A variant on traverse that allows the targets to be filtered out of the Witherable structure. Note that this introduces a change in types down the lens composition chain, which means that it is not a a valid optic at all. The use of Withering in the changed type also means that standard lens combinators don't fit

To address these issues, you can use unwithered to strip the Withering type back out. This allows the composed optic to be used with standard combinators from lens. In addition, the sequence withered . unwithered will act like a type-restricted version of traverse for all lawful instances of Witherable.

In some sense, this is a catch-like combinator. This marks the point where removing elements stops propagating and actually modifies the structure being focused.

Restore types in a lens composition chain that has had Withering introduced. Makes no changes to what elements are focused on.

A variant of withered for when you're already working in a Withering chain and want to change what structure elements are being removed from.

rewithered = unwithered . withered

The trivial optic in a Withering chain that removes everything.

The arguments are unused.

Remove elements from the current Withering context if they don't match the predicate. This is similar in concept to filtered from lens but instead of merely removing non-matching targets from the traversal, it removes those targets and their parents up to the next withered combinator.

Transform and effectfully filter elements matched by a specific Withering context, a la wither.

>>> witherOf (withered . _1) (\x -> do b <- readLn ; if b then pure (Just (show x)) else pure Nothing) [(1,2),(2,3),(3,4)]
False
True
True
[("2",3),("3",4)]

A version of witherOf with arguments re-arranged.

Transform and filter elements matched by a specific Withering context, a la mapMaybe. See witherOf for a more flexible version that works within arbitrary Applicative effects.

>>> mapMaybeOf (withered . _1) (\x -> if even x then Just (show x) else Nothing) [(1,2),(2,4),(3,6),(4,8)]
[("2",4),("4",8)]

Filter Nothing values out of a structure, like catMaybes.

>>> catMaybesOf withered [Just 1, Nothing, Just 2]
[1,2]

>>> catMaybesOf (withered . _2) [("a", Just 1), ("b", Nothing), ("c", Just 2)]
[("a",1),("c",2)]

Remove elements matched by a specific Withering context if they don't match a predicate returning a result in an arbitrary Applicative context.

>>> filterAOf (withered . _1) (const readLn) [(1,2),(2,4),(3,6),(4,8)]
False
True
True
False
[(2,4),(3,6)]

Remove elements matched by a specific Withering context if they don't match a predicate.

>>> filterOf (withered . _1) even [(1,2),(2,4),(3,6),(4,8)]
[(2,4),(4,8)]

Removes duplicates from a structure based on the focused element.

>>> ordNubOf (traverse . withered . _2) [[("z",3),("q",3)],[("apple", 1), ("bat", 1), ("cat", 2), ("dog", 2)]]
[[("z",3)],[("apple",1),("cat",2)]]

Removes duplicates from a structure based on applying a function to the focused element.

>>> ordNubOnOf (withered . _2) (`div` 2) [("apple", 1), ("bat", 2), ("cat", 3), ("dog", 4)]
[("apple",1),("bat",2),("dog",4)]

Removes duplicates from a structure based on the focused element. Often will be more efficient than ordNubOf if your data type supports Hashable

>>> hashNubOf (traverse . withered . _2) [[("z",3),("q",3)],[("apple", 1), ("bat", 1), ("cat", 2), ("dog", 2)]]
[[("z",3)],[("apple",1),("cat",2)]]

Removes duplicates from a structure based on applying a function to the focused element. Often will be more efficient than ordNubOnOf if your data type supports Hashable

>>> hashNubOnOf (withered . _2) (`div` 2) [("apple", 1), ("bat", 2), ("cat", 3), ("dog", 4)]
[("apple",1),("bat",2),("dog",4)]