mergeWithKey is a rather difficult bit of API from a user perspective. I wish it didn't exist. I'm not looking to remove it, but I don't think its availability is a great argument against something more user-friendly.
-1 on unionWithMapping/Key. There is already mergeWithKey (and there’s pull request to `these` to use it, I’d just hadn’t time to look it thru properly and apply).
- Oleg Grenrus (co-maintainer of these)
> On 07 Aug 2016, at 06:15, David Feuer <david.feuer@gmail.com> wrote:
>
> Cale Gibbard and Ryan Trinkle are interested in adding some more
> operations they say will be useful in their work. Note that
> unionWithMapping is intended to enable an efficient implementation of
> alignWith in Data.Align in the `these` package.
>
> Set:
>
> filterIncreasing :: Ord a => (a -> Bool) -> Set a -> Set a
> filterDecreasing :: Ord a => (a -> Bool) -> Set a -> Set a
> partitionIncreasing :: Ord a => (a -> Bool) -> Set a -> (Set a, Set a)
>
> Map:
>
> filterKeysIncreasing :: Ord k => (k -> Bool) -> Map k a -> Map k a
> filterKeysDecreasing :: Ord k => (k -> Bool) -> Map k a -> Map k a
> partitionKeysIncreasing :: Ord k => (k -> Bool) -> Map k a -> (Map k a, Map k a)
>
> traverseMaybe :: Applicative f => (a -> f (Maybe b)) -> Map k a -> f (Map k b)
>
> alterMany :: Ord k => (a -> Maybe b -> Maybe b) -> Map k a -> Map k b -> Map k b
>
> alterManyA :: (Applicative f, Ord k) => (a -> Maybe b -> f (Maybe b))
> -> Map k a -> Map k b -> f (Map k b)
>
> unionWithMapping :: Ord k => (a -> c) -> (b -> c) -> (a -> b -> c) ->
> Map k a -> Map k b -> Map k c
>
> unionWithKeyMapping :: Ord k => (k -> a -> c) -> (k -> b -> c) -> (k
> -> a -> b -> c)
> -> Map k a -> Map k b -> Map k c
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