content: {-# LANGUAGE CPP #-}
  -- |
  -- Module      : Streamly.Internal.Data.Stream.Transform
  -- Copyright   : (c) 2018 Composewell Technologies
  --               (c) Roman Leshchinskiy 2008-2010
  -- License     : BSD-3-Clause
  -- Maintainer  : streamly@composewell.com
  -- Stability   : experimental
  -- Portability : GHC
  
  -- A few functions in this module have been adapted from the vector package
  -- (c) Roman Leshchinskiy. See the notes in specific combinators.
  
  module Streamly.Internal.Data.Stream.Transform
      (
      -- * Mapping
      -- | Stateless one-to-one maps.
        sequence
  
      -- * Mapping Effects
      , tap
      , tapOffsetEvery
      , trace
      , trace_
  
      -- * Folding
      , foldrS
      , foldlS
  
      -- * Composable Scans
      , postscanl
      , scanl
      , scanlMany
      , scanr
      , pipe
  
      -- * Splitting
      , splitSepBy_
  
      -- * Ad-hoc Scans
      -- | Left scans. Stateful, mostly one-to-one maps.
      , scanlM'
      , scanlMAfter'
      , scanl'
      , scanlM
      , scanlBy
      , scanl1M'
      , scanl1'
      , scanl1M
      , scanl1
  
      , prescanl'
      , prescanlM'
  
      , postscanlBy
      , postscanlM
      , postscanl'
      , postscanlM'
      , postscanlMAfter'
  
      , postscanlx'
      , postscanlMx'
      , scanlMx'
      , scanlx'
  
      -- * Filtering
      -- delete is for once like insert, filter is for many like intersperse.
  
      -- | Produce a subset of the stream.
      , with
      , postscanlMaybe
      , filter -- retainBy
      , filterM
      , deleteBy -- deleteOnceBy/deleteFirstBy?
      , uniqBy
      , uniq
      , prune
      , repeated
  
      -- * Sampling
      -- | Value agnostic filtering.
      , sampleFromThen
      -- keepEvery/filterEvery -- sampling
      -- deleteEvery/dropEvery/removeEvery -- dual of intersperseEvery
      -- deintersperse - drop infixed elements
  
      -- * Trimming
      -- | Produce a subset of the stream trimmed at ends.
      , initNonEmpty
      , tailNonEmpty
      , drop
      , dropWhile
      , dropWhileM
  
      -- * Trimming from end
      -- | RingArray array based or buffering operations.
      --
      , takeWhileLast
      , takeWhileAround
      , dropLast
      , dropWhileLast
      , dropWhileAround
  
      -- * Inserting Elements
      -- insert is for once like delete, intersperse is for many like filter
      -- | Produce a superset of the stream. Value agnostic insertion.
      , intersperse
      , intersperseM
      , intersperseEveryM
      , intersperseEndByM
      , intersperseEndByEveryM
  
      -- Value aware insertion.
      , insertBy -- insertCmpBy
      -- insertBeforeBy
      -- insertAfterBy
      -- intersperseBeforeBy
      -- intersperseAfterBy
  
      -- * Inserting Side Effects
      , intersperseM_
      , intersperseEndByM_
      , intersperseBeginByM_
  
      , delay
      , delayPre
      , delayPost
  
      -- * Reordering
      -- | Produce strictly the same set but reordered.
      , reverse
      , reverseUnbox
      , reassembleBy
  
      -- * Position Indexing
      , indexed
      , indexedR
  
      -- * Time Indexing
      , timestampWith
      , timestamped
      , timeIndexWith
      , timeIndexed
  
      -- * Searching
      , findIndices
      , elemIndices
  
      -- * Rolling map
      -- | Map using the previous element.
      , rollingMap
      , rollingMapM
      , rollingMap2
  
      -- * Maybe Streams
      , mapMaybe
      , mapMaybeM
      , catMaybes
  
      -- * Either Streams
      , catLefts
      , catRights
      , catEithers
  
      -- * Deprecated
      , postscan
      , scan
      , scanMany
      , scanMaybe
      , intersperseMSuffix
      , intersperseMSuffixWith
      , intersperseMSuffix_
      , intersperseMPrefix_
      , strideFromThen
      , splitOn
      )
  where
  
  #include "deprecation.h"
  #include "inline.hs"
  
  import Control.Concurrent (threadDelay)
  import Control.Monad (void)
  import Control.Monad.IO.Class (MonadIO (liftIO))
  import Data.Either (fromLeft, isLeft, isRight, fromRight)
  import Data.Functor ((<&>))
  import Data.Maybe (fromJust, isJust)
  import Fusion.Plugin.Types (Fuse(..))
  
  import Streamly.Internal.Data.Fold.Type (Fold(..))
  import Streamly.Internal.Data.Pipe.Type (Pipe(..))
  import Streamly.Internal.Data.Scanl.Type (Scanl(..))
  import Streamly.Internal.Data.Scanr (Scanr(..))
  import Streamly.Internal.Data.SVar.Type (adaptState)
  import Streamly.Internal.Data.Time.Units (AbsTime, RelTime64)
  import Streamly.Internal.Data.Unbox (Unbox)
  import Streamly.Internal.System.IO (defaultChunkSize)
  
  -- import qualified Data.List as List
  import qualified Streamly.Internal.Data.Array.Type as A
  import qualified Streamly.Internal.Data.Fold.Type as FL
  import qualified Streamly.Internal.Data.Pipe.Type as Pipe
  import qualified Streamly.Internal.Data.StreamK.Type as K
  
  import Prelude hiding
         ( drop, dropWhile, filter, map, mapM, reverse
         , scanl, scanl1, scanr, sequence, take, takeWhile, zipWith)
  
  import Streamly.Internal.Data.Stream.Generate
      (absTimesWith, relTimesWith)
  import Streamly.Internal.Data.Stream.Type
  
  #include "DocTestDataStream.hs"
  
  ------------------------------------------------------------------------------
  -- Piping
  ------------------------------------------------------------------------------
  
  {-# ANN type PipeState Fuse #-}
  data PipeState st sc ps = PipeConsume st sc | PipeProduce st ps
  
  -- | Use a 'Pipe' to transform a stream.
  --
  {-# INLINE_NORMAL pipe #-}
  pipe :: Monad m => Pipe m a b -> Stream m a -> Stream m b
  pipe (Pipe consume produce initial) (Stream stream_step state) =
      Stream step (PipeConsume state initial)
  
      where
  
      {-# INLINE goConsume #-}
      goConsume st cs x = do
          res <- consume cs x
          return
              $ case res of
                  Pipe.YieldC s b -> Yield b (PipeConsume st s)
                  Pipe.SkipC s -> Skip (PipeConsume st s)
                  Pipe.Stop -> Stop
                  Pipe.YieldP ps b -> Yield b (PipeProduce st ps)
                  Pipe.SkipP ps -> Skip (PipeProduce st ps)
  
      {-# INLINE_LATE step #-}
      step gst (PipeConsume st cs) = do
          r <- stream_step (adaptState gst) st
          case r of
              Yield x s -> goConsume s cs x
              Skip s -> return $ Skip (PipeConsume s cs)
              Stop -> return Stop
      step _ (PipeProduce st ps) = do
          r <- produce ps
          return
              $ case r of
                  Pipe.YieldC cs b -> Yield b (PipeConsume st cs)
                  Pipe.SkipC cs -> Skip (PipeConsume st cs)
                  Pipe.Stop -> Stop
                  Pipe.YieldP ps1 b -> Yield b (PipeProduce st ps1)
                  Pipe.SkipP ps1 -> Skip (PipeProduce st ps1)
  
  {-# ANN type RunScanState Fuse #-}
  data RunScanState st sc ps = ScanConsume st sc
  
  -- | Use a lazy right 'Scanr' to transform a stream.
  --
  -- The following example extracts the input stream up to a point where the
  -- running average of elements is no more than 10:
  --
  -- >>> import Data.Maybe (fromJust)
  -- >>> let avg = Scanr.teeWith (/) Scanr.sum (fmap fromIntegral Scanr.length)
  -- >>> s = Stream.enumerateFromTo 1.0 100.0
  -- >>> :{
  --  Stream.fold Fold.toList
  --   $ fmap fst
  --   $ Stream.takeWhile (\(_,x) -> x <= 10)
  --   $ Stream.scanr (Scanr.tee Scanr.identity avg) s
  -- :}
  -- [1.0,2.0,3.0,4.0,5.0,6.0,7.0,8.0,9.0,10.0,11.0,12.0,13.0,14.0,15.0,16.0,17.0,18.0,19.0]
  --
  {-# INLINE_NORMAL scanr #-}
  scanr :: Monad m => Scanr m a b -> Stream m a -> Stream m b
  scanr (Scanr consume initial) (Stream stream_step state) =
      Stream step (ScanConsume state initial)
  
      where
  
      {-# INLINE_LATE step #-}
      step gst (ScanConsume st cs) = do
          r <- stream_step (adaptState gst) st
          case r of
              Yield x s -> do
                  res <- consume cs x
                  return
                      $ case res of
                          Yield b cs1 -> Yield b (ScanConsume s cs1)
                          Skip cs1 -> Skip (ScanConsume s cs1)
                          Stop -> Stop
              Skip s -> return $ Skip (ScanConsume s cs)
              Stop -> return Stop
  
  ------------------------------------------------------------------------------
  -- Transformation Folds
  ------------------------------------------------------------------------------
  
  -- Note, this is going to have horrible performance, because of the nature of
  -- the stream type (i.e. direct stream vs CPS). Its only for reference, it is
  -- likely be practically unusable.
  {-# INLINE_NORMAL foldlS #-}
  foldlS :: Monad m
      => (Stream m b -> a -> Stream m b) -> Stream m b -> Stream m a -> Stream m b
  foldlS fstep begin (Stream step state) = Stream step' (Left (state, begin))
    where
      step' gst (Left (st, acc)) = do
          r <- step (adaptState gst) st
          return $ case r of
              Yield x s -> Skip (Left (s, fstep acc x))
              Skip s -> Skip (Left (s, acc))
              Stop   -> Skip (Right acc)
  
      step' gst (Right (Stream stp stt)) = do
          r <- stp (adaptState gst) stt
          return $ case r of
              Yield x s -> Yield x (Right (Stream stp s))
              Skip s -> Skip (Right (Stream stp s))
              Stop   -> Stop
  
  ------------------------------------------------------------------------------
  -- Transformation by Mapping
  ------------------------------------------------------------------------------
  
