Imagine I have the following list:
lst :: [(Bool, Maybe Integer)]
lst = [(True, Just 3), (True, Nothing), (False, Just 12)]
Using the lens library, I want to extract the elements of the tuples, but I only want it to succeed when the second element is Just. I want some optic, split that works like this:
> lst ^.. folded.split (_1.to not) (_2._Just)
[(False, 3), (True, 12)]
I can implement split myself like this:
split :: Getting (First a) s a -> Getting (First b) s b -> Fold s (a, b)
split a b = folding (\x -> (,) <$> (x ^? a) <*> (x ^? b))
…which seems to work. However, this seems like I must be reinventing the wheel. Is there something already provided by the lens library that accomplishes this in an equally nice way?