Portability  nonportable (type families) 

Stability  experimental 
Maintainer  libraries@haskell.org 
 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 Error a where
 class Monad m => MonadError m where
 type ErrorType m
 throwError :: ErrorType m > m a
 catchError :: m a > (ErrorType m > m a) > m a
Documentation
class Error a where
Error IOException  
ErrorList a => Error [a]  A string can be thrown as an error. 
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 Data.Either
StringMonadError
class.
You can also define your own error type and/or use a monad type constructor
other than
or Data.Either
String
.
In these cases you will have to explicitly define instances of the Data.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) 