  -- |
  -- >>> sequence = Stream.mapM id
  --
  -- Replace the elements of a stream of monadic actions with the outputs of
  -- those actions.
  --
  -- >>> s = Stream.fromList [putStr "a", putStr "b", putStrLn "c"]
  -- >>> Stream.fold Fold.drain $ Stream.sequence s
  -- abc
  --
  {-# INLINE_NORMAL sequence #-}
  sequence :: Monad m => Stream m (m a) -> Stream m a
  sequence (Stream step state) = Stream step' state
    where
      {-# INLINE_LATE step' #-}
      step' gst st = do
           r <- step (adaptState gst) st
           case r of
               Yield x s -> x >>= \a -> return (Yield a s)
               Skip s    -> return $ Skip s
               Stop      -> return Stop
  
  ------------------------------------------------------------------------------
  -- Mapping side effects
  ------------------------------------------------------------------------------
  
  data TapState fs st a
      = TapInit | Tapping !fs st | TapDone st
  
  -- XXX Multiple yield points
  
  -- | Tap the data flowing through a stream into a 'Fold'. For example, you may
  -- add a tap to log the contents flowing through the stream. The fold is used
  -- only for effects, its result is discarded.
  --
  -- @
  --                   Fold m a b
  --                       |
  -- -----stream m a ---------------stream m a-----
  --
  -- @
  --
  -- >>> s = Stream.enumerateFromTo 1 2
  -- >>> Stream.fold Fold.drain $ Stream.tap (Fold.drainMapM print) s
  -- 1
  -- 2
  --
  -- Compare with 'trace'.
  --
  {-# INLINE tap #-}
  tap :: Monad m => Fold m a b -> Stream m a -> Stream m a
  tap (Fold fstep initial _ final) (Stream step state) = Stream step' TapInit
  
      where
  
      step' _ TapInit = do
          res <- initial
          return
              $ Skip
              $ case res of
                    FL.Partial s -> Tapping s state
                    FL.Done _ -> TapDone state
      step' gst (Tapping acc st) = do
          r <- step gst st
          case r of
              Yield x s -> do
                  res <- fstep acc x
                  return
                      $ Yield x
                      $ case res of
                            FL.Partial fs -> Tapping fs s
                            FL.Done _ -> TapDone s
              Skip s -> return $ Skip (Tapping acc s)
              Stop -> do
                  void $ final acc
                  return Stop
      step' gst (TapDone st) = do
          r <- step gst st
          return
              $ case r of
                    Yield x s -> Yield x (TapDone s)
                    Skip s -> Skip (TapDone s)
                    Stop -> Stop
  
  data TapOffState fs s a
      = TapOffInit
      | TapOffTapping !fs s Int
      | TapOffDone s
  
  -- XXX Multiple yield points
  {-# INLINE_NORMAL tapOffsetEvery #-}
  tapOffsetEvery :: Monad m
      => Int -> Int -> Fold m a b -> Stream m a -> Stream m a
  tapOffsetEvery offset n (Fold fstep initial _ final) (Stream step state) =
      Stream step' TapOffInit
  
      where
  
      {-# INLINE_LATE step' #-}
      step' _ TapOffInit = do
          res <- initial
          return
              $ Skip
              $ case res of
                    FL.Partial s -> TapOffTapping s state (offset `mod` n)
                    FL.Done _ -> TapOffDone state
      step' gst (TapOffTapping acc st count) = do
          r <- step gst st
          case r of
              Yield x s -> do
                  next <-
                      if count <= 0
                      then do
                          res <- fstep acc x
                          return
                              $ case res of
                                    FL.Partial sres ->
                                      TapOffTapping sres s (n - 1)
                                    FL.Done _ -> TapOffDone s
                      else return $ TapOffTapping acc s (count - 1)
                  return $ Yield x next
              Skip s -> return $ Skip (TapOffTapping acc s count)
              Stop -> do
                  void $ final acc
                  return Stop
      step' gst (TapOffDone st) = do
          r <- step gst st
          return
              $ case r of
                    Yield x s -> Yield x (TapOffDone s)
                    Skip s -> Skip (TapOffDone s)
                    Stop -> Stop
  
  -- | Apply a monadic function to each element flowing through the stream and
  -- discard the results.
  --
  -- >>> s = Stream.enumerateFromTo 1 2
  -- >>> Stream.fold Fold.drain $ Stream.trace print s
  -- 1
  -- 2
  --
  -- Compare with 'tap'.
  --
  {-# INLINE trace #-}
  trace :: Monad m => (a -> m b) -> Stream m a -> Stream m a
  trace f = mapM (\x -> void (f x) >> return x)
  
  -- | Perform a side effect before yielding each element of the stream and
  -- discard the results.
  --
  -- >>> s = Stream.enumerateFromTo 1 2
  -- >>> Stream.fold Fold.drain $ Stream.trace_ (print "got here") s
  -- "got here"
  -- "got here"
  --
  -- Same as 'intersperseMPrefix_' but always serial.
  --
  -- See also: 'trace'
  --
  -- /Pre-release/
  {-# INLINE trace_ #-}
  trace_ :: Monad m => m b -> Stream m a -> Stream m a
  trace_ eff = mapM (\x -> eff >> return x)
  
  ------------------------------------------------------------------------------
  -- Scanning with a Fold
  ------------------------------------------------------------------------------
  
  data ScanState s f = ScanInit s | ScanDo s !f | ScanDone
  
  -- NOTE: Lazy postscans can be useful e.g. to use a lazy postscan on "latest".
  -- We can keep the initial state undefined in lazy postscans which do not use
  -- it at all. Otherwise we have to wrap the accumulator in a Maybe type.
  -- Unfortunately, we cannot define lazy scans because the Partial constructor
  -- itself is strict.
  
  -- | Postscan a stream using the given fold. A postscan omits the initial
  -- (default) value of the accumulator and includes the final value.
  --
  -- >>> Stream.toList $ Stream.postscanl Scanl.latest (Stream.fromList [])
  -- []
  --
  -- Compare with 'scan' which includes the initial value as well:
  --
  -- >>> Stream.toList $ Stream.scanl Scanl.latest (Stream.fromList [])
  -- [Nothing]
  --
  -- The following example extracts the input stream up to a point where the
  -- running average of elements is no more than 10:
  --
  -- >>> import Data.Maybe (fromJust)
  -- >>> let avg = Scanl.teeWith (/) Scanl.sum (fmap fromIntegral Scanl.length)
  -- >>> s = Stream.enumerateFromTo 1.0 100.0
  -- >>> :{
  --  Stream.fold Fold.toList
  --   $ fmap (fromJust . fst)
  --   $ Stream.takeWhile (\(_,x) -> x <= 10)
  --   $ Stream.postscanl (Scanl.tee Scanl.latest avg) s
  -- :}
  -- [1.0,2.0,3.0,4.0,5.0,6.0,7.0,8.0,9.0,10.0,11.0,12.0,13.0,14.0,15.0,16.0,17.0,18.0,19.0]
  --
  {-# INLINE_NORMAL postscanl #-}
  postscanl :: Monad m => Scanl m a b -> Stream m a -> Stream m b
  postscanl (Scanl fstep initial extract final) (Stream sstep state) =
      Stream step (ScanInit state)
  
      where
  
      {-# INLINE_LATE step #-}
      step _ (ScanInit st) = do
          res <- initial
          return
              $ case res of
                    FL.Partial fs -> Skip $ ScanDo st fs
                    FL.Done b -> Yield b ScanDone
      step gst (ScanDo st fs) = do
          res <- sstep (adaptState gst) st
          case res of
              Yield x s -> do
                  r <- fstep fs x
                  case r of
                      FL.Partial fs1 -> do
                          !b <- extract fs1
                          return $ Yield b $ ScanDo s fs1
                      FL.Done b -> return $ Yield b ScanDone
              Skip s -> return $ Skip $ ScanDo s fs
              Stop -> final fs >> return Stop
      step _ ScanDone = return Stop
  
  {-# DEPRECATED postscan "Please use postscanl instead" #-}
  {-# INLINE_NORMAL postscan #-}
  postscan :: Monad m => FL.Fold m a b -> Stream m a -> Stream m b
  postscan (FL.Fold fstep initial extract final) =
      postscanl (Scanl fstep initial extract final)
  
  {-# INLINE scanlWith #-}
  scanlWith :: Monad m
      => Bool -> Scanl m a b -> Stream m a -> Stream m b
  scanlWith restart (Scanl fstep initial extract final) (Stream sstep state) =
      Stream step (ScanInit state)
  
      where
  
      {-# INLINE runStep #-}
      runStep st action = do
          res <- action
          case res of
              FL.Partial fs -> do
                  !b <- extract fs
                  return $ Yield b $ ScanDo st fs
              FL.Done b ->
                  let next = if restart then ScanInit st else ScanDone
                   in return $ Yield b next
  
      {-# INLINE_LATE step #-}
      step _ (ScanInit st) = runStep st initial
      step gst (ScanDo st fs) = do
          res <- sstep (adaptState gst) st
          case res of
              Yield x s -> runStep s (fstep fs x)
              Skip s -> return $ Skip $ ScanDo s fs
              Stop -> final fs >> return Stop
      step _ ScanDone = return Stop
  
  {-# DEPRECATED scanWith "Please use scanlWith instead" #-}
  {-# INLINE scanWith #-}
  scanWith :: Monad m
      => Bool -> Fold m a b -> Stream m a -> Stream m b
  scanWith restart (Fold fstep initial extract final) =
      scanlWith restart (Scanl fstep initial extract final)
  
  -- XXX It may be useful to have a version of scan where we can keep the
  -- accumulator independent of the value emitted. So that we do not necessarily
  -- have to keep a value in the accumulator which we are not using. We can pass
  -- an extraction function that will take the accumulator and the current value
  -- of the element and emit the next value in the stream. That will also make it
  -- possible to modify the accumulator after using it. In fact, the step function
  -- can return new accumulator and the value to be emitted. The signature would
  -- be more like mapAccumL.
  
