map package:base-prelude

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, ...]
>>> map (+1) [1, 2, 3]
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.
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.
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".
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_.
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. As of base 4.8.0.0, mapM_ is just traverse_, specialized to Monad.
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 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]
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.
Map over both arguments at the same time.
bimap f g ≡ first f . second g

Examples

>>> bimap toUpper (+1) ('j', 3)
('J',4)
>>> bimap toUpper (+1) (Left 'j')
Left 'J'
>>> bimap toUpper (+1) (Right 3)
Right 4
Map a function over all the elements of a container and concatenate the resulting lists.
Using ApplicativeDo: 'fmap f as' can be understood as the do expression
do a <- as
pure (f a)
with an inferred Functor constraint.
This function may be used as a value for fmap in a Functor instance, provided that traverse is defined. (Using fmapDefault with a Traversable instance defined only by sequenceA will result in infinite recursion.)
fmapDefault f ≡ runIdentity . traverse (Identity . f)
Map each element of the structure to a monoid, and combine the results.
A variant of foldMap that is strict in the accumulator.
This function may be used as a value for foldMap in a Foldable instance.
foldMapDefault f ≡ getConst . traverse (Const . f)
A generic monadic transformation that maps over the immediate subterms The default definition instantiates the type constructor c in the type of gfoldl to the monad datatype constructor, defining injection and projection using return and >>=.
Transformation of one immediate subterm with success
Transformation of at least one immediate subterm does not fail
A generic query that processes the immediate subterms and returns a list of results. The list is given in the same order as originally specified in the declaration of the data constructors.
A generic query that processes one child by index (zero-based)
A generic query with a left-associative binary operator
A generic query with a right-associative binary operator
A generic transformation that maps over the immediate subterms The default definition instantiates the type constructor c in the type of gfoldl to an identity datatype constructor, using the isomorphism pair as injection and projection.