Hello guys! A deadline of a discusssion on this has been reached. To review the discussion you can visit the archives. Following is a summarization. 1. Following is an implementation proposed by Schachaf Ben-Kiki, which in a combination with an `INLINABLE` pragma produces very impressive results by making even the primitive operations reimplemented in terms of it perform better: insert: ~ +5% increase using alterF delete: ~ +10% increase using alterF alterF :: (Ord k, Functor f) => k -> (Maybe a -> f (Maybe a)) -> Map k a -> f (Map k a) STRICT_1_OF_2(alterF) alterF k f = go where go Tip = maybe Tip (singleton k) <$> f Nothing go (Bin sx kx x l r) = case compare k kx of LT -> (\l' -> balance kx x l' r) <$> go l GT -> (\r' -> balance kx x l r') <$> go r EQ -> maybe (glue l r) (\x' -> Bin sx kx x' l r) <$> f (Just x) 2. `alterF` seems to be a mutually accepted title for the function 3. There was one downvote for implementing `alterF` with a changed order of parameters to "key -> lambda", as compared to "lambda -> key" of other modification functions in the library. Others seemed to be neutral about it. The implementation above is in that changed order. After some thinking my vote can be counted as a downvote too on that. 4. People seem to be indifferent to functions `insertLookupWithKey` and `updateLookupWithKey`, concerning their deprecation. The other proposed functions from the list following, however, generally are not wished to deprecate: insertWith, insertWithKey, adjust, adjustWithKey, update, updateWithKey, alter. Best regards, Nikita Volkov