  -- | Strict left scan. Scan a stream using the given fold. Scan includes
  -- the initial (default) value of the accumulator as well as the final value.
  -- Compare with 'postscan' which omits the initial value.
  --
  -- >>> s = Stream.fromList [1..10]
  -- >>> Stream.fold Fold.toList $ Stream.takeWhile (< 10) $ Stream.scanl Scanl.sum s
  -- [0,1,3,6]
  --
  -- See also: 'usingStateT'
  --
  
  -- EXPLANATION:
  -- >>> scanl' step z = Stream.scanl (Scanl.mkScanl step z)
  --
  -- Like 'map', 'scanl'' too is a one to one transformation,
  -- however it adds an extra element.
  --
  -- >>> s = Stream.fromList [1,2,3,4]
  -- >>> Stream.fold Fold.toList $ scanl' (+) 0 s
  -- [0,1,3,6,10]
  --
  -- >>> Stream.fold Fold.toList $ scanl' (flip (:)) [] s
  -- [[],[1],[2,1],[3,2,1],[4,3,2,1]]
  --
  -- The output of 'scanl'' is the initial value of the accumulator followed by
  -- all the intermediate steps and the final result of 'foldl''.
  --
  -- By streaming the accumulated state after each fold step, we can share the
  -- state across multiple stages of stream composition. Each stage can modify or
  -- extend the state, do some processing with it and emit it for the next stage,
  -- thus modularizing the stream processing. This can be useful in
  -- stateful or event-driven programming.
  --
  -- Consider the following monolithic example, computing the sum and the product
  -- of the elements in a stream in one go using a @foldl'@:
  --
  -- >>> foldl' step z = Stream.fold (Scanl.mkScanl step z)
  -- >>> foldl' (\(s, p) x -> (s + x, p * x)) (0,1) s
  -- (10,24)
  --
  -- Using @scanl'@ we can make it modular by computing the sum in the first
  -- stage and passing it down to the next stage for computing the product:
  --
  -- >>> :{
  --   foldl' (\(_, p) (s, x) -> (s, p * x)) (0,1)
  --   $ scanl' (\(s, _) x -> (s + x, x)) (0,1)
  --   $ Stream.fromList [1,2,3,4]
  -- :}
  -- (10,24)
  --
  -- IMPORTANT: 'scanl'' evaluates the accumulator to WHNF.  To avoid building
  -- lazy expressions inside the accumulator, it is recommended that a strict
  -- data structure is used for accumulator.
  --
  {-# INLINE_NORMAL scanl #-}
  scanl :: Monad m
      => Scanl m a b -> Stream m a -> Stream m b
  scanl = scanlWith False
  
  -- | Like 'scanl' but restarts scanning afresh when the scanning fold
  -- terminates.
  --
  {-# INLINE_NORMAL scanlMany #-}
  scanlMany :: Monad m
      => Scanl m a b -> Stream m a -> Stream m b
  scanlMany = scanlWith True
  
  {-# DEPRECATED scan "Please use scanl instead" #-}
  {-# INLINE_NORMAL scan #-}
  scan :: Monad m
      => FL.Fold m a b -> Stream m a -> Stream m b
  scan = scanWith False
  
  {-# DEPRECATED scanMany "Please use scanlMany instead" #-}
  {-# INLINE_NORMAL scanMany #-}
  scanMany :: Monad m
      => FL.Fold m a b -> Stream m a -> Stream m b
  scanMany = scanWith True
  
  ------------------------------------------------------------------------------
  -- Scanning - Prescans
  ------------------------------------------------------------------------------
  
  -- Adapted from the vector package.
  --
  -- XXX Is a prescan useful, discarding the last step does not sound useful?  I
  -- am not sure about the utility of this function, so this is implemented but
  -- not exposed. We can expose it if someone provides good reasons why this is
  -- useful.
  --
  -- XXX We have to execute the stream one step ahead to know that we are at the
  -- last step.  The vector implementation of prescan executes the last fold step
  -- but does not yield the result. This means we have executed the effect but
  -- discarded value. This does not sound right. In this implementation we are
  -- not executing the last fold step.
  {-# INLINE_NORMAL prescanlM' #-}
  prescanlM' :: Monad m => (b -> a -> m b) -> m b -> Stream m a -> Stream m b
  prescanlM' f mz (Stream step state) = Stream step' (state, mz)
    where
      {-# INLINE_LATE step' #-}
      step' gst (st, prev) = do
          r <- step (adaptState gst) st
          case r of
              Yield x s -> do
                  acc <- prev
                  return $ Yield acc (s, f acc x)
              Skip s -> return $ Skip (s, prev)
              Stop   -> return Stop
  
  {-# INLINE prescanl' #-}
  prescanl' :: Monad m => (b -> a -> b) -> b -> Stream m a -> Stream m b
  prescanl' f z = prescanlM' (\a b -> return (f a b)) (return z)
  
  ------------------------------------------------------------------------------
  -- Monolithic postscans (postscan followed by a map)
  ------------------------------------------------------------------------------
  
  -- The performance of a modular postscan followed by a map seems to be
  -- equivalent to this monolithic scan followed by map therefore we may not need
  -- this implementation. We just have it for performance comparison and in case
  -- modular version does not perform well in some situation.
  --
  {-# INLINE_NORMAL postscanlMx' #-}
  postscanlMx' :: Monad m
      => (x -> a -> m x) -> m x -> (x -> m b) -> Stream m a -> Stream m b
  postscanlMx' fstep begin done (Stream step state) = do
      Stream step' (state, begin)
    where
      {-# INLINE_LATE step' #-}
      step' gst (st, acc) = do
          r <- step (adaptState gst) st
          case r of
              Yield x s -> do
                  old <- acc
                  y <- fstep old x
                  v <- done y
                  v `seq` y `seq` return (Yield v (s, return y))
              Skip s -> return $ Skip (s, acc)
              Stop   -> return Stop
  
  {-# INLINE_NORMAL postscanlx' #-}
  postscanlx' :: Monad m
      => (x -> a -> x) -> x -> (x -> b) -> Stream m a -> Stream m b
  postscanlx' fstep begin done =
      postscanlMx' (\b a -> return (fstep b a)) (return begin) (return . done)
  
  -- XXX do we need consM strict to evaluate the begin value?
  {-# INLINE scanlMx' #-}
  scanlMx' :: Monad m
      => (x -> a -> m x) -> m x -> (x -> m b) -> Stream m a -> Stream m b
  scanlMx' fstep begin done s =
      (begin >>= \x -> x `seq` done x) `consM` postscanlMx' fstep begin done s
  
  {-# INLINE scanlx' #-}
  scanlx' :: Monad m
      => (x -> a -> x) -> x -> (x -> b) -> Stream m a -> Stream m b
  scanlx' fstep begin done =
      scanlMx' (\b a -> return (fstep b a)) (return begin) (return . done)
  
  ------------------------------------------------------------------------------
  -- postscans
  ------------------------------------------------------------------------------
  
  -- Adapted from the vector package.
  {-# INLINE_NORMAL postscanlM' #-}
  postscanlM' :: Monad m => (b -> a -> m b) -> m b -> Stream m a -> Stream m b
  postscanlM' fstep begin (Stream step state) =
      Stream step' Nothing
    where
      {-# INLINE_LATE step' #-}
      step' _ Nothing = do
          !x <- begin
          return $ Skip (Just (state, x))
  
      step' gst (Just (st, acc)) =  do
          r <- step (adaptState gst) st
          case r of
              Yield x s -> do
                  !y <- fstep acc x
                  return $ Yield y (Just (s, y))
              Skip s -> return $ Skip (Just (s, acc))
              Stop   -> return Stop
  
  {-# INLINE_NORMAL postscanl' #-}
  postscanl' :: Monad m => (a -> b -> a) -> a -> Stream m b -> Stream m a
  postscanl' f seed = postscanlM' (\a b -> return (f a b)) (return seed)
  
  {-# ANN type PScanAfterState Fuse #-}
  data PScanAfterState m st acc =
        PScanAfterStep st (m acc)
      | PScanAfterYield acc (PScanAfterState m st acc)
      | PScanAfterStop
  
  -- We can possibly have the "done" function as a Maybe to provide an option to
  -- emit or not emit the accumulator when the stream stops.
  --
  -- TBD: use a single Yield point
  --
  {-# INLINE_NORMAL postscanlMAfter' #-}
  postscanlMAfter' :: Monad m
      => (b -> a -> m b) -> m b -> (b -> m b) -> Stream m a -> Stream m b
  postscanlMAfter' fstep initial done (Stream step1 state1) = do
      Stream step (PScanAfterStep state1 initial)
  
      where
  
      {-# INLINE_LATE step #-}
      step gst (PScanAfterStep st acc) = do
          r <- step1 (adaptState gst) st
          case r of
              Yield x s -> do
                  !old <- acc
                  !y <- fstep old x
                  return (Skip $ PScanAfterYield y (PScanAfterStep s (return y)))
              Skip s -> return $ Skip $ PScanAfterStep s acc
              -- Strictness is important for fusion
              Stop -> do
                  !v <- acc
                  !res <- done v
                  return (Skip $ PScanAfterYield res PScanAfterStop)
      step _ (PScanAfterYield acc next) = return $ Yield acc next
      step _ PScanAfterStop = return Stop
  
  {-# INLINE_NORMAL postscanlM #-}
  postscanlM :: Monad m => (b -> a -> m b) -> m b -> Stream m a -> Stream m b
  postscanlM fstep begin (Stream step state) = Stream step' Nothing
    where
      {-# INLINE_LATE step' #-}
      step' _ Nothing = do
          r <- begin
          return $ Skip (Just (state, r))
  
      step' gst (Just (st, acc)) = do
          r <- step (adaptState gst) st
          case r of
              Yield x s -> do
                  y <- fstep acc x
                  return (Yield y (Just (s, y)))
              Skip s -> return $ Skip (Just (s, acc))
              Stop   -> return Stop
  
  {-# INLINE_NORMAL postscanlBy #-}
  postscanlBy :: Monad m => (a -> b -> a) -> a -> Stream m b -> Stream m a
  postscanlBy f seed = postscanlM (\a b -> return (f a b)) (return seed)
  
  -- | Like 'scanl'' but with a monadic step function and a monadic seed.
  --
  {-# INLINE_NORMAL scanlM' #-}
  scanlM' :: Monad m => (b -> a -> m b) -> m b -> Stream m a -> Stream m b
  scanlM' fstep begin (Stream step state) = Stream step' Nothing
    where
      {-# INLINE_LATE step' #-}
      step' _ Nothing = do
          !x <- begin
          return $ Yield x (Just (state, x))
      step' gst (Just (st, acc)) =  do
          r <- step (adaptState gst) st
          case r of
              Yield x s -> do
                  !y <- fstep acc x
                  return $ Yield y (Just (s, y))
              Skip s -> return $ Skip (Just (s, acc))
              Stop   -> return Stop
  
  -- | @scanlMAfter' accumulate initial done stream@ is like 'scanlM'' except
  -- that it provides an additional @done@ function to be applied on the
  -- accumulator when the stream stops. The result of @done@ is also emitted in
  -- the stream.
  --
  -- This function can be used to allocate a resource in the beginning of the
  -- scan and release it when the stream ends or to flush the internal state of
  -- the scan at the end.
  --
  -- /Pre-release/
  --
  {-# INLINE scanlMAfter' #-}
  scanlMAfter' :: Monad m
      => (b -> a -> m b) -> m b -> (b -> m b) -> Stream m a -> Stream m b
  scanlMAfter' fstep initial done s =
      initial `consM` postscanlMAfter' fstep initial done s
  
