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  1. 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.Merge.Strict

    Map covariantly over a WhenMatched f k x y.

  2. 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.Merge.Strict

    Map covariantly over a WhenMissing f k x.

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

    containers Data.Map.Strict

    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")])

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

    containers Data.Map.Strict

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

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

    containers Data.Map.Strict

    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")])

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

    containers Data.Map.Strict

    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")])

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

    containers Data.Map.Strict

    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")])

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

    containers Data.Map.Strict

    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"

  9. mapKeysMonotonic :: (k1 -> k2) -> Map k1 a -> Map k2 a

    containers Data.Map.Strict

    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

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

    containers Data.Map.Strict

    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.

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