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Control.Monad.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 non-portable (multi-parameter type classes)
Safe Haskell Safe
Language Haskell2010

Description

Deprecated: Use Control.Monad.Except instead

Computation type:
Computations which may fail or throw exceptions.
Binding strategy:
Failure records information about the cause/location of the failure. Failure values bypass the bound function, other values are used as inputs to the bound function.
Useful for:
Building computations from sequences of functions that may fail or using exception handling to structure error handling.
Zero and plus:
Zero is represented by an empty error and the plus operation executes its second argument if the first fails.
Example type:
Either String a

The Error monad (also called the Exception monad).

Monads with error handling

class Monad m => MonadError e m | m -> e where Source

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.)

Methods

throwError :: e -> m a Source

Is used within a monadic computation to begin exception processing.

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

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

Instance details

Defined in Control.Monad.Error.Class

Methods

throwError :: () -> Maybe a Source

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

MonadError IOException IO
Instance details

Defined in Control.Monad.Error.Class

Methods

throwError :: IOException -> IO a Source

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

MonadError e m => MonadError e (MaybeT m)
Instance details

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> MaybeT m a Source

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

MonadError e m => MonadError e (ListT m)
Instance details

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> ListT m a Source

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

MonadError e (Either e)
Instance details

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> Either e a Source

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

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

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> WriterT w m a Source

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

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

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> WriterT w m a Source

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

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

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> StateT s m a Source

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

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

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> StateT s m a Source

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

MonadError e m => MonadError e (IdentityT m)
Instance details

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> IdentityT m a Source

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

Monad m => MonadError e (ExceptT e m)

Since: mtl-2.2

Instance details

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> ExceptT e m a Source

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

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

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> ErrorT e m a Source

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

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

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> ReaderT r m a Source

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

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

Defined in Control.Monad.Error.Class

Methods

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

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

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

Defined in Control.Monad.Error.Class

Methods

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

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

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
Error IOException
Instance details

Defined in Control.Monad.Trans.Error

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

The ErrorT monad transformer

newtype ErrorT e (m :: Type -> Type) 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 (m (Either e a))
Instances
(Error e, MonadRWS r w s m) => MonadRWS r w s (ErrorT e m)
Instance details

Defined in Control.Monad.RWS.Class

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

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> ErrorT e m a Source

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

(Error e, MonadReader r m) => MonadReader r (ErrorT e m)
Instance details

Defined in Control.Monad.Reader.Class

Methods

ask :: ErrorT e m r Source

local :: (r -> r) -> ErrorT e m a -> ErrorT e m a Source

reader :: (r -> a) -> ErrorT e m a Source

(Error e, MonadState s m) => MonadState s (ErrorT e m)
Instance details

Defined in Control.Monad.State.Class

Methods

get :: ErrorT e m s Source

put :: s -> ErrorT e m () Source

state :: (s -> (a, s)) -> ErrorT e m a Source

(Error e, MonadWriter w m) => MonadWriter w (ErrorT e m)
Instance details

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> ErrorT e m a Source

tell :: w -> ErrorT e m () Source

listen :: ErrorT e m a -> ErrorT e m (a, w) Source

pass :: ErrorT e m (a, w -> w) -> ErrorT e m a Source

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

fail :: String -> 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

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

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

Defined in Control.Monad.Cont.Class

Methods

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

(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

runErrorT :: ErrorT e m a -> m (Either e a) 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.

module Control.Monad

module Control.Monad.Fix

module Control.Monad.Trans

Example 1: Custom Error Data Type

Here is an example that demonstrates the use of a custom Error data type with the throwError and catchError exception mechanism from MonadError. The example throws an exception if the user enters an empty string or a string longer than 5 characters. Otherwise it prints length of the string.

-- This is the type to represent length calculation error.
data LengthError = EmptyString  -- Entered string was empty.
          | StringTooLong Int   -- A string is longer than 5 characters.
                                -- Records a length of the string.
          | OtherError String   -- Other error, stores the problem description.

-- We make LengthError an instance of the Error class
-- to be able to throw it as an exception.
instance Error LengthError where
  noMsg    = OtherError "A String Error!"
  strMsg s = OtherError s

-- Converts LengthError to a readable message.
instance Show LengthError where
  show EmptyString = "The string was empty!"
  show (StringTooLong len) =
      "The length of the string (" ++ (show len) ++ ") is bigger than 5!"
  show (OtherError msg) = msg

-- For our monad type constructor, we use Either LengthError
-- which represents failure using Left LengthError
-- or a successful result of type a using Right a.
type LengthMonad = Either LengthError

main = do
  putStrLn "Please enter a string:"
  s <- getLine
  reportResult (calculateLength s)

-- Wraps length calculation to catch the errors.
-- Returns either length of the string or an error.
calculateLength :: String -> LengthMonad Int
calculateLength s = (calculateLengthOrFail s) `catchError` Left

-- Attempts to calculate length and throws an error if the provided string is
-- empty or longer than 5 characters.
-- The processing is done in Either monad.
calculateLengthOrFail :: String -> LengthMonad Int
calculateLengthOrFail [] = throwError EmptyString
calculateLengthOrFail s | len > 5 = throwError (StringTooLong len)
                        | otherwise = return len
  where len = length s

-- Prints result of the string length calculation.
reportResult :: LengthMonad Int -> IO ()
reportResult (Right len) = putStrLn ("The length of the string is " ++ (show len))
reportResult (Left e) = putStrLn ("Length calculation failed with error: " ++ (show e))

Example 2: Using ErrorT Monad Transformer

ErrorT monad transformer can be used to add error handling to another monad. Here is an example how to combine it with an IO monad:

import Control.Monad.Error

-- An IO monad which can return String failure.
-- It is convenient to define the monad type of the combined monad,
-- especially if we combine more monad transformers.
type LengthMonad = ErrorT String IO

main = do
  -- runErrorT removes the ErrorT wrapper
  r <- runErrorT calculateLength
  reportResult r

-- Asks user for a non-empty string and returns its length.
-- Throws an error if user enters an empty string.
calculateLength :: LengthMonad Int
calculateLength = do
  -- all the IO operations have to be lifted to the IO monad in the monad stack
  liftIO $ putStrLn "Please enter a non-empty string: "
  s <- liftIO getLine
  if null s
    then throwError "The string was empty!"
    else return $ length s

-- Prints result of the string length calculation.
reportResult :: Either String Int -> IO ()
reportResult (Right len) = putStrLn ("The length of the string is " ++ (show len))
reportResult (Left e) = putStrLn ("Length calculation failed with error: " ++ (show e))

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.6.1/docs/html/libraries/mtl-2.2.2/Control-Monad-Error.html