  -- >>> scanl' f z xs = z `Stream.cons` postscanl' f z xs
  
  -- | Strict left scan. Like 'map', 'scanl'' too is a one to one transformation,
  -- however it adds an extra element.
  --
  -- >>> Stream.toList $ Stream.scanl' (+) 0 $ Stream.fromList [1,2,3,4]
  -- [0,1,3,6,10]
  --
  -- >>> Stream.toList $ Stream.scanl' (flip (:)) [] $ Stream.fromList [1,2,3,4]
  -- [[],[1],[2,1],[3,2,1],[4,3,2,1]]
  --
  -- The output of 'scanl'' is the initial value of the accumulator followed by
  -- all the intermediate steps and the final result of 'foldl''.
  --
  -- By streaming the accumulated state after each fold step, we can share the
  -- state across multiple stages of stream composition. Each stage can modify or
  -- extend the state, do some processing with it and emit it for the next stage,
  -- thus modularizing the stream processing. This can be useful in
  -- stateful or event-driven programming.
  --
  -- Consider the following monolithic example, computing the sum and the product
  -- of the elements in a stream in one go using a @foldl'@:
  --
  -- >>> Stream.fold (Fold.foldl' (\(s, p) x -> (s + x, p * x)) (0,1)) $ Stream.fromList [1,2,3,4]
  -- (10,24)
  --
  -- Using @scanl'@ we can make it modular by computing the sum in the first
  -- stage and passing it down to the next stage for computing the product:
  --
  -- >>> :{
  --   Stream.fold (Fold.foldl' (\(_, p) (s, x) -> (s, p * x)) (0,1))
  --   $ Stream.scanl' (\(s, _) x -> (s + x, x)) (0,1)
  --   $ Stream.fromList [1,2,3,4]
  -- :}
  -- (10,24)
  --
  -- IMPORTANT: 'scanl'' evaluates the accumulator to WHNF.  To avoid building
  -- lazy expressions inside the accumulator, it is recommended that a strict
  -- data structure is used for accumulator.
  --
  -- >>> scanl' step z = Stream.scanl (Scanl.mkScanl step z)
  -- >>> scanl' f z xs = Stream.scanlM' (\a b -> return (f a b)) (return z) xs
  --
  -- See also: 'usingStateT'
  --
  {-# INLINE scanl' #-}
  scanl' :: Monad m => (b -> a -> b) -> b -> Stream m a -> Stream m b
  scanl' f seed = scanlM' (\a b -> return (f a b)) (return seed)
  
  {-# INLINE_NORMAL scanlM #-}
  scanlM :: Monad m => (b -> a -> m b) -> m b -> Stream m a -> Stream m b
  scanlM fstep begin (Stream step state) = Stream step' Nothing
    where
      {-# INLINE_LATE step' #-}
      step' _ Nothing = do
          x <- begin
          return $ Yield x (Just (state, x))
      step' gst (Just (st, acc)) = do
          r <- step (adaptState gst) st
          case r of
              Yield x s -> do
                  y <- fstep acc x
                  return $ Yield y (Just (s, y))
              Skip s -> return $ Skip (Just (s, acc))
              Stop   -> return Stop
  
  {-# INLINE scanlBy #-}
  scanlBy :: Monad m => (b -> a -> b) -> b -> Stream m a -> Stream m b
  scanlBy f seed = scanlM (\a b -> return (f a b)) (return seed)
  
  -- Adapted from the vector package
  {-# INLINE_NORMAL scanl1M #-}
  scanl1M :: Monad m => (a -> a -> m a) -> Stream m a -> Stream m a
  scanl1M fstep (Stream step state) = Stream step' (state, Nothing)
    where
      {-# INLINE_LATE step' #-}
      step' gst (st, Nothing) = do
          r <- step gst st
          case r of
              Yield x s -> return $ Yield x (s, Just x)
              Skip s -> return $ Skip (s, Nothing)
              Stop   -> return Stop
  
      step' gst (st, Just acc) = do
          r <- step gst st
          case r of
              Yield y s -> do
                  z <- fstep acc y
                  return $ Yield z (s, Just z)
              Skip s -> return $ Skip (s, Just acc)
              Stop   -> return Stop
  
  {-# INLINE scanl1 #-}
  scanl1 :: Monad m => (a -> a -> a) -> Stream m a -> Stream m a
  scanl1 f = scanl1M (\x y -> return (f x y))
  
  -- Adapted from the vector package
  
  -- | Like 'scanl1'' but with a monadic step function.
  --
  {-# INLINE_NORMAL scanl1M' #-}
  scanl1M' :: Monad m => (a -> a -> m a) -> Stream m a -> Stream m a
  scanl1M' fstep (Stream step state) = Stream step' (state, Nothing)
    where
      {-# INLINE_LATE step' #-}
      step' gst (st, Nothing) = do
          r <- step gst st
          case r of
              Yield x s -> x `seq` return $ Yield x (s, Just x)
              Skip s -> return $ Skip (s, Nothing)
              Stop   -> return Stop
  
      step' gst (st, Just acc) = acc `seq` do
          r <- step gst st
          case r of
              Yield y s -> do
                  z <- fstep acc y
                  z `seq` return $ Yield z (s, Just z)
              Skip s -> return $ Skip (s, Just acc)
              Stop   -> return Stop
  
  -- | Like 'scanl'' but for a non-empty stream. The first element of the stream
  -- is used as the initial value of the accumulator. Does nothing if the stream
  -- is empty.
  --
  -- >>> Stream.toList $ Stream.scanl1' (+) $ Stream.fromList [1,2,3,4]
  -- [1,3,6,10]
  --
  {-# INLINE scanl1' #-}
  scanl1' :: Monad m => (a -> a -> a) -> Stream m a -> Stream m a
  scanl1' f = scanl1M' (\x y -> return (f x y))
  
  -------------------------------------------------------------------------------
  -- Filtering
  -------------------------------------------------------------------------------
  
  -- | Modify a @Stream m a -> Stream m a@ stream transformation that accepts a
  -- predicate @(a -> b)@ to accept @((s, a) -> b)@ instead, provided a
  -- transformation @Stream m a -> Stream m (s, a)@. Convenient to filter with
  -- index or time.
  --
  -- >>> filterWithIndex = Stream.with Stream.indexed Stream.filter
  --
  -- /Pre-release/
  {-# INLINE with #-}
  with :: Monad m =>
         (Stream m a -> Stream m (s, a))
      -> (((s, a) -> b) -> Stream m (s, a) -> Stream m (s, a))
      -> (((s, a) -> b) -> Stream m a -> Stream m a)
  with f comb g = fmap snd . comb g . f
  
  -- Adapted from the vector package
  
  -- | Same as 'filter' but with a monadic predicate.
  --
  -- >>> f p x = p x >>= \r -> return $ if r then Just x else Nothing
  -- >>> filterM p = Stream.mapMaybeM (f p)
  --
  {-# INLINE_NORMAL filterM #-}
  filterM :: Monad m => (a -> m Bool) -> Stream m a -> Stream m a
  filterM f (Stream step state) = Stream step' state
    where
      {-# INLINE_LATE step' #-}
      step' gst st = do
          r <- step gst st
          case r of
              Yield x s -> do
                  b <- f x
                  return $ if b
                           then Yield x s
                           else Skip s
              Skip s -> return $ Skip s
              Stop   -> return Stop
  
  -- | Include only those elements that pass a predicate.
  --
  -- >>> filter p = Stream.filterM (return . p)
  -- >>> filter p = Stream.mapMaybe (\x -> if p x then Just x else Nothing)
  -- >>> filter p = Stream.postscanlMaybe (Scanl.filtering p)
  --
  {-# INLINE filter #-}
  filter :: Monad m => (a -> Bool) -> Stream m a -> Stream m a
  filter f = filterM (return . f)
  -- filter p = scanMaybe (FL.filtering p)
  
  -- | Drop repeated elements that are adjacent to each other using the supplied
  -- comparison function.
  --
  -- >>> uniq = Stream.uniqBy (==)
  --
  -- To strip duplicate path separators:
  --
  -- >>> input = Stream.fromList "//a//b"
  -- >>> f x y = x == '/' && y == '/'
  -- >>> Stream.fold Fold.toList $ Stream.uniqBy f input
  -- "/a/b"
  --
  -- Space: @O(1)@
  --
  -- /Pre-release/
  --
  {-# INLINE uniqBy #-}
  uniqBy :: Monad m =>
      (a -> a -> Bool) -> Stream m a -> Stream m a
  -- uniqBy eq = scanMaybe (FL.uniqBy eq)
  uniqBy eq = catMaybes . rollingMap f
  
      where
  
      f pre curr =
          case pre of
              Nothing -> Just curr
              Just x -> if x `eq` curr then Nothing else Just curr
  
  -- Adapted from the vector package
  
  -- | Drop repeated elements that are adjacent to each other.
  --
  -- >>> uniq = Stream.uniqBy (==)
  --
  {-# INLINE_NORMAL uniq #-}
  uniq :: (Eq a, Monad m) => Stream m a -> Stream m a
  -- uniq = scanMaybe FL.uniq
  uniq (Stream step state) = Stream step' (Nothing, state)
    where
      {-# INLINE_LATE step' #-}
      step' gst (Nothing, st) = do
          r <- step gst st
          case r of
              Yield x s -> return $ Yield x (Just x, s)
              Skip  s   -> return $ Skip  (Nothing, s)
              Stop      -> return Stop
      step' gst (Just x, st)  = do
           r <- step gst st
           case r of
               Yield y s | x == y   -> return $ Skip (Just x, s)
                         | otherwise -> return $ Yield y (Just y, s)
               Skip  s   -> return $ Skip (Just x, s)
               Stop      -> return Stop
  
  -- | Deletes the first occurrence of the element in the stream that satisfies
  -- the given equality predicate.
  --
  -- >>> input = Stream.fromList [1,3,3,5]
  -- >>> Stream.fold Fold.toList $ Stream.deleteBy (==) 3 input
  -- [1,3,5]
  --
  {-# INLINE_NORMAL deleteBy #-}
  deleteBy :: Monad m => (a -> a -> Bool) -> a -> Stream m a -> Stream m a
  -- deleteBy cmp x = scanMaybe (FL.deleteBy cmp x)
  deleteBy eq x (Stream step state) = Stream step' (state, False)
    where
      {-# INLINE_LATE step' #-}
      step' gst (st, False) = do
          r <- step gst st
          case r of
              Yield y s -> return $
                  if eq x y then Skip (s, True) else Yield y (s, False)
              Skip s -> return $ Skip (s, False)
              Stop   -> return Stop
  
      step' gst (st, True) = do
          r <- step gst st
          case r of
              Yield y s -> return $ Yield y (s, True)
              Skip s -> return $ Skip (s, True)
              Stop   -> return Stop
  
  -- | Strip all leading and trailing occurrences of an element passing a
  -- predicate and make all other consecutive occurrences uniq.
  --
  -- >> prune p = Stream.dropWhileAround p $ Stream.uniqBy (x y -> p x && p y)
  --
  -- @
  -- > Stream.prune isSpace (Stream.fromList "  hello      world!   ")
  -- "hello world!"
  --
  -- @
  --
  -- Space: @O(1)@
  --
  -- /Unimplemented/
  {-# INLINE prune #-}
  prune ::
      -- (Monad m, Eq a) =>
      (a -> Bool) -> Stream m a -> Stream m a
  prune = error "Not implemented yet!"
  
