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Within LTS Haskell 24.33 (ghc-9.10.3)

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1. mapMaybeWithKey :: (k -> v1 -> Maybe v2) -> HashMap k v1 -> HashMap k v2

   rio	RIO.HashMap

   Transform this map by applying a function to every value and retaining only some of them.

2. mapWithKey :: (k -> v1 -> v2) -> HashMap k v1 -> HashMap k v2

   rio	RIO.HashMap

   Transform this map by applying a function to every value.

3. mapAccumL :: Traversable t => (s -> a -> (s, b)) -> s -> t a -> (s, t b)

   rio	RIO.List

   The mapAccumL function behaves like a combination of fmap and foldl; it applies a function to each element of a structure, passing an accumulating parameter from left to right, and returning a final value of this accumulator together with the new structure.

   Examples

   Basic usage:

   >>> mapAccumL (\a b -> (a + b, a)) 0 [1..10]
   (55,[0,1,3,6,10,15,21,28,36,45])
   

   >>> mapAccumL (\a b -> (a <> show b, a)) "0" [1..5]
   ("012345",["0","01","012","0123","01234"])
   

4. mapAccumR :: Traversable t => (s -> a -> (s, b)) -> s -> t a -> (s, t b)

   rio	RIO.List

   The mapAccumR function behaves like a combination of fmap and foldr; it applies a function to each element of a structure, passing an accumulating parameter from right to left, and returning a final value of this accumulator together with the new structure.

   Examples

   Basic usage:

   >>> mapAccumR (\a b -> (a + b, a)) 0 [1..10]
   (55,[54,52,49,45,40,34,27,19,10,0])
   

   >>> mapAccumR (\a b -> (a <> show b, a)) "0" [1..5]
   ("054321",["05432","0543","054","05","0"])
   

5. mapAccum :: (a -> b -> (a, c)) -> a -> Map k b -> (a, Map k c)

   rio	RIO.Map

   The function mapAccum threads an accumulating argument through the map in ascending order of keys.

   let f a b = (a ++ b, b ++ "X")
   mapAccum f "Everything: " (fromList [(5,"a"), (3,"b")]) == ("Everything: ba", fromList [(3, "bX"), (5, "aX")])
   

6. mapAccumRWithKey :: (a -> k -> b -> (a, c)) -> a -> Map k b -> (a, Map k c)

   rio	RIO.Map

   The function mapAccumRWithKey threads an accumulating argument through the map in descending order of keys.

7. mapAccumWithKey :: (a -> k -> b -> (a, c)) -> a -> Map k b -> (a, Map k c)

   rio	RIO.Map

   The function mapAccumWithKey threads an accumulating argument through the map in ascending order of keys.

   let f a k b = (a ++ " " ++ (show k) ++ "-" ++ b, b ++ "X")
   mapAccumWithKey f "Everything:" (fromList [(5,"a"), (3,"b")]) == ("Everything: 3-b 5-a", fromList [(3, "bX"), (5, "aX")])
   

8. mapEither :: (a -> Either b c) -> Map k a -> (Map k b, Map k c)

   rio	RIO.Map

   Map values and separate the Left and Right results.

   let f a = if a < "c" then Left a else Right a
   mapEither f (fromList [(5,"a"), (3,"b"), (1,"x"), (7,"z")])
   == (fromList [(3,"b"), (5,"a")], fromList [(1,"x"), (7,"z")])
   
   mapEither (\ a -> Right a) (fromList [(5,"a"), (3,"b"), (1,"x"), (7,"z")])
   == (empty, fromList [(5,"a"), (3,"b"), (1,"x"), (7,"z")])
   

9. mapEitherWithKey :: (k -> a -> Either b c) -> Map k a -> (Map k b, Map k c)

   rio	RIO.Map

   Map keys/values and separate the Left and Right results.

   let f k a = if k < 5 then Left (k * 2) else Right (a ++ a)
   mapEitherWithKey f (fromList [(5,"a"), (3,"b"), (1,"x"), (7,"z")])
   == (fromList [(1,2), (3,6)], fromList [(5,"aa"), (7,"zz")])
   
   mapEitherWithKey (\_ a -> Right a) (fromList [(5,"a"), (3,"b"), (1,"x"), (7,"z")])
   == (empty, fromList [(1,"x"), (3,"b"), (5,"a"), (7,"z")])
   

10. mapKeys :: Ord k2 => (k1 -> k2) -> Map k1 a -> Map k2 a

    rio	RIO.Map

    mapKeys f s is the map obtained by applying f to each key of s. The size of the result may be smaller if f maps two or more distinct keys to the same new key. In this case the value at the greatest of the original keys is retained.

    mapKeys (+ 1) (fromList [(5,"a"), (3,"b")])                        == fromList [(4, "b"), (6, "a")]
    mapKeys (\ _ -> 1) (fromList [(1,"b"), (2,"a"), (3,"d"), (4,"c")]) == singleton 1 "c"
    mapKeys (\ _ -> 3) (fromList [(1,"b"), (2,"a"), (3,"d"), (4,"c")]) == singleton 3 "c"
    

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