| Safe Haskell | Safe-Inferred |
|---|---|
| Language | Haskell98 |
Data.Attempt
Contents
Description
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 Attempt v
- class FromAttempt a where
- fromAttempt :: Attempt v -> a v
- fa :: FromAttempt a => Attempt v -> a v
- joinAttempt :: (FromAttempt m, Monad m) => m (Attempt v) -> m v
- attempt :: (forall e. Exception e => e -> b) -> (a -> b) -> Attempt a -> b
- makeHandler :: [AttemptHandler v] -> v -> forall e. Exception e => e -> v
- data AttemptHandler v = forall e . Exception e => AttemptHandler (e -> v)
- isFailure :: Attempt v -> Bool
- isSuccess :: Attempt v -> Bool
- fromSuccess :: Attempt v -> v
- successes :: [Attempt v] -> [v]
- failures :: [Attempt v] -> [SomeException]
- partitionAttempts :: [Attempt v] -> ([SomeException], [v])
- attemptIO :: (Exception eIn, Exception eOut) => (eIn -> eOut) -> IO v -> IO (Attempt v)
- module Control.Failure
Data type and type class
class FromAttempt a where Source
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.
Methods
fromAttempt :: Attempt v -> a v Source
Instances
fa :: FromAttempt a => Attempt v -> a v Source
A shortcut for fromAttempt.
joinAttempt :: (FromAttempt m, Monad m) => m (Attempt v) -> m v Source
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
Arguments
| :: (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 -> v Source
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.
Constructors
| forall e . Exception e => AttemptHandler (e -> v) |
Individual Attempts
fromSuccess :: Attempt v -> v Source
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
failures :: [Attempt v] -> [SomeException] Source
Returns only the Failure values, each wrapped in a SomeException.
partitionAttempts :: [Attempt v] -> ([SomeException], [v]) Source
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
module Control.Failure