  -- Possible implementation:
  -- @repeated =
  --      Stream.catMaybes . Stream.parseMany (Parser.groupBy (==) Fold.repeated)@
  --
  -- 'Fold.repeated' should return 'Just' when repeated, and 'Nothing' for a
  -- single element.
  
  -- | Emit only repeated elements, once.
  --
  -- /Unimplemented/
  repeated :: -- (Monad m, Eq a) =>
      Stream m a -> Stream m a
  repeated = undefined
  
  ------------------------------------------------------------------------------
  -- Sampling
  ------------------------------------------------------------------------------
  
  -- XXX We can implement this using addition instead of "mod" to make it more
  -- efficient.
  
  -- | @sampleFromThen offset stride@ takes the element at @offset@ index and
  -- then every element at strides of @stride@.
  --
  -- >>> Stream.fold Fold.toList $ Stream.sampleFromThen 2 3 $ Stream.enumerateFromTo 0 10
  -- [2,5,8]
  --
  {-# INLINE sampleFromThen #-}
  sampleFromThen, strideFromThen :: Monad m =>
      Int -> Int -> Stream m a -> Stream m a
  sampleFromThen offset stride =
      with indexed filter
          (\(i, _) -> i >= offset && (i - offset) `mod` stride == 0)
  
  RENAME(strideFromThen,sampleFromThen)
  
  ------------------------------------------------------------------------------
  -- Trimming
  ------------------------------------------------------------------------------
  
  -- | init for non-empty streams, fails for empty stream case.
  --
  {-# INLINE initNonEmpty #-}
  initNonEmpty :: Monad m => Stream m a -> Stream m a
  initNonEmpty (Stream step1 state1) = Stream step (Nothing, state1)
  
      where
  
      step gst (Nothing, s1) = do
          r <- step1 (adaptState gst) s1
          return $
              case r of
                  Yield x s -> Skip (Just x, s)
                  Skip s -> Skip (Nothing, s)
                  Stop -> error "initNonEmpty: empty Stream"
  
      step gst (Just a, s1) = do
          r <- step1 (adaptState gst) s1
          return $
              case r of
                  Yield x s -> Yield a (Just x, s)
                  Skip s -> Skip (Just a, s)
                  Stop -> Stop
  
  -- | tail for non-empty streams, fails for empty stream case.
  --
  -- See also 'tail' for a non-partial version of this function..
  {-# INLINE tailNonEmpty #-}
  tailNonEmpty :: Monad m => Stream m a -> Stream m a
  tailNonEmpty (Stream step1 state1) = Stream step (Nothing, state1)
  
      where
  
      step gst (Nothing, s1) = do
          r <- step1 (adaptState gst) s1
          return $
              case r of
                  Yield x s -> Skip (Just x, s)
                  Skip s -> Skip (Nothing, s)
                  Stop -> error "tailNonEmpty: empty Stream"
  
      step gst (Just a, s1) = do
          r <- step1 (adaptState gst) s1
          return $
              case r of
                  Yield x s -> Yield x (Just x, s)
                  Skip s -> Skip (Just a, s)
                  Stop -> Stop
  
  -- | Take all consecutive elements at the end of the stream for which the
  -- predicate is true.
  --
  -- O(n) space, where n is the number elements taken.
  --
  -- /Unimplemented/
  {-# INLINE takeWhileLast #-}
  takeWhileLast :: -- Monad m =>
      (a -> Bool) -> Stream m a -> Stream m a
  takeWhileLast = undefined -- fromStreamD $ D.takeWhileLast n $ toStreamD m
  
  -- | Like 'takeWhile' and 'takeWhileLast' combined.
  --
  -- O(n) space, where n is the number elements taken from the end.
  --
  -- /Unimplemented/
  {-# INLINE takeWhileAround #-}
  takeWhileAround :: -- Monad m =>
      (a -> Bool) -> Stream m a -> Stream m a
  takeWhileAround = undefined -- fromStreamD $ D.takeWhileAround n $ toStreamD m
  
  -- Adapted from the vector package
  
  -- | Discard first 'n' elements from the stream and take the rest.
  --
  {-# INLINE_NORMAL drop #-}
  drop :: Monad m => Int -> Stream m a -> Stream m a
  drop n (Stream step state) = Stream step' (state, Just n)
    where
      {-# INLINE_LATE step' #-}
      step' gst (st, Just i)
        | i > 0 = do
            r <- step gst st
            return $
              case r of
                Yield _ s -> Skip (s, Just (i - 1))
                Skip s    -> Skip (s, Just i)
                Stop      -> Stop
        | otherwise = return $ Skip (st, Nothing)
  
      step' gst (st, Nothing) = do
        r <- step gst st
        return $
          case r of
            Yield x s -> Yield x (s, Nothing)
            Skip  s   -> Skip (s, Nothing)
            Stop      -> Stop
  
  -- Adapted from the vector package
  data DropWhileState s a
      = DropWhileDrop s
      | DropWhileYield a s
      | DropWhileNext s
  
  -- | Same as 'dropWhile' but with a monadic predicate.
  --
  {-# INLINE_NORMAL dropWhileM #-}
  dropWhileM :: Monad m => (a -> m Bool) -> Stream m a -> Stream m a
  -- dropWhileM p = scanMaybe (FL.droppingWhileM p)
  dropWhileM f (Stream step state) = Stream step' (DropWhileDrop state)
    where
      {-# INLINE_LATE step' #-}
      step' gst (DropWhileDrop st) = do
          r <- step gst st
          case r of
              Yield x s -> do
                  b <- f x
                  if b
                  then return $ Skip (DropWhileDrop s)
                  else return $ Skip (DropWhileYield x s)
              Skip s -> return $ Skip (DropWhileDrop s)
              Stop -> return Stop
  
      step' gst (DropWhileNext st) =  do
          r <- step gst st
          case r of
              Yield x s -> return $ Skip (DropWhileYield x s)
              Skip s    -> return $ Skip (DropWhileNext s)
              Stop      -> return Stop
  
      step' _ (DropWhileYield x st) = return $ Yield x (DropWhileNext st)
  
  -- | Drop elements in the stream as long as the predicate succeeds and then
  -- take the rest of the stream.
  --
  {-# INLINE dropWhile #-}
  dropWhile :: Monad m => (a -> Bool) -> Stream m a -> Stream m a
  -- dropWhile p = scanMaybe (FL.droppingWhile p)
  dropWhile f = dropWhileM (return . f)
  
  -- | Drop @n@ elements at the end of the stream.
  --
  -- O(n) space, where n is the number elements dropped.
  --
  -- /Unimplemented/
  {-# INLINE dropLast #-}
  dropLast :: -- Monad m =>
      Int -> Stream m a -> Stream m a
  dropLast = undefined -- fromStreamD $ D.dropLast n $ toStreamD m
  
  -- | Drop all consecutive elements at the end of the stream for which the
  -- predicate is true.
  --
  -- O(n) space, where n is the number elements dropped.
  --
  -- /Unimplemented/
  {-# INLINE dropWhileLast #-}
  dropWhileLast :: -- Monad m =>
      (a -> Bool) -> Stream m a -> Stream m a
  dropWhileLast = undefined -- fromStreamD $ D.dropWhileLast n $ toStreamD m
  
  -- | Like 'dropWhile' and 'dropWhileLast' combined.
  --
  -- O(n) space, where n is the number elements dropped from the end.
  --
  -- /Unimplemented/
  {-# INLINE dropWhileAround #-}
  dropWhileAround :: -- Monad m =>
      (a -> Bool) -> Stream m a -> Stream m a
  dropWhileAround = undefined -- fromStreamD $ D.dropWhileAround n $ toStreamD m
  
  ------------------------------------------------------------------------------
  -- Inserting Elements
  ------------------------------------------------------------------------------
  
  -- | @insertBy cmp elem stream@ inserts @elem@ before the first element in
  -- @stream@ that is less than @elem@ when compared using @cmp@.
  --
  -- >>> insertBy cmp x = Stream.mergeBy cmp (Stream.fromPure x)
  --
  -- >>> input = Stream.fromList [1,3,5]
  -- >>> Stream.fold Fold.toList $ Stream.insertBy compare 2 input
  -- [1,2,3,5]
  --
  {-# INLINE_NORMAL insertBy #-}
  insertBy :: Monad m => (a -> a -> Ordering) -> a -> Stream m a -> Stream m a
  insertBy cmp a (Stream step state) = Stream step' (state, False, Nothing)
    where
      {-# INLINE_LATE step' #-}
      step' gst (st, False, _) = do
          r <- step gst st
          case r of
              Yield x s -> case cmp a x of
                  GT -> return $ Yield x (s, False, Nothing)
                  _  -> return $ Yield a (s, True, Just x)
              Skip s -> return $ Skip (s, False, Nothing)
              Stop   -> return $ Yield a (st, True, Nothing)
  
      step' _ (_, True, Nothing) = return Stop
  
      step' gst (st, True, Just prev) = do
          r <- step gst st
          case r of
              Yield x s -> return $ Yield prev (s, True, Just x)
              Skip s    -> return $ Skip (s, True, Just prev)
              Stop      -> return $ Yield prev (st, True, Nothing)
  
  data LoopState x s = FirstYield s
                     | InterspersingYield s
                     | YieldAndCarry x s
  
