Control.Monad.Error.Class

As handle is flipped catch, handleError is flipped catchError.

Lifts an Either e into any MonadError e.

do { val <- liftEither =<< action1; action2 }

where action1 returns an Either to represent errors.

MonadError analogue of the mapExceptT function. The computation is unwrapped, a function is applied to the Either, and the result is lifted into the second MonadError instance.

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.

MonadError analogue to the try function.

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.

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.)