| 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 | libraries@haskell.org |
| Stability | experimental |
| Portability | non-portable (multi-parameter type classes) |
| Safe Haskell | Safe |
| Language | Haskell2010 |
Control.Monad.Error
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:
-
EitherString a
The Error monad (also called the Exception monad).
Synopsis
-
class
Monad
m =>
MonadError
e m | m -> e
where
- throwError :: e -> m a
- catchError :: m a -> (e -> m a) -> m a
- class Error a where
- newtype ErrorT e (m :: Type -> Type ) a = ErrorT (m ( Either e a))
- runErrorT :: ErrorT e m a -> m ( Either e a)
- mapErrorT :: (m ( Either e a) -> n ( Either e' b)) -> ErrorT e m a -> ErrorT e' n b
- module Control.Monad
- module Control.Monad.Fix
- module Control.Monad.Trans
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
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
Either
String
MonadError
class.
You can also define your own error type and/or use a monad type constructor
other than
or
Either
String
.
In these cases you will have to explicitly define instances of the
Either
IOError
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
Minimal complete definition
Nothing
Methods
Creates an exception without a message.
The default implementation is
.
strMsg
""
Instances
| Error IOException | |
|
Defined in Control.Monad.Trans.Error |
|
| ErrorList a => Error [a] |
A string can be thrown as an error. |
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.
Instances
| ( Error e, MonadRWS r w s m) => MonadRWS r w s ( ErrorT e m) Source # | |
|
Defined in Control.Monad.RWS.Class |
|
| ( Monad m, Error e) => MonadError e ( ErrorT e m) Source # | |
|
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) Source # | |
| ( Error e, MonadState s m) => MonadState s ( ErrorT e m) Source # | |
| ( Error e, MonadWriter w m) => MonadWriter w ( ErrorT e m) Source # | |
| MonadTrans ( ErrorT e) | |
| ( Monad m, Error e) => Monad ( ErrorT e m) | |
| Functor m => Functor ( ErrorT e m) | |
| ( MonadFix m, Error e) => MonadFix ( ErrorT e m) | |
| ( Monad m, Error e) => MonadFail ( ErrorT e m) | |
| ( Functor m, Monad m) => Applicative ( ErrorT e m) | |
|
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) | |
|
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 # |
|
| Traversable f => Traversable ( ErrorT e f) | |
|
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) | |
| ( Eq e, Eq1 m) => Eq1 ( ErrorT e m) | |
| ( Ord e, Ord1 m) => Ord1 ( ErrorT e m) | |
|
Defined in Control.Monad.Trans.Error |
|
| ( Read e, Read1 m) => Read1 ( ErrorT e m) | |
|
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) | |
| ( Error e, MonadIO m) => MonadIO ( ErrorT e m) | |
| ( Functor m, Monad m, Error e) => Alternative ( ErrorT e m) | |
| ( Monad m, Error e) => MonadPlus ( ErrorT e m) | |
| ( Error e, MonadCont m) => MonadCont ( ErrorT e m) Source # | |
| ( Eq e, Eq1 m, Eq a) => Eq ( ErrorT e m a) | |
| ( Ord e, Ord1 m, Ord a) => Ord ( ErrorT e m a) | |
|
Defined in Control.Monad.Trans.Error Methods compare :: ErrorT e m a -> ErrorT e m a -> Ordering Source # (<) :: ErrorT e m a -> ErrorT e m a -> Bool Source # (<=) :: ErrorT e m a -> ErrorT e m a -> Bool Source # (>) :: ErrorT e m a -> ErrorT e m a -> Bool Source # (>=) :: ErrorT e m a -> ErrorT e m a -> Bool Source # max :: ErrorT e m a -> ErrorT e m a -> ErrorT e m a Source # min :: ErrorT e m a -> ErrorT e m a -> ErrorT e m a Source # |
|
| ( Read e, Read1 m, Read a) => Read ( ErrorT e m a) | |
| ( Show e, Show1 m, Show a) => Show ( ErrorT e m a) | |
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
monad transformer can be used to add error handling to another monad.
Here is an example how to combine it with an
ErrorT
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))