mtl- Monad transformer library

Portabilitynon-portable (multi-parameter type classes)




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:
Data.Either String a

The Error monad (also called the Exception monad).



newtype ErrorT e m 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.

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)))




runErrorT :: m (Either e a)


(Error e, MonadRWS r w s m) => MonadRWS r w s (ErrorT e m) 
(Error e, MonadWriter w m) => MonadWriter w (ErrorT e m) 
(Monad m, Error e) => MonadError e (ErrorT e m) 
(Error e, MonadState s m) => MonadState s (ErrorT e m) 
(Error e, MonadReader r m) => MonadReader r (ErrorT e m) 
Error e => MonadTrans (ErrorT e) 
(Monad m, Error e) => Monad (ErrorT e m) 
Monad m => Functor (ErrorT e m) 
(MonadFix m, Error e) => MonadFix (ErrorT e m) 
(Monad m, Error e) => MonadPlus (ErrorT e m) 
(Error e, MonadIO m) => MonadIO (ErrorT e m) 
(Error e, MonadCont m) => MonadCont (ErrorT e m) 

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

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

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 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))