  -- | Effectful variant of 'intersperse'. Insert an effect and its output
  -- between successive elements of a stream. It does nothing if stream has less
  -- than two elements.
  --
  -- Definition:
  --
  -- >>> intersperseM x = Stream.interleaveSepBy (Stream.repeatM x)
  --
  {-# INLINE_NORMAL intersperseM #-}
  intersperseM :: Monad m => m a -> Stream m a -> Stream m a
  intersperseM m (Stream step state) = Stream step' (FirstYield state)
    where
      {-# INLINE_LATE step' #-}
      step' gst (FirstYield st) = do
          r <- step gst st
          return $
              case r of
                  Yield x s -> Skip (YieldAndCarry x s)
                  Skip s -> Skip (FirstYield s)
                  Stop -> Stop
  
      step' gst (InterspersingYield st) = do
          r <- step gst st
          case r of
              Yield x s -> do
                  a <- m
                  return $ Yield a (YieldAndCarry x s)
              Skip s -> return $ Skip $ InterspersingYield s
              Stop -> return Stop
  
      step' _ (YieldAndCarry x st) = return $ Yield x (InterspersingYield st)
  
  -- | Insert a pure value between successive elements of a stream. It does
  -- nothing if stream has less than two elements.
  --
  -- Definition:
  --
  -- >>> intersperse x = Stream.intersperseM (return x)
  -- >>> intersperse x = Stream.unfoldEachSepBy x Unfold.identity
  -- >>> intersperse x = Stream.unfoldEachSepBySeq x Unfold.identity
  -- >>> intersperse x = Stream.interleaveSepBy (Stream.repeat x)
  --
  -- Example:
  --
  -- >>> f x y = Stream.toList $ Stream.intersperse x $ Stream.fromList y
  -- >>> f ',' "abc"
  -- "a,b,c"
  -- >>> f ',' "a"
  -- "a"
  --
  {-# INLINE intersperse #-}
  intersperse :: Monad m => a -> Stream m a -> Stream m a
  intersperse a = intersperseM (return a)
  
  -- | Perform a side effect between two successive elements of a stream. It does
  -- nothing if the stream has less than two elements.
  --
  -- >>> f x y = Stream.fold Fold.drain $ Stream.trace putChar $ Stream.intersperseM_ x $ Stream.fromList y
  -- >>> f (putChar '.') "abc"
  -- a.b.c
  -- >>> f (putChar '.') "a"
  -- a
  --
  -- /Pre-release/
  {-# INLINE_NORMAL intersperseM_ #-}
  intersperseM_ :: Monad m => m b -> Stream m a -> Stream m a
  intersperseM_ m (Stream step1 state1) = Stream step (Left (pure (), state1))
    where
      {-# INLINE_LATE step #-}
      step gst (Left (eff, st)) = do
          r <- step1 gst st
          case r of
              Yield x s -> eff >> return (Yield x (Right s))
              Skip s -> return $ Skip (Left (eff, s))
              Stop -> return Stop
  
      step _ (Right st) = return $ Skip $ Left (void m, st)
  
  -- | Intersperse a monadic action into the input stream after every @n@
  -- elements.
  --
  -- Definition:
  --
  -- >> intersperseEveryM n x = Stream.interleaveEverySepBy n (Stream.repeatM x)
  --
  -- Idioms:
  --
  -- >>> intersperseM = Stream.intersperseEveryM 1
  -- >>> intersperse x = Stream.intersperseEveryM 1 (return x)
  --
  -- Usage:
  --
  -- >> input = Stream.fromList "hello"
  -- >> Stream.toList $ Stream.intersperseEveryM 2 (return ',') input
  -- "he,ll,o"
  --
  -- /Unimplemented/
  {-# INLINE intersperseEveryM #-}
  intersperseEveryM :: -- Monad m =>
      Int -> m a -> Stream m a -> Stream m a
  intersperseEveryM _n _f _xs = undefined
  
  data SuffixState s a
      = SuffixElem s
      | SuffixSuffix s
      | SuffixYield a (SuffixState s a)
  
  -- | Insert an effect and its output after every element of a stream.
  --
  -- Definition:
  --
  -- >>> intersperseEndByM x = Stream.interleaveEndBy (Stream.repeatM x)
  --
  -- Usage:
  --
  -- >>> f x y = Stream.toList $ Stream.intersperseEndByM (pure x) $ Stream.fromList y
  -- >>> f ',' "abc"
  -- "a,b,c,"
  -- >>> f ',' "a"
  -- "a,"
  --
  -- /Pre-release/
  {-# INLINE_NORMAL intersperseEndByM #-}
  intersperseEndByM, intersperseMSuffix :: forall m a. Monad m =>
      m a -> Stream m a -> Stream m a
  intersperseEndByM action (Stream step state) = Stream step' (SuffixElem state)
      where
      {-# INLINE_LATE step' #-}
      step' gst (SuffixElem st) = do
          r <- step gst st
          return $ case r of
              Yield x s -> Skip (SuffixYield x (SuffixSuffix s))
              Skip s -> Skip (SuffixElem s)
              Stop -> Stop
  
      step' _ (SuffixSuffix st) = do
          action >>= \r -> return $ Skip (SuffixYield r (SuffixElem st))
  
      step' _ (SuffixYield x next) = return $ Yield x next
  
  RENAME(intersperseMSuffix,intersperseEndByM)
  
  -- | Insert an effect after every element of a stream.
  --
  -- Example:
  --
  -- >>> f x y = Stream.fold Fold.drain $ Stream.trace putChar $ Stream.intersperseEndByM_ x $ Stream.fromList y
  -- >>> f (putChar '.') "abc"
  -- a.b.c.
  -- >>> f (putChar '.') "a"
  -- a.
  --
  -- /Pre-release/
  --
  {-# INLINE_NORMAL intersperseEndByM_ #-}
  intersperseEndByM_, intersperseMSuffix_ :: Monad m => m b -> Stream m a -> Stream m a
  intersperseEndByM_ m (Stream step1 state1) = Stream step (Left state1)
    where
      {-# INLINE_LATE step #-}
      step gst (Left st) = do
          r <- step1 gst st
          case r of
              Yield x s -> return $ Yield x (Right s)
              Skip s -> return $ Skip $ Left s
              Stop -> return Stop
  
      step _ (Right st) = m >> return (Skip (Left st))
  
  RENAME(intersperseMSuffix_,intersperseEndByM_)
  
  data SuffixSpanState s a
      = SuffixSpanElem s Int
      | SuffixSpanSuffix s
      | SuffixSpanYield a (SuffixSpanState s a)
      | SuffixSpanLast
      | SuffixSpanStop
  
  -- | Like 'intersperseEndByM' but intersperses an effectful action into the
  -- input stream after every @n@ elements and also after the last element.
  --
  -- Example:
  --
  -- >>> input = Stream.fromList "hello"
  -- >>> Stream.toList $ Stream.intersperseEndByEveryM 2 (return ',') input
  -- "he,ll,o,"
  -- >>> f n x y = Stream.toList $ Stream.intersperseEndByEveryM n (pure x) $ Stream.fromList y
  -- >>> f 2 ',' "abcdef"
  -- "ab,cd,ef,"
  -- >>> f 2 ',' "abcdefg"
  -- "ab,cd,ef,g,"
  -- >>> f 2 ',' "a"
  -- "a,"
  --
  -- /Pre-release/
  --
  {-# INLINE_NORMAL intersperseEndByEveryM #-}
  intersperseEndByEveryM, intersperseMSuffixWith :: forall m a. Monad m
      => Int -> m a -> Stream m a -> Stream m a
  intersperseEndByEveryM n action (Stream step state) =
      Stream step' (SuffixSpanElem state n)
      where
      {-# INLINE_LATE step' #-}
      step' gst (SuffixSpanElem st i) | i > 0 = do
          r <- step gst st
          return $ case r of
              Yield x s -> Skip (SuffixSpanYield x (SuffixSpanElem s (i - 1)))
              Skip s -> Skip (SuffixSpanElem s i)
              Stop -> if i == n then Stop else Skip SuffixSpanLast
      step' _ (SuffixSpanElem st _) = return $ Skip (SuffixSpanSuffix st)
  
      step' _ (SuffixSpanSuffix st) = do
          action >>= \r -> return $ Skip (SuffixSpanYield r (SuffixSpanElem st n))
  
      step' _ SuffixSpanLast = do
          action >>= \r -> return $ Skip (SuffixSpanYield r SuffixSpanStop)
  
      step' _ (SuffixSpanYield x next) = return $ Yield x next
  
      step' _ SuffixSpanStop = return Stop
  
  RENAME(intersperseMSuffixWith,intersperseEndByEveryM)
  
  -- | Insert a side effect before every element of a stream.
  --
  -- Definition:
  --
  -- >>> intersperseBeginByM_ = Stream.trace_
  -- >>> intersperseBeginByM_ m = Stream.mapM (\x -> void m >> return x)
  --
  -- Usage:
  --
  -- >>> f x y = Stream.fold Fold.drain $ Stream.trace putChar $ Stream.intersperseBeginByM_ x $ Stream.fromList y
  -- >>> f (putChar '.') "abc"
  -- .a.b.c
  --
  -- Same as 'trace_'.
  --
  -- /Pre-release/
  --
  {-# INLINE intersperseBeginByM_ #-}
  intersperseBeginByM_, intersperseMPrefix_ :: Monad m =>
      m b -> Stream m a -> Stream m a
  intersperseBeginByM_ m = mapM (\x -> void m >> return x)
  
  RENAME(intersperseMPrefix_,intersperseBeginByM_)
  
  ------------------------------------------------------------------------------
  -- Inserting Time
  ------------------------------------------------------------------------------
  
  -- XXX This should be in Prelude, should we export this as a helper function?
  
