
Control.Monad.Error  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  


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.
Here are some examples of use:
 wraps IO action that can throw an error e
type ErrorWithIO e a = ErrorT e IO a
==> ErrorT (IO (Either e a))
 IO monad wrapped in StateT inside of ErrorT
type ErrorAndStateWithIO e s a = ErrorT e (StateT s IO) a
==> ErrorT (StateT s IO (Either e a))
==> ErrorT (StateT (s > IO (Either e a,s)))
 Constructors  ErrorT   runErrorT :: (m (Either e a))  

 Instances  


mapErrorT :: (m (Either e a) > n (Either e' b)) > ErrorT e m a > ErrorT e' n b 

Example 1: Custom Error Data Type


Here is an example that demonstrates the use of a custom Error data type with
the ErrorMonad's throwError and catchError exception mechanism.
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 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))


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