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/Haskell 8

Control.Monad.Trans.Error

Copyright (c) Michael Weber <[email protected]> 2001
(c) Jeff Newbern 2003-2006
(c) Andriy Palamarchuk 2006
License BSD-style (see the file LICENSE)
Maintainer [email protected]
Stability experimental
Portability portable
Safe Haskell Safe
Language Haskell98

Description

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 Source

An exception to be thrown.

Minimal complete definition: noMsg or strMsg.

Minimal complete definition

Nothing

Methods

noMsg :: a Source

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

strMsg :: String -> a Source

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

Instances
Instances details
Error IOException
Instance details

Defined in Control.Monad.Trans.Error

Methods

noMsg :: IOException Source

strMsg :: String -> IOException Source

ErrorList a => Error [a]

A string can be thrown as an error.
Instance details

Defined in Control.Monad.Trans.Error

Methods

noMsg :: [a] Source

strMsg :: String -> [a] Source

class ErrorList a where Source

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

Methods

listMsg :: String -> [a] Source

Instances
Instances details
ErrorList Char
Instance details

Defined in Control.Monad.Trans.Error

Methods

listMsg :: String -> [Char] Source

newtype ErrorT e m a Source

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

Instances
Instances details
MonadTrans (ErrorT e)
Instance details

Defined in Control.Monad.Trans.Error

Methods

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

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

Defined in Control.Monad.Trans.Error

Methods

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

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

return :: a -> ErrorT e m a Source

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

Defined in Control.Monad.Trans.Error

Methods

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

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

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

Defined in Control.Monad.Trans.Error

Methods

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

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

Defined in Control.Monad.Trans.Error

Methods

fail :: String -> ErrorT e m a Source

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

Defined in Control.Monad.Trans.Error

Methods

pure :: a -> ErrorT e m a Source

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

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

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

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

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

Defined in Control.Monad.Trans.Error

Methods

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

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

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

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

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

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

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

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

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

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

null :: ErrorT e f a -> Bool Source

length :: ErrorT e f a -> Int Source

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

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

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

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

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

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

Defined in Control.Monad.Trans.Error

Methods

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

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

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

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

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

Defined in Control.Monad.Trans.Error

Methods

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

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

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

Defined in Control.Monad.Trans.Error

Methods

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

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

Defined in Control.Monad.Trans.Error

Methods

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

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

Defined in Control.Monad.Trans.Error

Methods

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

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

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

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

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

Defined in Control.Monad.Trans.Error

Methods

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

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

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

Defined in Control.Monad.Trans.Error

Methods

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

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

Defined in Control.Monad.Trans.Error

Methods

empty :: ErrorT e m a Source

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

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

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

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

Defined in Control.Monad.Trans.Error

Methods

mzero :: ErrorT e m a Source

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

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

Defined in Control.Monad.Trans.Error

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)
Instance details

Defined in Control.Monad.Trans.Error

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)
Instance details

Defined in Control.Monad.Trans.Error

Methods

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

readList :: ReadS [ErrorT e m a] Source

readPrec :: ReadPrec (ErrorT e m a) Source

readListPrec :: ReadPrec [ErrorT e m a] Source

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

Defined in Control.Monad.Trans.Error

Methods

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

show :: ErrorT e m a -> String Source

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

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

Map the unwrapped computation using the given function.

Error operations

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

Signal an error value e.

catchError Source

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.

Lifting other operations

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

Lift a callCC operation to the new monad.

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

Lift a listen operation to the new monad.

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

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)
Instance details

Methods

empty :: Either e a Source

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

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

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

Error e => MonadPlus (Either e)
Instance details

Methods

mzero :: Either e a Source

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

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.8.3/docs/html/libraries/transformers-0.5.6.2/Control-Monad-Trans-Error.html