Add intersections, intersectionsWith

[Icelandjack]

Data.Map and Data.IntMap have the functions

unions     :: Ord k =>                  [Map k a] -> Map k a
unionsWith :: Ord k => (a -> a -> a) -> [Map k a] -> Map k a 

unions     ::                  [IntMap a] -> IntMap a 
unionsWith :: (a -> a -> a) -> [IntMap a] -> IntMap a 

should there be corresponding functions for intersection?

intersections     :: Ord k =>                  [Map k a] -> Map k a
intersectionsWith :: Ord k => (a -> a -> a) -> [Map k a] -> Map k a 

intersections     ::                  [IntMap a] -> IntMap a 
intersectionsWith :: (a -> a -> a) -> [IntMap a] -> IntMap a 

They could all be given Foldable constraints.

There are some similar functions, from data-interval

intersections :: (Ord r, Num r) => [Interval r] -> Interval r 

and fixplate

intersectionsWith :: (Ord hash, Ord k) => (v -> v -> v) -> [HashTable hash k v] -> HashTable hash k v 

[treeowl]

I don't really see any need for unions; what's wrong with foldl' union mempty? intersections as you propose it is much worse, because it's
partial. Specifically, intersections [] = _|_. You could certainly work
around this in various ways, but I don't, as yet, see the point. Am I
missing something?

On Nov 23, 2016 3:05 AM, "Icelandjack" [email protected] wrote:

Data.Map and Data.IntMap have the functions

unions :: Ord k => [Map k a] -> Map k a
unionsWith :: Ord k => (a -> a -> a) -> [Map k a] -> Map k a
unions :: [IntMap a] -> IntMap a unionsWith :: (a -> a -> a) -> [IntMap a] -> IntMap a

should there be corresponding functions for intersection?

intersections :: Ord k => [Map k a] -> Map k a
intersectionsWith :: Ord k => (a -> a -> a) -> [Map k a] -> Map k a
intersections :: [IntMap a] -> IntMap a intersectionsWith :: (a -> a -> a) -> [IntMap a] -> IntMap a

They could all be given Foldable constraints.

There are some similar functions, from data-interval
https://hackage.haskell.org/package/data-interval-0.6.0/candidate/docs/Data-Interval.html#v:intersections

intersections :: (Ord r, Num r) => [Interval r] -> Interval r

and fixplate
https://hackage.haskell.org/package/fixplate-0.1.6/docs/Data-Generics-Fixplate-Util-Hash-Table.html#v:intersectionsWith

intersectionsWith :: (Ord hash, Ord k) => (v -> v -> v) -> [HashTable hash k v] -> HashTable hash k v

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[Icelandjack]

unions, unionsWith are simple folds that are in containers already. I didn't think deeply about this but we can get around the empty case, as you said

intersections     ::                  NonEmpty (IntMap a) -> IntMap a 
intersectionsWith :: (a -> a -> a) -> NonEmpty (IntMap a) -> IntMap a

Is this more/less useful than unions, unionsWith? Who knows

[phadej]

NonEmpty is only in base-4.9, otherwise intersectionsWith would be nice indeed, for completeness. Though bike-shedding: unions is []-specialised fold, and I use UnionsWith, I can imagine newtype for intersection as well.

[fumieval]

Somewhat related: #699

[sjakobi]

Also related: #137