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.

Example type:

Either String a

The Error monad (also called the Exception monad).

Since: mtl-2.2.1

Please do not confuse ExceptT and throwError with Exception / SomeException and catch, respectively. The latter are for exceptions built into GHC, by default, and are mostly used from within the IO monad. They do not interact with the "exceptions" in this package at all. This package allows you to define a new kind of exception control mechanism which does not necessarily need your code to be placed in the IO monad.

In short, all "catching" mechanisms in this library will be unable to catch exceptions thrown by functions in the Control.Exception module, and vice-versa.

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.

-- 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)

-- Attempts to calculate length and throws an error if the provided string is
-- empty or longer than 5 characters.
-- (Throwing an error in this monad means returning a 'Left'.)
calculateLength :: String -> LengthMonad Int
calculateLength [] = throwError EmptyString
calculateLength 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))

ExceptT 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.Except

-- 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 = ExceptT String IO

main = do
  -- runExceptT removes the ExceptT wrapper
  r <- runExceptT 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))