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mapAccumL :: Traversable t => (s -> a -> (s, b)) -> s -> t a -> (s, t b)relude Relude.Foldable.Reexport 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"])mapAccumR :: Traversable t => (s -> a -> (s, b)) -> s -> t a -> (s, t b)relude Relude.Foldable.Reexport 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"])mapM :: (Traversable t, Monad m) => (a -> m b) -> t a -> m (t b)relude Relude.Foldable.Reexport 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.mapM_ :: (Foldable t, Monad m) => (a -> m b) -> t a -> m ()relude Relude.Foldable.Reexport 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.
mapMaybeM :: Monad m => (a -> m (Maybe b)) -> [a] -> m [b]relude Relude.Monad.Maybe The monadic version of the mapMaybe function.
>>> :{ evenInHalf :: Int -> IO (Maybe Int) evenInHalf n | even n = pure $ Just $ n `div` 2 | otherwise = pure Nothing :}>>> mapMaybeM evenInHalf [1..10] [1,2,3,4,5]
mapAndUnzipM :: Applicative m => (a -> m (b, c)) -> [a] -> m ([b], [c])relude Relude.Monad.Reexport 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.
mapMaybe :: (a -> Maybe b) -> [a] -> [b]relude Relude.Monad.Reexport 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]
mappend :: Monoid a => a -> a -> arelude Relude.Monoid 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.
mapExcept :: (Either e a -> Either e' b) -> Except e a -> Except e' bselective Control.Selective.Trans.Except No documentation available.
mapExceptT :: (m (Either e a) -> n (Either e' b)) -> ExceptT e m a -> ExceptT e' n bselective Control.Selective.Trans.Except No documentation available.