A typeclass and some combinators to aid in the lifting of errors into your preferred context.
As an example, assuming we have the following error type:
data MyError
= WrongInt Int
| WrapLibError LibErrorThis package provides a combinator for straight forward translation of a general error case into a more specific type:
-- Given a function:
fn :: MonadIO m => Int -> m (Maybe b)
-- Working in some MonadError context..
... :: MonadError MyError m => m b
-- Handle the `Nothing` case by using an explicit constructor, with no intermediate boilerplate.
g :: (MonadIO m, MonadError MyError m) => Int -> m b
g n = fn n <!?> WrongInt nYou're able to run a function on the error as well:
-- Given...
fn :: MonadIO m => Int -> Either LibError b
-- Provide an `(e -> e')` function as required. In this case, wrapping a general
-- library error with our more relevant constructor.
g :: (MonadIO m, MonadError MyError m) => Int -> m b
g n = fn n <%!?> WrapLibError
