Except

ExceptT

newtype ExceptT e (m :: * -> *) a

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

MonadReader r m => MonadReader r (ExceptT e m)

Since: mtl-2.2

Defined in Control.Monad.Reader.Class

Methods

ask :: ExceptT e m r

local :: (r -> r) -> ExceptT e m a -> ExceptT e m a

reader :: (r -> a) -> ExceptT e m a

MonadState s m => MonadState s (ExceptT e m)

Since: mtl-2.2

Defined in Control.Monad.State.Class

Methods

get :: ExceptT e m s

put :: s -> ExceptT e m ()

state :: (s -> (a, s)) -> ExceptT e m a

(Functor f, MonadFree f m) => MonadFree f (ExceptT e m)

Defined in Control.Monad.Free.Class

Methods

wrap :: f (ExceptT e m a) -> ExceptT e m a

Monad m => MonadError e (ExceptT e m)

Since: mtl-2.2

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> ExceptT e m a

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

MonadWriter w m => MonadWriter w (ExceptT e m)

Since: mtl-2.2

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> ExceptT e m a

tell :: w -> ExceptT e m ()

listen :: ExceptT e m a -> ExceptT e m (a, w)

pass :: ExceptT e m (a, w -> w) -> ExceptT e m a

MonadBase b m => MonadBase b (ExceptT e m)

Defined in Control.Monad.Base

Methods

liftBase :: b α -> ExceptT e m α

MonadTrans (ExceptT e)

Defined in Control.Monad.Trans.Except

Methods

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

MMonad (ExceptT e)

Defined in Control.Monad.Morph

Methods

embed :: Monad n => (forall a. m a -> ExceptT e n a) -> ExceptT e m b -> ExceptT e n b

MFunctor (ExceptT e :: (* -> *) -> * -> *)

Defined in Control.Monad.Morph

Methods

hoist :: Monad m => (forall a. m a -> n a) -> ExceptT e m b -> ExceptT e n b

Monad m => Monad (ExceptT e m)

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

fail :: String -> ExceptT e m a

Functor m => Functor (ExceptT e m)

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)

Defined in Control.Monad.Trans.Except

Methods

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

MonadFail m => MonadFail (ExceptT e m)

Defined in Control.Monad.Trans.Except

Methods

fail :: String -> ExceptT e m a

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

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)

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

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)

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)

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

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]

Contravariant m => Contravariant (ExceptT e m)

Defined in Data.Functor.Contravariant

Methods

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

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

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

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) => Eq1 (ExceptT e m)

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)

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)

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)

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)

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)

Defined in Control.Monad.Trans.Except

Methods

liftIO :: IO a -> ExceptT e m a

Divisible m => Divisible (ExceptT e m)

Defined in Data.Functor.Contravariant.Divisible

Methods

divide :: (a -> (b, c)) -> ExceptT e m b -> ExceptT e m c -> ExceptT e m a

conquer :: ExceptT e m a

MonadThrow m => MonadThrow (ExceptT e m)

Throws exceptions into the base monad.

Defined in Control.Monad.Catch

Methods

throwM :: Exception e0 => e0 -> ExceptT e m a

MonadCatch m => MonadCatch (ExceptT e m)

Catches exceptions from the base monad.

Defined in Control.Monad.Catch

Methods

catch :: Exception e0 => ExceptT e m a -> (e0 -> ExceptT e m a) -> ExceptT e m a

MonadMask m => MonadMask (ExceptT e m)

Since: exceptions-0.9.0

Defined in Control.Monad.Catch

Methods

mask :: ((forall a. ExceptT e m a -> ExceptT e m a) -> ExceptT e m b) -> ExceptT e m b

uninterruptibleMask :: ((forall a. ExceptT e m a -> ExceptT e m a) -> ExceptT e m b) -> ExceptT e m b

generalBracket :: ExceptT e m a -> (a -> ExitCase b -> ExceptT e m c) -> (a -> ExceptT e m b) -> ExceptT e m (b, c)

PrimMonad m => PrimMonad (ExceptT e m)

