attempt- Concrete data type for handling extensible exceptions as failures.




A universal data type for computations which may fail. Errors are reported using extensible exceptions. These exceptions are not explicitly stated; if you want this kind of functionality, something like control-monad-exception might be a more appropriate fit.


Data type and type class

data Attempt v Source

Contains either a Success value or a Failure exception.


Success v 
forall e . Exception e => Failure e 

class FromAttempt a whereSource

Any type which can be converted from an Attempt. The included instances are your "usual suspects" for dealing with error handling. They include:

IO: For the IO instance, any exceptions in the Attempt are thrown as runtime exceptions.

Maybe: Returns Nothing on Failure, or Just on Success.

List: Returns the empty list on Failure, or a singleton list on Success.

Either String: Returns Left (show exception) on Failure, or Right on Success.

Either Exception: Returns Left exception on Failure, or Right on Success.


fromAttempt :: Attempt v -> a vSource

fa :: FromAttempt a => Attempt v -> a vSource

A shortcut for fromAttempt.

joinAttempt :: (FromAttempt m, Monad m) => m (Attempt v) -> m vSource

This is not a simple translation of the Control.Monad.join function. Instead, for Monads which are instances of FromAttempt, it removes the inner Attempt type, reporting errors as defined in the FromAttempt instance.

For example, join (Just (failureString "foo")) == Nothing.

General handling of Attempts



:: (forall e. Exception e => e -> b)

error handler

-> (a -> b)

success handler

-> Attempt a 
-> b 

Process either the exception or value in an Attempt to produce a result.

This function is modeled after maybe and either. The first argument must accept any instances of Exception. If you want to handle multiple types of exceptions, see makeHandler. The second argument converts the success value.

Note that this function does not expose all the data available in an Attempt value. Notably, the monadic stack trace is not passed on to the error handler. If desired, use the monadicStackTrace function to extract it.

makeHandler :: [AttemptHandler v] -> v -> forall e. Exception e => e -> vSource

Convert multiple AttemptHandlers and a default value into an exception handler.

This is a convenience function when you want to have special handling for a few types of Exceptions and provide another value for anything else.

data AttemptHandler v Source

A simple wrapper value necesary due to the Haskell type system. Wraps a function from a *specific* Exception type to some value.


forall e . Exception e => AttemptHandler (e -> v) 

Individual Attempts

isFailure :: Attempt v -> BoolSource

Tests for a Failure value.

isSuccess :: Attempt v -> BoolSource

Tests for a Success value.

fromSuccess :: Attempt v -> vSource

This is an unsafe, partial function which should only be used if you either know that a function will succeed or don't mind the occassional runtime exception.

Lists of Attempts

successes :: [Attempt v] -> [v]Source

Returns only the Success values.

failures :: [Attempt v] -> [SomeException]Source

Returns only the Failure values, each wrapped in a SomeException.

partitionAttempts :: [Attempt v] -> ([SomeException], [v])Source

Return all of the Failures and Successes separately in a tuple.

Runtime exceptions

attemptIO :: (Exception eIn, Exception eOut) => (eIn -> eOut) -> IO v -> IO (Attempt v)Source

Catches runtime (ie, IO) exceptions and inserts them into an Attempt.

Like handle, the first argument to this function must explicitly state the type of its input.

Reexport the Failure class