unionWith package:nonempty-containers

unionWith :: (a -> a -> a) -> NEIntMap a -> NEIntMap a -> NEIntMap a
nonempty-containers Data.IntMap.NonEmpty
O(m*log(n/m + 1)), m <= n. Union with a combining function. 
unionWith (++) (fromList ((5, "a") :| [(3, "b")])) (fromList ((5, "A") :| [(7, "C")])) == fromList ((3, "b") :| [(5, "aA"), (7, "C")])
unionWith :: Ord k => (a -> a -> a) -> NEMap k a -> NEMap k a -> NEMap k a
nonempty-containers Data.Map.NonEmpty
O(m*log(n/m + 1)), m <= n. Union with a combining function. 
unionWith (++) (fromList ((5, "a") :| [(3, "b")])) (fromList ((5, "A") :| [(7, "C")])) == fromList ((3, "b") :| [(5, "aA"), (7, "C")])
unionWithKey :: (Key -> a -> a -> a) -> NEIntMap a -> NEIntMap a -> NEIntMap a
nonempty-containers Data.IntMap.NonEmpty
O(m*log(n/m + 1)), m <= n. Union with a combining function, given the matching key. 
let f key left_value right_value = (show key) ++ ":" ++ left_value ++ "|" ++ right_value
unionWithKey f (fromList ((5, "a") :| [(3, "b")])) (fromList ((5, "A") :| [(7, "C")])) == fromList ((3, "b") :| [(5, "5:a|A"), (7, "C")])
unionWithKey :: Ord k => (k -> a -> a -> a) -> NEMap k a -> NEMap k a -> NEMap k a
nonempty-containers Data.Map.NonEmpty
O(m*log(n/m + 1)), m <= n. Union with a combining function, given the matching key. 
let f key left_value right_value = (show key) ++ ":" ++ left_value ++ "|" ++ right_value
unionWithKey f (fromList ((5, "a") :| [(3, "b")])) (fromList ((5, "A") :| [(7, "C")])) == fromList ((3, "b") :| [(5, "5:a|A"), (7, "C")])