  -- | Block the current thread for specified number of seconds.
  {-# INLINE sleep #-}
  sleep :: MonadIO m => Double -> m ()
  sleep n = liftIO $ threadDelay $ round $ n * 1000000
  
  -- | Introduce a delay of specified seconds between elements of the stream.
  --
  -- Definition:
  --
  -- >>> sleep n = liftIO $ threadDelay $ round $ n * 1000000
  -- >>> delay = Stream.intersperseM_ . sleep
  --
  -- Example:
  --
  -- >>> input = Stream.enumerateFromTo 1 3
  -- >>> Stream.fold (Fold.drainMapM print) $ Stream.delay 1 input
  -- 1
  -- 2
  -- 3
  --
  {-# INLINE delay #-}
  delay :: MonadIO m => Double -> Stream m a -> Stream m a
  delay = intersperseM_ . sleep
  
  -- | Introduce a delay of specified seconds after consuming an element of a
  -- stream.
  --
  -- Definition:
  --
  -- >>> sleep n = liftIO $ threadDelay $ round $ n * 1000000
  -- >>> delayPost = Stream.intersperseEndByM_ . sleep
  --
  -- Example:
  --
  -- >>> input = Stream.enumerateFromTo 1 3
  -- >>> Stream.fold (Fold.drainMapM print) $ Stream.delayPost 1 input
  -- 1
  -- 2
  -- 3
  --
  -- /Pre-release/
  --
  {-# INLINE delayPost #-}
  delayPost :: MonadIO m => Double -> Stream m a -> Stream m a
  delayPost n = intersperseMSuffix_ $ liftIO $ threadDelay $ round $ n * 1000000
  
  -- | Introduce a delay of specified seconds before consuming an element of a
  -- stream.
  --
  -- Definition:
  --
  -- >>> sleep n = liftIO $ threadDelay $ round $ n * 1000000
  -- >>> delayPre = Stream.intersperseBeginByM_ . sleep
  --
  -- Example:
  --
  -- >>> input = Stream.enumerateFromTo 1 3
  -- >>> Stream.fold (Fold.drainMapM print) $ Stream.delayPre 1 input
  -- 1
  -- 2
  -- 3
  --
  -- /Pre-release/
  --
  {-# INLINE delayPre #-}
  delayPre :: MonadIO m => Double -> Stream m a -> Stream m a
  delayPre = intersperseMPrefix_. sleep
  
  ------------------------------------------------------------------------------
  -- Reordering
  ------------------------------------------------------------------------------
  
  -- | Returns the elements of the stream in reverse order.  The stream must be
  -- finite. Note that this necessarily buffers the entire stream in memory.
  --
  -- Definition:
  --
  -- >>> reverse m = Stream.concatEffect $ Stream.fold Fold.toListRev m >>= return . Stream.fromList
  --
  {-# INLINE_NORMAL reverse #-}
  reverse :: Monad m => Stream m a -> Stream m a
  reverse m = concatEffect $ fold FL.toListRev m <&> fromList
  {-
  reverse m = Stream step Nothing
      where
      {-# INLINE_LATE step #-}
      step _ Nothing = do
          xs <- foldl' (flip (:)) [] m
          return $ Skip (Just xs)
      step _ (Just (x:xs)) = return $ Yield x (Just xs)
      step _ (Just []) = return Stop
  -}
  
  -- | Like 'reverse' but several times faster, requires an 'Unbox' instance.
  --
  -- /O(n) space/
  --
  -- /Pre-release/
  {-# INLINE reverseUnbox #-}
  reverseUnbox :: (MonadIO m, Unbox a) => Stream m a -> Stream m a
  reverseUnbox =
      A.concatRev -- unfoldMany A.readerRev
          . fromStreamK
          . K.reverse
          . toStreamK
          . A.chunksOf defaultChunkSize
  
  -- | Buffer until the next element in sequence arrives. The function argument
  -- determines the difference in sequence numbers. This could be useful in
  -- implementing sequenced streams, for example, TCP reassembly.
  --
  -- /Unimplemented/
  --
  {-# INLINE reassembleBy #-}
  reassembleBy
      :: -- Monad m =>
         Fold m a b
      -> (a -> a -> Int)
      -> Stream m a
      -> Stream m b
  reassembleBy = undefined
  
  ------------------------------------------------------------------------------
  -- Position Indexing
  ------------------------------------------------------------------------------
  
  -- Adapted from the vector package
  
  -- |
  -- >>> f = Scanl.mkScanl (\(i, _) x -> (i + 1, x)) (-1,undefined)
  -- >>> indexed = Stream.postscanl f
  -- >>> indexed = Stream.zipWith (,) (Stream.enumerateFrom 0)
  -- >>> indexedR n = fmap (\(i, a) -> (n - i, a)) . indexed
  --
  -- Pair each element in a stream with its index, starting from index 0.
  --
  -- >>> Stream.fold Fold.toList $ Stream.indexed $ Stream.fromList "hello"
  -- [(0,'h'),(1,'e'),(2,'l'),(3,'l'),(4,'o')]
  --
  {-# INLINE_NORMAL indexed #-}
  indexed :: Monad m => Stream m a -> Stream m (Int, a)
  -- indexed = scanMaybe FL.indexing
  indexed (Stream step state) = Stream step' (state, 0)
    where
      {-# INLINE_LATE step' #-}
      step' gst (st, i) = i `seq` do
           r <- step (adaptState gst) st
           case r of
               Yield x s -> return $ Yield (i, x) (s, i+1)
               Skip    s -> return $ Skip (s, i)
               Stop      -> return Stop
  
  -- Adapted from the vector package
  
  -- |
  -- >>> f n = Scanl.mkScanl (\(i, _) x -> (i - 1, x)) (n + 1,undefined)
  -- >>> indexedR n = Stream.postscanl (f n)
  --
  -- >>> s n = Stream.enumerateFromThen n (n - 1)
  -- >>> indexedR n = Stream.zipWith (,) (s n)
  --
  -- Pair each element in a stream with its index, starting from the
  -- given index @n@ and counting down.
  --
  -- >>> Stream.fold Fold.toList $ Stream.indexedR 10 $ Stream.fromList "hello"
  -- [(10,'h'),(9,'e'),(8,'l'),(7,'l'),(6,'o')]
  --
  {-# INLINE_NORMAL indexedR #-}
  indexedR :: Monad m => Int -> Stream m a -> Stream m (Int, a)
  -- indexedR n = scanMaybe (FL.indexingRev n)
  indexedR m (Stream step state) = Stream step' (state, m)
    where
      {-# INLINE_LATE step' #-}
      step' gst (st, i) = i `seq` do
           r <- step (adaptState gst) st
           case r of
               Yield x s -> let i' = i - 1
                            in return $ Yield (i, x) (s, i')
               Skip    s -> return $ Skip (s, i)
               Stop      -> return Stop
  
  -------------------------------------------------------------------------------
  -- Time Indexing
  -------------------------------------------------------------------------------
  
  -- Note: The timestamp stream must be the second stream in the zip so that the
  -- timestamp is generated after generating the stream element and not before.
  -- If we do not do that then the following example will generate the same
  -- timestamp for first two elements:
  --
  -- Stream.fold Fold.toList $ Stream.timestamped $ Stream.delay $ Stream.enumerateFromTo 1 3
  
  -- | Pair each element in a stream with an absolute timestamp, using a clock of
  -- specified granularity.  The timestamp is generated just before the element
  -- is consumed.
  --
  -- >>> Stream.fold Fold.toList $ Stream.timestampWith 0.01 $ Stream.delay 1 $ Stream.enumerateFromTo 1 3
  -- [(AbsTime (TimeSpec {sec = ..., nsec = ...}),1),(AbsTime (TimeSpec {sec = ..., nsec = ...}),2),(AbsTime (TimeSpec {sec = ..., nsec = ...}),3)]
  --
  -- /Pre-release/
  --
  {-# INLINE timestampWith #-}
  timestampWith :: (MonadIO m)
      => Double -> Stream m a -> Stream m (AbsTime, a)
  timestampWith g stream = zipWith (flip (,)) stream (absTimesWith g)
  
  -- TBD: check performance vs a custom implementation without using zipWith.
  --
  -- /Pre-release/
  --
  {-# INLINE timestamped #-}
  timestamped :: (MonadIO m)
      => Stream m a -> Stream m (AbsTime, a)
  timestamped = timestampWith 0.01
  
  -- | Pair each element in a stream with relative times starting from 0, using a
  -- clock with the specified granularity. The time is measured just before the
  -- element is consumed.
  --
  -- >>> Stream.fold Fold.toList $ Stream.timeIndexWith 0.01 $ Stream.delay 1 $ Stream.enumerateFromTo 1 3
  -- [(RelTime64 (NanoSecond64 ...),1),(RelTime64 (NanoSecond64 ...),2),(RelTime64 (NanoSecond64 ...),3)]
  --
  -- /Pre-release/
  --
  {-# INLINE timeIndexWith #-}
  timeIndexWith :: (MonadIO m)
      => Double -> Stream m a -> Stream m (RelTime64, a)
  timeIndexWith g stream = zipWith (flip (,)) stream (relTimesWith g)
  
  -- | Pair each element in a stream with relative times starting from 0, using a
  -- 10 ms granularity clock. The time is measured just before the element is
  -- consumed.
  --
  -- >>> Stream.fold Fold.toList $ Stream.timeIndexed $ Stream.delay 1 $ Stream.enumerateFromTo 1 3
  -- [(RelTime64 (NanoSecond64 ...),1),(RelTime64 (NanoSecond64 ...),2),(RelTime64 (NanoSecond64 ...),3)]
  --
  -- /Pre-release/
  --
  {-# INLINE timeIndexed #-}
  timeIndexed :: (MonadIO m)
      => Stream m a -> Stream m (RelTime64, a)
  timeIndexed = timeIndexWith 0.01
  
  ------------------------------------------------------------------------------
  -- Searching
  ------------------------------------------------------------------------------
  
  -- | Find all the indices where the element in the stream satisfies the given
  -- predicate.
  --
  -- >>> findIndices p = Stream.postscanlMaybe (Scanl.findIndices p)
  --
  {-# INLINE_NORMAL findIndices #-}
  findIndices :: Monad m => (a -> Bool) -> Stream m a -> Stream m Int
  findIndices p (Stream step state) = Stream step' (state, 0)
    where
      {-# INLINE_LATE step' #-}
      step' gst (st, i) = i `seq` do
        r <- step (adaptState gst) st
        return $ case r of
            Yield x s -> if p x then Yield i (s, i+1) else Skip (s, i+1)
            Skip s -> Skip (s, i)
            Stop   -> Stop
  
  -- | Find all the indices where the value of the element in the stream is equal
  -- to the given value.
  --
  -- >>> elemIndices a = Stream.findIndices (== a)
  --
  {-# INLINE elemIndices #-}
  elemIndices :: (Monad m, Eq a) => a -> Stream m a -> Stream m Int
  elemIndices a = findIndices (== a)
  
  ------------------------------------------------------------------------------
  -- Rolling map
  ------------------------------------------------------------------------------
  
  data RollingMapState s a = RollingMapGo s a
  
  -- | Like 'rollingMap' but with an effectful map function.
  --
  -- /Pre-release/
  --
  {-# INLINE rollingMapM #-}
  rollingMapM :: Monad m => (Maybe a -> a -> m b) -> Stream m a -> Stream m b
  -- rollingMapM f = scanMaybe (FL.slide2 $ Window.rollingMapM f)
  rollingMapM f (Stream step1 state1) = Stream step (RollingMapGo state1 Nothing)
  
      where
  
      step gst (RollingMapGo s1 curr) = do
          r <- step1 (adaptState gst) s1
          case r of
              Yield x s -> do
                  !res <- f curr x
                  return $ Yield res $ RollingMapGo s (Just x)
              Skip s -> return $ Skip $ RollingMapGo s curr
              Stop   -> return Stop
  
