mtl-2.3.1: Monad classes for transformers, using functional dependencies
Copyright (c) Michael Weber 2001(c) Jeff Newbern 2003-2006(c) Andriy Palamarchuk 2006(c) Edward Kmett 2012 BSD-style (see the file LICENSE) libraries@haskell.org experimental non-portable (multi-parameter type classes) Safe Haskell2010

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

Synopsis

# Documentation

class Monad m => MonadError e m | m -> e where Source #

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 Source #

Is used within a monadic computation to begin exception processing.

catchError :: m a -> (e -> m a) -> m a Source #

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.

#### Instances

Instances details

liftEither :: MonadError e m => Either e a -> m a Source #

Lifts an Either e into any MonadError e.

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

where action1 returns an Either to represent errors.

Since: mtl-2.2.2

tryError :: MonadError e m => m a -> m (Either e a) Source #

MonadError analogue to the try function.

withError :: MonadError e m => (e -> e) -> m a -> m a Source #

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.

handleError :: MonadError e m => (e -> m a) -> m a -> m a Source #

As handle is flipped catch, handleError is flipped catchError.

mapError :: (MonadError e m, MonadError e' n) => (m (Either e a) -> n (Either e' b)) -> m a -> n b Source #

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.

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

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.

Since: mtl-2.3.1