Copyright	(c) Michael Weber <michael.weber@post.rwth-aachen.de> 2001, (c) Jeff Newbern 2003-2006, (c) Andriy Palamarchuk 2006
License	BSD-style (see the file LICENSE)
Maintainer	R.Paterson@city.ac.uk
Stability	experimental
Portability	portable
Safe Haskell	Safe
Language	Haskell98

Control.Monad.Trans.Error

Contents

The ErrorT monad transformer
Error operations
Lifting other operations
Examples

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.

Synopsis

The ErrorT monad transformer

class Error a where Source

An exception to be thrown.

Minimal complete definition: noMsg or strMsg.

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

Error IOException
Methods noMsg :: IOException Source strMsg :: String -> IOException Source
ErrorList a => Error [a]	A string can be thrown as an 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.

Minimal complete definition

listMsg

Methods

listMsg :: String -> [a] Source

Instances

ErrorList Char
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 runErrorT :: m (Either e a)

Instances

MonadTrans (ErrorT e)
Methods lift :: Monad m => m a -> ErrorT e m a Source
(Monad m, Error e) => Monad (ErrorT e m)
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 fail :: String -> ErrorT e m a Source
Functor m => Functor (ErrorT e m)
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)
Methods mfix :: (a -> ErrorT e m a) -> ErrorT e m a Source
(Monad m, Error e) => MonadFail (ErrorT e m)
Methods fail :: String -> ErrorT e m a Source
(Functor m, Monad m) => Applicative (ErrorT e m)
Methods pure :: a -> ErrorT e m a Source (<>) :: ErrorT e m (a -> b) -> ErrorT e m a -> ErrorT e m b 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)
Methods fold :: Monoid m => ErrorT e f m -> 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)
Methods traverse :: Applicative f => (a -> f b) -> ErrorT e f a -> f (ErrorT e f b) Source sequenceA :: Applicative f => ErrorT e f (f a) -> f (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
(Eq e, Eq1 m) => Eq1 (ErrorT e m)
Methods liftEq :: (a -> b -> Bool) -> ErrorT e m a -> ErrorT e m b -> Bool Source
(Ord e, Ord1 m) => Ord1 (ErrorT e m)
Methods liftCompare :: (a -> b -> Ordering) -> ErrorT e m a -> ErrorT e m b -> Ordering Source
(Read e, Read1 m) => Read1 (ErrorT e m)
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
(Show e, Show1 m) => Show1 (ErrorT e m)
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)
Methods liftIO :: IO a -> ErrorT e m a Source
(Functor m, Monad m, Error e) => Alternative (ErrorT e m)
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)
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)
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) 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)
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.

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

Error operations

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

Signal an error value e.

runErrorT (throwError e) = return (Left e)

```
throwError e >>= m = throwError 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.

```
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 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)))