
Control.Monad.Error.Class  Portability  nonportable (multiparameter type classes)  Stability  experimental  Maintainer  libraries@haskell.org 



Description 
 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).


Synopsis 



Documentation 

class Error a where 
An exception to be thrown.
An instance must redefine at least one of noMsg, strMsg.
  Methods  noMsg :: a  Creates an exception without a message.
Default implementation is strMsg "".
  strMsg :: String > a  Creates an exception with a message.
Default implementation is noMsg.

  Instances  


class Monad m => MonadError e m  m > e where 
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 Error
and/or MonadError classes.
  Methods  throwError :: e > m a  Is used within a monadic computation to begin exception processing.
  catchError :: m a > (e > m a) > m a  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.

  Instances  


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