Portability  nonportable (type families) 

Stability  experimental 
Maintainer  ross@soi.city.ac.uk 
Safe Haskell  SafeInferred 
 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).
 class Monad m => MonadError m where
 type ErrorType m
 throwError :: ErrorType m > m a
 catchError :: m a > (ErrorType m > m a) > m a
 class Error a
 newtype ErrorT e m a = ErrorT {}
 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 m whereSource
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
StringMonadError
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
IOErrorError
and/or MonadError
classes.
throwError :: ErrorType m > m aSource
Is used within a monadic computation to begin exception processing.
catchError :: m a > (ErrorType m > m a) > m aSource
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 doblock must have the same return type.
MonadError IO  
Error e => MonadError (Either e)  
MonadError m => MonadError (MaybeT m)  
MonadError m => MonadError (ListT m)  
MonadError m => MonadError (IdentityT m)  
(Monoid w, MonadError m) => MonadError (WriterT w m)  
(Monoid w, MonadError m) => MonadError (WriterT w m)  
MonadError m => MonadError (StateT s m)  
MonadError m => MonadError (StateT s m)  
MonadError m => MonadError (ReaderT r m)  
(Monad m, Error e) => MonadError (ErrorT e m)  
(Monoid w, MonadError m) => MonadError (RWST r w s m)  
(Monoid w, MonadError m) => MonadError (RWST r w s m) 
class Error a
Error IOException  
ErrorList a => Error [a]  A string can be thrown as an error. 
The ErrorT monad transformer
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.
Error e => 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) => MonadPlus (ErrorT e m)  
(Functor m, Monad m) => Applicative (ErrorT e m)  
Foldable f => Foldable (ErrorT e f)  
Traversable f => Traversable (ErrorT e f)  
(Functor m, Monad m, Error e) => Alternative (ErrorT e m)  
(Error e, MonadIO m) => MonadIO (ErrorT e m)  
(Error e, MonadCont m) => MonadCont (ErrorT e m)  
(Monad m, Error e) => MonadError (ErrorT e m)  
(Error e, MonadReader m) => MonadReader (ErrorT e m)  
(Error e, MonadState m) => MonadState (ErrorT e m)  
(Error e, MonadWriter m) => MonadWriter (ErrorT e m)  
(Error e, MonadRWS m) => MonadRWS (ErrorT e m) 
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 nonempty 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 nonempty 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))