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GHC 9.10.3 · lts/ghc-9.10.x · c74966e · 2026-09-27

Modulemtl-2.3.1Haskell2010

Control.Monad.Except

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).

  • 2 types
  • 1 class
  • 12 values
  • Packagemtl-2.3.1
  • Exports15
  • LanguageHaskell2010
  • LicenceBSD-3-Clause
  • SourceExcept.hs

Warning

7 declarations

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.

classclass Monad m => MonadError e (m :: Type -> Type) | 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 MonadError class. (If you are using the deprecated Control.Monad.Error or Control.Monad.Trans.Error, you may also have to define an Error instance.)

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 do-block must have the same return type.

Instances16MonadError, …
valuewithError :: MonadError e m => (e -> e) -> m a -> m a
#

MonadError analogue to the withExceptT function. Modify the value (but not the type) of an error. The type is fixed because of the functional dependency m -> e. If you need to change the type of e use mapError or modifyError.

valuemodifyError :: MonadError e' m => (e -> e') -> ExceptT e m a -> m a
#

A different MonadError analogue to the withExceptT function. Modify the value (and possibly the type) of an error in an ExceptT-transformed monad, while stripping the ExceptT layer.

This is useful for adapting the MonadError constraint of a computation.

For example:

data DatabaseError = ...

performDatabaseQuery :: (MonadError DatabaseError m, ...) => m PersistedValue

data AppError
  = MkDatabaseError DatabaseError
  | ...

app :: (MonadError AppError m, ...) => m ()

Given these types, performDatabaseQuery cannot be used directly inside app, because the error types don't match. Using modifyError, an equivalent function with a different error type can be constructed:

performDatabaseQuery' :: (MonadError AppError m, ...) => m PersistedValue
performDatabaseQuery' = modifyError MkDatabaseError performDatabaseQuery

Since the error types do match, performDatabaseQuery' _can_ be used in app, assuming all other constraints carry over.

This works by instantiating the m in the type of performDatabaseQuery to ExceptT DatabaseError m', which satisfies the MonadError DatabaseError constraint. Immediately, the ExceptT DatabaseError layer is unwrapped, producing Either a DatabaseError or a PersistedValue. If it's the former, the error is wrapped in MkDatabaseError and re-thrown in the inner monad, otherwise the result value is returned.

The ExceptT monad transformer

8 declarations
newtypenewtype ExceptT e (m :: Type -> Type) a
#

A monad transformer that adds exceptions to other monads.

ExceptT constructs a monad parameterized over two things:

  • e - The exception type.

  • m - The inner monad.

The return function yields a computation that produces the given value, while >>= sequences two subcomputations, exiting on the first exception.

Constructors

Instances33MonadRWS, Generic1, MonadAccum, MonadError, MonadReader, MonadState, …
typetype Except e = ExceptT e Identity
#

The parameterizable exception monad.

Computations are either exceptions or normal values.

The return function returns a normal value, while >>= exits on the first exception. For a variant that continues after an error and collects all the errors, see Errors.

valuewithExcept :: (e -> e') -> Except e a -> Except e' a
#

Transform any exceptions thrown by the computation using the given function (a specialization of withExceptT).

Example 1: Custom Error Data Type

0 declarations

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))

Example 2: Using ExceptT Monad Transformer

0 declarations

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))