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

containers	Data.Map.Strict.Internal

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")])

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

containers Data.Map.Strict.Internal

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"
```
mapKeysMonotonic :: (k1 -> k2) -> Map k1 a -> Map k2 a

containers Data.Map.Strict.Internal

mapKeysMonotonic f s == mapKeys f s, but works only when f is strictly monotonic. That is, for any values x and y, if x < y then f x < f y. The precondition is not checked. Semi-formally, we have:
```
and [x < y ==> f x < f y | x <- ls, y <- ls]
==> mapKeysMonotonic f s == mapKeys f s
where ls = keys s
```
This means that f maps distinct original keys to distinct resulting keys. This function has better performance than mapKeys.
```
mapKeysMonotonic (\ k -> k * 2) (fromList [(5,"a"), (3,"b")]) == fromList [(6, "b"), (10, "a")]
valid (mapKeysMonotonic (\ k -> k * 2) (fromList [(5,"a"), (3,"b")])) == True
valid (mapKeysMonotonic (\ _ -> 1)     (fromList [(5,"a"), (3,"b")])) == False
```
mapKeysWith :: Ord k2 => (a -> a -> a) -> (k1 -> k2) -> Map k1 a -> Map k2 a

containers Data.Map.Strict.Internal

mapKeysWith c 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 associated values will be combined using c. The value at the greater of the two original keys is used as the first argument to c.
```
mapKeysWith (++) (\ _ -> 1) (fromList [(1,"b"), (2,"a"), (3,"d"), (4,"c")]) == singleton 1 "cdab"
mapKeysWith (++) (\ _ -> 3) (fromList [(1,"b"), (2,"a"), (3,"d"), (4,"c")]) == singleton 3 "cdab"
```
Also see the performance note on fromListWith.
mapMaybe :: (a -> Maybe b) -> Map k a -> Map k b

containers Data.Map.Strict.Internal

Map values and collect the Just results.
```
let f x = if x == "a" then Just "new a" else Nothing
mapMaybe f (fromList [(5,"a"), (3,"b")]) == singleton 5 "new a"
```
mapMaybeMissing :: forall (f :: Type -> Type) k x y . Applicative f => (k -> x -> Maybe y) -> WhenMissing f k x y

containers Data.Map.Strict.Internal

Map over the entries whose keys are missing from the other map, optionally removing some. This is the most powerful SimpleWhenMissing tactic, but others are usually more efficient.
```
mapMaybeMissing :: (k -> x -> Maybe y) -> SimpleWhenMissing k x y
```
```
mapMaybeMissing f = traverseMaybeMissing (\k x -> pure (f k x))
```
but mapMaybeMissing uses fewer unnecessary Applicative operations.

mapMaybeWithKey :: (k -> a -> Maybe b) -> Map k a -> Map k b

containers	Data.Map.Strict.Internal

Map keys/values and collect the Just results.

let f k _ = if k < 5 then Just ("key : " ++ (show k)) else Nothing
mapMaybeWithKey f (fromList [(5,"a"), (3,"b")]) == singleton 3 "key : 3"

mapMissing :: forall (f :: Type -> Type) k x y . Applicative f => (k -> x -> y) -> WhenMissing f k x y

containers Data.Map.Strict.Internal

Map over the entries whose keys are missing from the other map.
```
mapMissing :: (k -> x -> y) -> SimpleWhenMissing k x y
```
```
mapMissing f = mapMaybeMissing (\k x -> Just $ f k x)
```
but mapMissing is somewhat faster.
mapWhenMatched :: forall (f :: Type -> Type) a b k x y . Functor f => (a -> b) -> WhenMatched f k x y a -> WhenMatched f k x y b

containers Data.Map.Strict.Internal

Map covariantly over a WhenMatched f k x y.
mapWhenMissing :: forall (f :: Type -> Type) a b k x . Functor f => (a -> b) -> WhenMissing f k x a -> WhenMissing f k x b

containers Data.Map.Strict.Internal

Map covariantly over a WhenMissing f k x.

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