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Within LTS Haskell 24.17 (ghc-9.10.3)
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mapAccumM :: (Monad m, Traversable t) => (s -> a -> m (s, b)) -> s -> t a -> m (s, t b)base Data.Traversable 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])mapAccumR :: Traversable t => (s -> a -> (s, b)) -> s -> t a -> (s, t b)base Data.Traversable 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)base Data.Traversable 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.mapFB :: (elt -> lst -> lst) -> (a -> elt) -> a -> lst -> lstbase GHC.Base No documentation available.
mapM :: Monad m => (a -> m b) -> [a] -> m [b]base GHC.Base mappend :: Monoid a => a -> a -> abase GHC.Base 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.
mapAccumL :: (acc -> Word8 -> (acc, Word8)) -> acc -> ByteString -> (acc, ByteString)bytestring Data.ByteString The mapAccumL function behaves like a combination of map and foldl; it applies a function to each element of a ByteString, passing an accumulating parameter from left to right, and returning a final value of this accumulator together with the new ByteString.
mapAccumR :: (acc -> Word8 -> (acc, Word8)) -> acc -> ByteString -> (acc, ByteString)bytestring Data.ByteString The mapAccumR function behaves like a combination of map and foldr; it applies a function to each element of a ByteString, passing an accumulating parameter from right to left, and returning a final value of this accumulator together with the new ByteString.
mapAccumL :: (acc -> Char -> (acc, Char)) -> acc -> ByteString -> (acc, ByteString)bytestring Data.ByteString.Char8 The mapAccumL function behaves like a combination of map and foldl; it applies a function to each element of a ByteString, passing an accumulating parameter from left to right, and returning a final value of this accumulator together with the new ByteString.
mapAccumR :: (acc -> Char -> (acc, Char)) -> acc -> ByteString -> (acc, ByteString)bytestring Data.ByteString.Char8 The mapAccumR function behaves like a combination of map and foldr; it applies a function to each element of a ByteString, passing an accumulating parameter from right to left, and returning a final value of this accumulator together with the new ByteString.