map -package:containers -package:text package:aeson

Map a function over all values in the map.

map f xs is the list obtained by applying f to each element of xs, i.e.,

map f [x1, x2, ..., xn] == [f x1, f x2, ..., f xn]
map f [x1, x2, ...] == [f x1, f x2, ...]

this means that map id == id

Examples

>>> map (+1) [1, 2, 3]
[2,3,4]

>>> map id [1, 2, 3]
[1,2,3]

>>> map (\n -> 3 * n + 1) [1, 2, 3]
[4,7,10]

Transform the keys and values of a KeyMap.

Map values and collect the Just results.

Map values and collect the Just results.

Map a function over all values in the map.

Same as fmap. Provided for the consistency with ToJSONKeyFunction.

Map each element of a structure to a monadic action, evaluate these actions from left to right, and collect the results. For a version that ignores the results see mapM_.

Examples

mapM is literally a traverse with a type signature restricted to Monad. Its implementation may be more efficient due to additional power of Monad.

Map each element of a structure to a monadic action, evaluate these actions from left to right, and ignore the results. For a version that doesn't ignore the results see mapM. mapM_ is just like traverse_, but specialised to monadic actions.

An associative operation NOTE: This method is redundant and has the default implementation mappend = (<>) since base-4.11.0.0. Should it be implemented manually, since mappend is a synonym for (<>), it is expected that the two functions are defined the same way. In a future GHC release mappend will be removed from Monoid.

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"])

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"])

The mapMaybe function is a version of map which can throw out elements. In particular, the functional argument returns something of type Maybe b. If this is Nothing, no element is added on to the result list. If it is Just b, then b is included in the result list.

Examples

Using mapMaybe f x is a shortcut for catMaybes $ map f x in most cases:

>>> import GHC.Internal.Text.Read ( readMaybe )

>>> let readMaybeInt = readMaybe :: String -> Maybe Int

>>> mapMaybe readMaybeInt ["1", "Foo", "3"]
[1,3]

>>> catMaybes $ map readMaybeInt ["1", "Foo", "3"]
[1,3]

If we map the Just constructor, the entire list should be returned:

>>> mapMaybe Just [1,2,3]
[1,2,3]

The mapAndUnzipM function maps its first argument over a list, returning the result as a pair of lists. This function is mainly used with complicated data structures or a state monad.

This function maps one exception into another as proposed in the paper "A semantics for imprecise exceptions".

The mapAccumM function behaves like a combination of mapM and mapAccumL that traverses the structure while evaluating the actions and passing an accumulating parameter from left to right. It returns a final value of this accumulator together with the new structure. The accumulator is often used for caching the intermediate results of a computation.

Examples

Basic usage:

>>> let expensiveDouble a = putStrLn ("Doubling " <> show a) >> pure (2 * a)

>>> :{
mapAccumM (\cache a -> case lookup a cache of
Nothing -> expensiveDouble a >>= \double -> pure ((a, double):cache, double)
Just double -> pure (cache, double)
) [] [1, 2, 3, 1, 2, 3]
:}
Doubling 1
Doubling 2
Doubling 3
([(3,6),(2,4),(1,2)],[2,4,6,2,4,6])

mapM f is equivalent to sequence . map f.

An isomorphism holds when lifted into a functor. For example, if a list contains a bunch of a's which are each isomorphic to a b, the whole list of a's is isomorphic to a list of b's.

>>> ["1","2","3"] ^. mapping _Show :: [Int]
[1,2,3]

>>> ([1,2,3] :: [Int]) ^. from (mapping _Show)
["1","2","3"]

This also hold across different functors:

>>> let l = mapping @[] @Maybe _Show

>>> :t l
l :: (Read b, Show b) => Iso [String] (Maybe String) [b] (Maybe b)

>>> ["1","2","3"] & l %~ Just . sum
Just "6"

Constructs a new array derived from the original array by applying a function to each of the elements.

Constructs a new array derived from the original array by applying a function to each of the indices.

Map elements to monadic actions, sequence them left-to-right, and discard the results.

Maps an IO-performing function over any Traversable data type, performing all the IO actions concurrently, and returning the original data structure with the arguments replaced by the results. If any of the actions throw an exception, then all other actions are cancelled and the exception is re-thrown. For example, mapConcurrently works with lists:

pages <- mapConcurrently getURL ["url1", "url2", "url3"]

If you just have a list of actions, run them concurrently with

results <- mapConcurrently id [act1, act2, act3]

NOTE: mapConcurrently will immediately spawn a thread for each element of the Traversable, so running this on large inputs can lead to resource exhaustion (of memory, file descriptors, or other limited resources). To avoid unbounded resource usage, see Control.Concurrent.Stream.

mapConcurrently_ is mapConcurrently with the return value discarded; a concurrent equivalent of mapM_.

Concurrent map over a list of values, using a bounded number of threads.