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concatMap :: forall (m :: Type -> Type) b a c . Monad m => (b -> Fold m a c) -> Fold m a b -> Fold m a c

streamly-core Streamly.Internal.Data.Fold

Map a Fold returning function on the result of a Fold and run the returned fold. This operation can be used to express data dependencies between fold operations. Let's say the first element in the stream is a count of the following elements that we have to add, then:
```
>>> import Data.Maybe (fromJust)

>>> count = fmap fromJust Fold.one

>>> total n = Fold.take n Fold.sum

>>> Stream.fold (Fold.concatMap total count) $ Stream.fromList [10,9..1]
45
```
This does not fuse completely, see refold for a fusible alternative. Time: O(n^2) where n is the number of compositions. See also: foldIterateM, refold

demuxKvToMap :: (Monad m, Ord k) => (k -> m (Fold m a b)) -> Fold m (k, a) (Map k b)

streamly-core	Streamly.Internal.Data.Fold

Fold a stream of key value pairs using a function that maps keys to folds. Definition:

>>> demuxKvToMap f = Fold.demuxToContainer fst (Fold.lmap snd . f)

Example:

>>> import Data.Map (Map)

>>> :{
let f "SUM" = return Fold.sum
f _ = return Fold.product
input = Stream.fromList [("SUM",1),("PRODUCT",2),("SUM",3),("PRODUCT",4)]
in Stream.fold (Fold.demuxKvToMap f) input :: IO (Map String Int)
:}
fromList [("PRODUCT",8),("SUM",4)]

Pre-release

demuxToMap :: (Monad m, Ord k) => (a -> k) -> (a -> m (Fold m a b)) -> Fold m a (Map k b)

streamly-core Streamly.Internal.Data.Fold

This collects all the results of demux in a Map.
demuxToMapIO :: (MonadIO m, Ord k) => (a -> k) -> (a -> m (Fold m a b)) -> Fold m a (Map k b)

streamly-core Streamly.Internal.Data.Fold

Same as demuxToMap but uses demuxIO for better performance.
drainMapM :: Monad m => (a -> m b) -> Fold m a ()

streamly-core Streamly.Internal.Data.Fold

Definitions:
```
>>> drainMapM f = Fold.lmapM f Fold.drain

>>> drainMapM f = Fold.foldMapM (void . f)
```
Drain all input after passing it through a monadic function. This is the dual of mapM_ on stream producers.
foldMap :: forall (m :: Type -> Type) b a . (Monad m, Monoid b) => (a -> b) -> Fold m a b

streamly-core Streamly.Internal.Data.Fold

Definition:
```
>>> foldMap f = Fold.lmap f Fold.mconcat
```
Make a fold from a pure function that folds the output of the function using mappend and mempty.
```
>>> sum = Fold.foldMap Data.Monoid.Sum

>>> Stream.fold sum $ Stream.enumerateFromTo 1 10
Sum {getSum = 55}
```
foldMapM :: (Monad m, Monoid b) => (a -> m b) -> Fold m a b

streamly-core Streamly.Internal.Data.Fold

Definition:
```
>>> foldMapM f = Fold.lmapM f Fold.mconcat
```
Make a fold from a monadic function that folds the output of the function using mappend and mempty.
```
>>> sum = Fold.foldMapM (return . Data.Monoid.Sum)

>>> Stream.fold sum $ Stream.enumerateFromTo 1 10
Sum {getSum = 55}
```
kvToMap :: forall (m :: Type -> Type) k a b . (Monad m, Ord k) => Fold m a b -> Fold m (k, a) (Map k b)

streamly-core Streamly.Internal.Data.Fold

Given an input stream of key value pairs and a fold for values, fold all the values belonging to each key. Useful for map/reduce, bucketizing the input in different bins or for generating histograms. Definition:
```
>>> kvToMap = Fold.toMap fst . Fold.lmap snd
```
Example:
```
>>> :{
let input = Stream.fromList [("ONE",1),("ONE",1.1),("TWO",2), ("TWO",2.2)]
in Stream.fold (Fold.kvToMap Fold.toList) input
:}
fromList [("ONE",[1.0,1.1]),("TWO",[2.0,2.2])]
```
Pre-release
lmap :: forall a b (m :: Type -> Type) r . (a -> b) -> Fold m b r -> Fold m a r

streamly-core Streamly.Internal.Data.Fold

lmap f fold maps the function f on the input of the fold. Definition:
```
>>> lmap = Fold.lmapM return
```
Example:
```
>>> sumSquared = Fold.lmap (\x -> x * x) Fold.sum

>>> Stream.fold sumSquared (Stream.enumerateFromTo 1 100)
338350
```
lmapM :: Monad m => (a -> m b) -> Fold m b r -> Fold m a r

streamly-core Streamly.Internal.Data.Fold

lmapM f fold maps the monadic function f on the input of the fold.

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