Control.Monad.Trans.Error

Deprecated: Use Control.Monad.Trans.Except instead

This monad transformer adds the ability to fail or throw exceptions to a monad.

A sequence of actions succeeds, producing a value, only if all the actions in the sequence are successful. If one fails with an error, the rest of the sequence is skipped and the composite action fails with that error.

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

Note: This module will be removed in a future release. Instead, use Control.Monad.Trans.Except, which does not restrict the exception type, and also includes a base exception monad.

The ErrorT monad transformer

class Error a where

An exception to be thrown.

Minimal complete definition: noMsg or strMsg.

Methods

noMsg :: a

Creates an exception without a message. The default implementation is strMsg "".

strMsg :: String -> a

Creates an exception with a message. The default implementation of strMsg s is noMsg.

Instances

Error IOException

Methods

noMsg :: IOException

strMsg :: String -> IOException

ErrorList a => Error [a]

A string can be thrown as an error.

Methods

noMsg :: [a]

strMsg :: String -> [a]

class ErrorList a where

Workaround so that we can have a Haskell 98 instance Error String.

Minimal complete definition

listMsg

Methods

listMsg :: String -> [a]

Instances

ErrorList Char

Methods

listMsg :: String -> [Char]

newtype ErrorT e m a

The error monad transformer. It can be used to add error handling to other monads.

The ErrorT Monad structure is parameterized over two things:

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

The return function yields a successful computation, while >>= sequences two subcomputations, failing on the first error.

Constructors

ErrorT

Fields

• runErrorT :: m (Either e a)

Instances

MonadTrans (ErrorT e)

Methods

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

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

Methods

(>>=) :: ErrorT e m a -> (a -> ErrorT e m b) -> ErrorT e m b

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

return :: a -> ErrorT e m a

fail :: String -> ErrorT e m a

Functor m => Functor (ErrorT e m)

Methods

fmap :: (a -> b) -> ErrorT e m a -> ErrorT e m b

(<$) :: a -> ErrorT e m b -> ErrorT e m a

(MonadFix m, Error e) => MonadFix (ErrorT e m)

Methods

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

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

Methods

fail :: String -> ErrorT e m a

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

Methods

pure :: a -> ErrorT e m a

(<*>) :: ErrorT e m (a -> b) -> ErrorT e m a -> ErrorT e m b

liftA2 :: (a -> b -> c) -> ErrorT e m a -> ErrorT e m b -> ErrorT e m c

(*>) :: ErrorT e m a -> ErrorT e m b -> ErrorT e m b

(<*) :: ErrorT e m a -> ErrorT e m b -> ErrorT e m a

Foldable f => Foldable (ErrorT e f)

Methods

fold :: Monoid m => ErrorT e f m -> m

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

foldr :: (a -> b -> b) -> b -> ErrorT e f a -> b

foldr' :: (a -> b -> b) -> b -> ErrorT e f a -> b

foldl :: (b -> a -> b) -> b -> ErrorT e f a -> b

foldl' :: (b -> a -> b) -> b -> ErrorT e f a -> b

foldr1 :: (a -> a -> a) -> ErrorT e f a -> a

foldl1 :: (a -> a -> a) -> ErrorT e f a -> a

toList :: ErrorT e f a -> [a]

null :: ErrorT e f a -> Bool

length :: ErrorT e f a -> Int

elem :: Eq a => a -> ErrorT e f a -> Bool

maximum :: Ord a => ErrorT e f a -> a

minimum :: Ord a => ErrorT e f a -> a

sum :: Num a => ErrorT e f a -> a

product :: Num a => ErrorT e f a -> a

Traversable f => Traversable (ErrorT e f)

Methods

traverse :: Applicative f => (a -> f b) -> ErrorT e f a -> f (ErrorT e f b)

sequenceA :: Applicative f => ErrorT e f (f a) -> f (ErrorT e f a)

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

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

(Eq e, Eq1 m) => Eq1 (ErrorT e m)

Methods

liftEq :: (a -> b -> Bool) -> ErrorT e m a -> ErrorT e m b -> Bool

(Ord e, Ord1 m) => Ord1 (ErrorT e m)

Methods

liftCompare :: (a -> b -> Ordering) -> ErrorT e m a -> ErrorT e m b -> Ordering

(Read e, Read1 m) => Read1 (ErrorT e m)

Methods

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

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

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

liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [ErrorT e m a]

(Show e, Show1 m) => Show1 (ErrorT e m)

Methods

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

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

(Error e, MonadIO m) => MonadIO (ErrorT e m)

Methods

liftIO :: IO a -> ErrorT e m a

(Functor m, Monad m, Error e) => Alternative (ErrorT e m)

Methods

empty :: ErrorT e m a

(<|>) :: ErrorT e m a -> ErrorT e m a -> ErrorT e m a

some :: ErrorT e m a -> ErrorT e m [a]

many :: ErrorT e m a -> ErrorT e m [a]

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

Methods

mzero :: ErrorT e m a

mplus :: ErrorT e m a -> ErrorT e m a -> ErrorT e m a

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

Methods

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

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

(Ord e, Ord1 m, Ord a) => Ord (ErrorT e m a)

Methods

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

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

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

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

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

max :: ErrorT e m a -> ErrorT e m a -> ErrorT e m a

min :: ErrorT e m a -> ErrorT e m a -> ErrorT e m a

(Read e, Read1 m, Read a) => Read (ErrorT e m a)

Methods

readsPrec :: Int -> ReadS (ErrorT e m a)

readList :: ReadS [ErrorT e m a]

readPrec :: ReadPrec (ErrorT e m a)

readListPrec :: ReadPrec [ErrorT e m a]

(Show e, Show1 m, Show a) => Show (ErrorT e m a)

Methods

showsPrec :: Int -> ErrorT e m a -> ShowS

show :: ErrorT e m a -> String

showList :: [ErrorT e m a] -> ShowS

mapErrorT :: (m (Either e a) -> n (Either e' b)) -> ErrorT e m a -> ErrorT e' n b

Map the unwrapped computation using the given function.

• runErrorT (mapErrorT f m) = f (runErrorT m)

Error operations

throwError :: Monad m => e -> ErrorT e m a

Signal an error value e.

• runErrorT (throwError e) = return (Left e)

• throwError e >>= m = throwError e

catchError

Arguments

:: Monad m

=> ErrorT e m a

the inner computation

-> (e -> ErrorT e m a)

a handler for errors in the inner computation

-> ErrorT e m a

Handle an error.

• catchError h (lift m) = lift m

• catchError h (throwError e) = h e

Lifting other operations

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

Lift a callCC operation to the new monad.

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

Lift a listen operation to the new monad.

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

Lift a pass operation to the new monad.

Examples

Wrapping an IO action that can throw an error e:

type ErrorWithIO e a = ErrorT e IO a
==> ErrorT (IO (Either e a))

An IO monad wrapped in StateT inside of ErrorT:

type ErrorAndStateWithIO e s a = ErrorT e (StateT s IO) a
==> ErrorT (StateT s IO (Either e a))
==> ErrorT (StateT (s -> IO (Either e a,s)))

Orphan instances

Error e => Alternative (Either e)

Methods

empty :: Either e a

(<|>) :: Either e a -> Either e a -> Either e a

some :: Either e a -> Either e [a]

many :: Either e a -> Either e [a]

Error e => MonadPlus (Either e)

Methods

mzero :: Either e a

mplus :: Either e a -> Either e a -> Either e a