Defined in Control.Monad.Primitive

Associated Types

type PrimState (ExceptT e m) :: *

Methods

primitive :: (State# (PrimState (ExceptT e m)) -> (#State# (PrimState (ExceptT e m)), a#)) -> ExceptT e m a

(Functor m, Monad m) => Apply (ExceptT e m)

Defined in Data.Functor.Bind.Class

Methods

(<.>) :: ExceptT e m (a -> b) -> ExceptT e m a -> ExceptT e m b

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

(<.) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m a

liftF2 :: (a -> b -> c) -> ExceptT e m a -> ExceptT e m b -> ExceptT e m c

MonadManaged m => MonadManaged (ExceptT e m)

Defined in Control.Monad.Managed

Methods

using :: Managed a -> ExceptT e m a

MonadCont m => MonadCont (ExceptT e m)

Since: mtl-2.2

Defined in Control.Monad.Cont.Class

Methods

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

Pointed m => Pointed (ExceptT e m)

Defined in Data.Pointed

Methods

point :: a -> ExceptT e m a

(Bind f, Monad f, Semigroup e, Monoid e) => Plus (ExceptT e f)

Defined in Data.Functor.Plus

Methods

zero :: ExceptT e f a

(Bind f, Monad f, Semigroup e) => Alt (ExceptT e f)

Defined in Data.Functor.Alt

Methods

(<!>) :: ExceptT e f a -> ExceptT e f a -> ExceptT e f a

some :: Applicative (ExceptT e f) => ExceptT e f a -> ExceptT e f [a]

many :: Applicative (ExceptT e f) => ExceptT e f a -> ExceptT e f [a]

(Functor m, Monad m) => Bind (ExceptT e m)

Defined in Data.Functor.Bind.Class

Methods

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

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

Zoom m n s t => Zoom (ExceptT e m) (ExceptT e n) s t

Defined in Control.Lens.Zoom

Methods

zoom :: LensLike' (Zoomed (ExceptT e m) c) t s -> ExceptT e m c -> ExceptT e n c

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

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)

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)

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)

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

Wrapped (ExceptT e m a)

Defined in Control.Lens.Wrapped

Associated Types

type Unwrapped (ExceptT e m a) :: *

Methods

_Wrapped' :: Iso' (ExceptT e m a) (Unwrapped (ExceptT e m a))

t ~ ExceptT e' m' a' => Rewrapped (ExceptT e m a) t

Defined in Control.Lens.Wrapped

type Zoomed (ExceptT e m)

Defined in Control.Lens.Zoom

type Zoomed (ExceptT e m) = FocusingErr e (Zoomed m)

type PrimState (ExceptT e m)

Defined in Control.Monad.Primitive

type PrimState (ExceptT e m) = PrimState m

type Unwrapped (ExceptT e m a)

Defined in Control.Lens.Wrapped

type Unwrapped (ExceptT e m a) = m (Either e a)

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

The inverse of ExceptT.

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

Map the unwrapped computation using the given function.

• runExceptT (mapExceptT f m) = f (runExceptT m)

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

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

MonadError

class Monad m => MonadError e (m :: * -> *) | m -> e where

The strategy of combining computations that can throw exceptions by bypassing bound functions from the point an exception is thrown to the point that it is handled.

Is parameterized over the type of error information and the monad type constructor. It is common to use Either String as the monad type constructor for an error monad in which error descriptions take the form of strings. In that case and many other common cases the resulting monad is already defined as an instance of the MonadError class. You can also define your own error type and/or use a monad type constructor other than Either String or Either IOError. In these cases you will have to explicitly define instances of the MonadError class. (If you are using the deprecated Control.Monad.Error or Control.Monad.Trans.Error, you may also have to define an Error instance.)

Minimal complete definition

throwError, catchError

Methods

throwError :: e -> m a

Is used within a monadic computation to begin exception processing.

catchError :: m a -> (e -> m a) -> m a

A handler function to handle previous errors and return to normal execution. A common idiom is:

do { action1; action2; action3 } `catchError` handler

where the action functions can call throwError. Note that handler and the do-block must have the same return type.

