**difference** +containers

*O(n+m)*. Difference between two maps (based on keys).
> difference (fromList [(5, "a"), (3, "b")]) (fromList [(5, "A"), (7, "C")]) == singleton 3 "b"

*O(n+m)*. Difference between two sets.

*O(n+m)*. Difference of two sets. The implementation uses an efficient *hedge* algorithm comparable with *hedge-union*.

*O(n+m)*. Difference of two maps. Return elements of the first map not existing in the second map. The implementation uses an efficient *hedge* algorithm comparable with *hedge-union*.
> difference (fromList [(5, "a"), (3, "b")]) (fromList [(5, "A"), (7, "C")]) == singleton 3 "b"

*O(n+m)*. Difference with a combining function.
> let f al ar = if al == "b" then Just (al ++ ":" ++ ar) else Nothing
> differenceWith f (fromList [(5, "a"), (3, "b")]) (fromList [(5, "A"), (3, "B"), (7, "C")])
> == singleton 3 "b:B"

*O(n+m)*. Difference with a combining function. When two equal keys are encountered, the combining function is applied to the values of these keys. If it returns Nothing, the element is discarded (proper set difference). If it returns (Just y), the element is updated with a new value y. The implementation uses an efficient *hedge* algorithm comparable with *hedge-union*.
> let f al ar = if al == "b" then Just (al ++ ":" ++ ar) else Nothing
> differenceWith f (fromList [(5, "a"), (3, "b")]) (fromList [(5, "A"), (3, "B"), (7, "C")])
> == singleton 3 "b:B"
*O(n+m)*. Difference with a combining function. When two equal keys are encountered, the combining function is applied to the key and both values. If it returns Nothing, the element is discarded (proper set difference). If it returns (Just y), the element is updated with a new value y.
> let f k al ar = if al == "b" then Just ((show k) ++ ":" ++ al ++ "|" ++ ar) else Nothing
> differenceWithKey f (fromList [(5, "a"), (3, "b")]) (fromList [(5, "A"), (3, "B"), (10, "C")])
> == singleton 3 "3:b|B"
*O(n+m)*. Difference with a combining function. When two equal keys are encountered, the combining function is applied to the key and both values. If it returns Nothing, the element is discarded (proper set difference). If it returns (Just y), the element is updated with a new value y. The implementation uses an efficient *hedge* algorithm comparable with *hedge-union*.
> let f k al ar = if al == "b" then Just ((show k) ++ ":" ++ al ++ "|" ++ ar) else Nothing
> differenceWithKey f (fromList [(5, "a"), (3, "b")]) (fromList [(5, "A"), (3, "B"), (10, "C")])
> == singleton 3 "3:b|B"