Expanded unionWith for different value types

[Benjamin-McRae-Tracsis]

Hi, I think there's space for a unionWith with a signature like unionWith' :: Hashable k => (a -> c) -> (b -> c) -> (a -> b -> c) -> HM.HashMap k a -> HM.HashMap k b -> HM.HashMap k c.

This differs from the existing unionWith by allowing different ways to transforming the input maps, and being more specific in how to combine maps with differing value types.

My current usage is generally HM.unionWith' This That These. This would be a more powerful function.

[phadej]

That's what alignWith from semialign package does. The (a -> c) -> (b -> c) -> (a -> b -> c) is a way to represent These a b -> c.

(EDIT: HM.unionWith' This That These is align).

So if you are already using these, I don't see a reason for you to not depend on semialign.

[Benjamin-McRae-Tracsis]

The case I was using was just one example which could use semialign's alignWith, but I feel like it would be a useful function to have without relying on other packages.

[phadej]

FWIW, https://hackage.haskell.org/package/containers-0.7/docs/Data-Map-Merge-Lazy.html#v:merge has in its docs

unionWithKey f = merge preserveMissing preserveMissing (zipWithMatched f)

The alignWith in semialign is defined as

    alignWith f = Map.merge (Map.mapMissing (\_ x ->  f (This x)))
                            (Map.mapMissing (\_ y ->  f (That y)))
                            (Map.zipWithMatched (\_ x y -> f (These x y)))

so the building blocks are there.

[Benjamin-McRae-Tracsis]

merge-style function would be nice to add to unordered-containers so we could do things like this, yes.