Instances

MonadError () Maybe

Since: mtl-2.2.2

Defined in Control.Monad.Error.Class

Methods

throwError :: () -> Maybe a

catchError :: Maybe a -> (() -> Maybe a) -> Maybe a

MonadError IOException IO

Defined in Control.Monad.Error.Class

Methods

throwError :: IOException -> IO a

catchError :: IO a -> (IOException -> IO a) -> IO a

MonadError e m => MonadError e (LogicT m)

Defined in Control.Monad.Logic

Methods

throwError :: e -> LogicT m a

catchError :: LogicT m a -> (e -> LogicT m a) -> LogicT m a

MonadError e m => MonadError e (ListT m)

Defined in List.Transformer

Methods

throwError :: e -> ListT m a

catchError :: ListT m a -> (e -> ListT m a) -> ListT m a

MonadError e m => MonadError e (MaybeT m)

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> MaybeT m a

catchError :: MaybeT m a -> (e -> MaybeT m a) -> MaybeT m a

MonadError e m => MonadError e (ListT m)

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> ListT m a

catchError :: ListT m a -> (e -> ListT m a) -> ListT m a

MonadError e (Either e)

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> Either e a

catchError :: Either e a -> (e -> Either e a) -> Either e a

(Functor m, MonadError e m) => MonadError e (Free m)

Defined in Control.Monad.Free

Methods

throwError :: e -> Free m a

catchError :: Free m a -> (e -> Free m a) -> Free m a

(Monoid w, MonadError e m) => MonadError e (WriterT w m)

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> WriterT w m a

catchError :: WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a

(Monoid w, MonadError e m) => MonadError e (WriterT w m)

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> WriterT w m a

catchError :: WriterT w m a -> (e -> WriterT w m a) -> WriterT w m a

MonadError e m => MonadError e (StateT s m)

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> StateT s m a

catchError :: StateT s m a -> (e -> StateT s m a) -> StateT s m a

MonadError e m => MonadError e (StateT s m)

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> StateT s m a

catchError :: StateT s m a -> (e -> StateT s m a) -> StateT s m a

MonadError e m => MonadError e (IdentityT m)

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> IdentityT m a

catchError :: IdentityT m a -> (e -> IdentityT m a) -> IdentityT m a

Monad m => MonadError e (ExceptT e m)

Since: mtl-2.2

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> ExceptT e m a

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

(Monad m, Error e) => MonadError e (ErrorT e m)

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> ErrorT e m a

catchError :: ErrorT e m a -> (e -> ErrorT e m a) -> ErrorT e m a

(Functor f, MonadError e m) => MonadError e (FreeT f m)

Defined in Control.Monad.Trans.Free

Methods

throwError :: e -> FreeT f m a

catchError :: FreeT f m a -> (e -> FreeT f m a) -> FreeT f m a

(Stream s, MonadError e' m) => MonadError e' (ParsecT e s m)

Defined in Text.Megaparsec.Internal

Methods

throwError :: e' -> ParsecT e s m a

catchError :: ParsecT e s m a -> (e' -> ParsecT e s m a) -> ParsecT e s m a

MonadError e m => MonadError e (ReaderT r m)

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> ReaderT r m a

catchError :: ReaderT r m a -> (e -> ReaderT r m a) -> ReaderT r m a

(Monoid w, MonadError e m) => MonadError e (RWST r w s m)

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> RWST r w s m a

catchError :: RWST r w s m a -> (e -> RWST r w s m a) -> RWST r w s m a

(Monoid w, MonadError e m) => MonadError e (RWST r w s m)

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> RWST r w s m a

catchError :: RWST r w s m a -> (e -> RWST r w s m a) -> RWST r w s m a

liftEither :: MonadError e m => Either e a -> m a

Lifts an Either e into any MonadError e.

do { val <- liftEither =<< action1; action2 }

where action1 returns an Either to represent errors.

Since: mtl-2.2.2