extensible-effects- An Alternative to Monad Transformers

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Exception-producing and exception-handling effects



newtype Exc e v Source #


exceptions of the type e; no resumption


Exc e 

type Fail = Exc () Source #

throwError :: Member (Exc e) r => e -> Eff r a Source #

Throw an exception in an effectful computation. The type is inferred.

die :: Member Fail r => Eff r a Source #

Makes an effect fail, preventing future effects from happening.

runError :: Eff (Exc e ': r) a -> Eff r (Either e a) Source #

Run a computation that might produce an exception.

runFail :: Eff (Fail ': r) a -> Eff r (Maybe a) Source #

Runs a failable effect, such that failed computation return Nothing, and Just the return value on success.

catchError :: Member (Exc e) r => Eff r a -> (e -> Eff r a) -> Eff r a Source #

Run a computation that might produce exceptions, and give it a way to deal with the exceptions that come up. The handler is allowed to rethrow the exception

onFail Source #


:: Eff (Fail ': r) a

The fallible computation.

-> Eff r a

The computation to run on failure.

-> Eff r a 

Add a default value (i.e. failure handler) to a fallible computation. This hides the fact that a failure happened.

rethrowError :: Member (Exc e') r => (e -> e') -> Eff (Exc e ': r) a -> Eff r a Source #

Run a computation until it produces an exception, and convert and throw that exception in a new context.

liftEither :: Member (Exc e) r => Either e a -> Eff r a Source #

Treat Lefts as exceptions and Rights as return values.

liftEitherM :: (Member (Exc e) r, SetMember Lift (Lift m) r) => m (Either e a) -> Eff r a Source #

liftEither in a lifted Monad

liftMaybe :: Member Fail r => Maybe a -> Eff r a Source #

Lift a maybe into the Fail effect, causing failure if it's Nothing.

liftMaybeM :: (Member Fail r, SetMember Lift (Lift m) r) => m (Maybe a) -> Eff r a Source #

liftMaybe in a lifted Monad

ignoreFail :: Eff (Fail ': r) a -> Eff r () Source #

Ignores a failure event. Since the event can fail, you cannot inspect its return type, because it has none on failure. To inspect it, use runFail.