Methods

mzero :: ExceptT e m a #

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

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

Defined in Control.Monad.Trans.Except

Methods

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

readList :: ReadS [ExceptT e m a] #

readPrec :: ReadPrec (ExceptT e m a) #

readListPrec :: ReadPrec [ExceptT e m a] #

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

Defined in Control.Monad.Trans.Except

Methods

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

show :: ExceptT e m a -> String #

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

Generic (ExceptT e m a) Source # 
Instance details

Defined in Control.Monad.Trans.Except

Associated Types

type Rep (ExceptT e m a) :: Type -> Type #

Methods

from :: ExceptT e m a -> Rep (ExceptT e m a) x #

to :: Rep (ExceptT e m a) x -> ExceptT e m a #

type Rep1 (ExceptT e m :: Type -> Type) Source # 
Instance details

Defined in Control.Monad.Trans.Except

type Rep1 (ExceptT e m :: Type -> Type) = D1 ('MetaData "ExceptT" "Control.Monad.Trans.Except" "transformers-0.6.0.4-6wgqQgktLayLloKShWoG1E" 'True) (C1 ('MetaCons "ExceptT" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (m :.: Rec1 (Either e))))
type Rep (ExceptT e m a) Source # 
Instance details

Defined in Control.Monad.Trans.Except

type Rep (ExceptT e m a) = D1 ('MetaData "ExceptT" "Control.Monad.Trans.Except" "transformers-0.6.0.4-6wgqQgktLayLloKShWoG1E" 'True) (C1 ('MetaCons "ExceptT" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (m (Either e a)))))

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.

handleE :: Monad m => (e -> ExceptT e' m a) -> ExceptT e m a -> ExceptT e' m a Source #

The same as flip catchE, which is useful in situations where the code for the handler is shorter.

tryE :: Monad m => ExceptT e m a -> ExceptT e m (Either e a) Source #

Similar to catchE, but returns an Either result which is (Right a) if no exception was thown, or (Left ex) if an exception ex was thrown.

finallyE :: Monad m => ExceptT e m a -> ExceptT e m () -> ExceptT e m a Source #

finallyE a b executes computation a followed by computation b, even if a exits early by throwing an exception. In the latter case, the exception is re-thrown after b has been executed.

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.