  -- rollingMap is a special case of an incremental sliding fold. It can be
  -- written as:
  --
  -- > fld f = slidingWindow 1 (Scanl.mkScanl (\_ (x,y) -> f y x)
  -- > rollingMap f = Stream.postscan (fld f) undefined
  
  -- | Apply a function on every two successive elements of a stream. The first
  -- argument of the map function is the previous element and the second argument
  -- is the current element. When the current element is the first element, the
  -- previous element is 'Nothing'.
  --
  -- /Pre-release/
  --
  {-# INLINE rollingMap #-}
  rollingMap :: Monad m => (Maybe a -> a -> b) -> Stream m a -> Stream m b
  -- rollingMap f = scanMaybe (FL.slide2 $ Window.rollingMap f)
  rollingMap f = rollingMapM (\x y -> return $ f x y)
  
  -- | Like 'rollingMap' but requires at least two elements in the stream,
  -- returns an empty stream otherwise.
  --
  -- This is the stream equivalent of the list idiom @zipWith f xs (tail xs)@.
  --
  -- /Pre-release/
  --
  {-# INLINE rollingMap2 #-}
  rollingMap2 :: Monad m => (a -> a -> b) -> Stream m a -> Stream m b
  rollingMap2 f = catMaybes . rollingMap g
  
      where
  
      g Nothing _ = Nothing
      g (Just x) y = Just (f x y)
  
  ------------------------------------------------------------------------------
  -- Maybe Streams
  ------------------------------------------------------------------------------
  
  -- XXX Will this always fuse properly?
  
  -- | Map a 'Maybe' returning function to a stream, filter out the 'Nothing'
  -- elements, and return a stream of values extracted from 'Just'.
  --
  -- Equivalent to:
  --
  -- >>> mapMaybe f = Stream.catMaybes . fmap f
  --
  {-# INLINE_NORMAL mapMaybe #-}
  mapMaybe :: Monad m => (a -> Maybe b) -> Stream m a -> Stream m b
  mapMaybe f = fmap fromJust . filter isJust . map f
  
  -- | Like 'mapMaybe' but maps a monadic function.
  --
  -- Equivalent to:
  --
  -- >>> mapMaybeM f = Stream.catMaybes . Stream.mapM f
  --
  -- >>> mapM f = Stream.mapMaybeM (\x -> Just <$> f x)
  --
  {-# INLINE_NORMAL mapMaybeM #-}
  mapMaybeM :: Monad m => (a -> m (Maybe b)) -> Stream m a -> Stream m b
  mapMaybeM f = fmap fromJust . filter isJust . mapM f
  
  -- | In a stream of 'Maybe's, discard 'Nothing's and unwrap 'Just's.
  --
  -- >>> catMaybes = Stream.mapMaybe id
  -- >>> catMaybes = fmap fromJust . Stream.filter isJust
  --
  -- /Pre-release/
  --
  {-# INLINE catMaybes #-}
  catMaybes :: Monad m => Stream m (Maybe a) -> Stream m a
  -- catMaybes = fmap fromJust . filter isJust
  catMaybes (Stream step state) = Stream step1 state
  
      where
  
      {-# INLINE_LATE step1 #-}
      step1 gst st = do
          r <- step (adaptState gst) st
          case r of
              Yield x s -> do
                  return
                      $ case x of
                          Just a -> Yield a s
                          Nothing -> Skip s
              Skip s -> return $ Skip s
              Stop -> return Stop
  
  -- | Use a filtering scan on a stream.
  --
  -- >>> postscanlMaybe f = Stream.catMaybes . Stream.postscanl f
  --
  {-# INLINE postscanlMaybe #-}
  postscanlMaybe :: Monad m => Scanl m a (Maybe b) -> Stream m a -> Stream m b
  postscanlMaybe f = catMaybes . postscanl f
  
  {-# DEPRECATED scanMaybe "Use postscanlMaybe instead" #-}
  {-# INLINE scanMaybe #-}
  scanMaybe :: Monad m => Fold m a (Maybe b) -> Stream m a -> Stream m b
  scanMaybe f = catMaybes . postscan f
  
  ------------------------------------------------------------------------------
  -- Either streams
  ------------------------------------------------------------------------------
  
  -- | Discard 'Right's and unwrap 'Left's in an 'Either' stream.
  --
  -- >>> catLefts = fmap (fromLeft undefined) . Stream.filter isLeft
  --
  -- /Pre-release/
  --
  {-# INLINE catLefts #-}
  catLefts :: Monad m => Stream m (Either a b) -> Stream m a
  catLefts = fmap (fromLeft undefined) . filter isLeft
  
  -- | Discard 'Left's and unwrap 'Right's in an 'Either' stream.
  --
  -- >>> catRights = fmap (fromRight undefined) . Stream.filter isRight
  --
  -- /Pre-release/
  --
  {-# INLINE catRights #-}
  catRights :: Monad m => Stream m (Either a b) -> Stream m b
  catRights = fmap (fromRight undefined) . filter isRight
  
  -- | Remove the either wrapper and flatten both lefts and as well as rights in
  -- the output stream.
  --
  -- >>> catEithers = fmap (either id id)
  --
  -- /Pre-release/
  --
  {-# INLINE catEithers #-}
  catEithers :: Monad m => Stream m (Either a a) -> Stream m a
  catEithers = fmap (either id id)
  
  ------------------------------------------------------------------------------
  -- Splitting
  ------------------------------------------------------------------------------
  
  -- Design note: If we use splitSepBy_ on an empty stream what should be the
  -- result? Let's try the splitOn function in the "split" package:
  --
  -- > splitOn "a" ""
  -- [""]
  --
  -- Round tripping the result through intercalate gives identity:
  --
  -- > intercalate "a" [""]
  -- ""
  --
  -- Now let's try intercalate on empty list:
  --
  -- > intercalate "a" []
  -- ""
  --
  -- Round tripping it with splitOn is not identity:
  --
  -- > splitOn "a" ""
  -- [""]
  --
  -- Because intercalate flattens the two layers, both [] and [""] produce the
  -- same result after intercalate. Therefore, inverse of intercalate is not
  -- possible. We have to choose one of the two options for splitting an empty
  -- stream.
  --
  -- Choosing empty stream as the result of splitting empty stream makes better
  -- sense. This is different from the split package's choice. Splitting an empty
  -- stream resulting into a non-empty stream seems a bit odd. Also, splitting
  -- empty stream to empty stream is consistent with splitEndBy operation as
  -- well.
  
  {-# ANN type SplitSepBy Fuse #-}
  data SplitSepBy s fs b a
      = SplitSepByInit s
      | SplitSepByInitFold0 s
      | SplitSepByInitFold1 s fs
      | SplitSepByCheck s a fs
      | SplitSepByNext s fs
      | SplitSepByYield b (SplitSepBy s fs b a)
      | SplitSepByDone
  
  -- | Split on an infixed separator element, dropping the separator.  The
  -- supplied 'Fold' is applied on the split segments.  Splits the stream on
  -- separator elements determined by the supplied predicate, separator is
  -- considered as infixed between two segments:
  --
  -- Definition:
  --
  --
  -- Usage:
  --
  -- >>> splitOn p xs = Stream.fold Fold.toList $ Stream.splitSepBy_ p Fold.toList (Stream.fromList xs)
  -- >>> splitOn (== '.') "a.b"
  -- ["a","b"]
  --
  -- Splitting an empty stream results in an empty stream i.e. zero splits:
  --
  -- >>> splitOn (== '.') ""
  -- []
  --
  -- If the stream does not contain the separator then it results in a single
  -- split:
  --
  -- >>> splitOn (== '.') "abc"
  -- ["abc"]
  --
  -- If one or both sides of the separator are missing then the empty segment on
  -- that side is folded to the default output of the fold:
  --
  -- >>> splitOn (== '.') "."
  -- ["",""]
  --
  -- >>> splitOn (== '.') ".a"
  -- ["","a"]
  --
  -- >>> splitOn (== '.') "a."
  -- ["a",""]
  --
  -- >>> splitOn (== '.') "a..b"
  -- ["a","","b"]
  --
  -- 'splitSepBy_' is an inverse of 'unfoldEachSepBy':
  --
  -- > Stream.unfoldEachSepBy '.' Unfold.fromList . Stream.splitSepBy_ (== '.') Fold.toList === id
  --
  -- Assuming the input stream does not contain the separator:
  --
  -- > Stream.splitSepBy_ (== '.') Fold.toList . Stream.unfoldEachSepBy '.' Unfold.fromList === id
  --
  {-# INLINE splitSepBy_ #-}
  splitSepBy_ :: Monad m => (a -> Bool) -> Fold m a b -> Stream m a -> Stream m b
  -- We can express the infix splitting in terms of optional suffix split
  -- fold.  After applying a suffix split fold repeatedly if the last segment
  -- ends with a suffix then we need to return the default output of the fold
  -- after that to make it an infix split.
  --
  -- Alternately, we can also express it using an optional prefix split fold.
  -- If the first segment starts with a prefix then we need to emit the
  -- default output of the fold before that to make it an infix split, and
  -- then apply prefix split fold repeatedly.
  --
  splitSepBy_ predicate (Fold fstep initial _ final) (Stream step1 state1) =
      Stream step (SplitSepByInit state1)
  
      where
  
      -- Note: there is a question of whether we should initialize the fold
      -- before we run the stream or only after the stream yields an element. If
      -- we initialize it before then we may have to discard an effect if the
      -- stream does not yield anything. If we initialize it after then we may
      -- have to discard the stream element if the fold terminates without
      -- consuming anything. Though the state machine is simpler if we initialize
      -- the fold first. Also, in most common cases the fold is not effectful.
      -- On the other hand, in most cases the fold will not terminate without
      -- consuming anything. So both ways are similar.
      {-# INLINE_LATE step #-}
      step _ (SplitSepByInit st) = do
          fres <- initial
          return
              $ Skip
              $ case fres of
                    FL.Done b -> SplitSepByYield b (SplitSepByInit st)
                    FL.Partial fs -> SplitSepByInitFold1 st fs
  
      step _ (SplitSepByInitFold0 st) = do
          fres <- initial
          return
              $ Skip
              $ case fres of
                    FL.Done b -> SplitSepByYield b (SplitSepByInitFold0 st)
                    FL.Partial fs -> SplitSepByNext st fs
  
      step gst (SplitSepByInitFold1 st fs) = do
          r <- step1 (adaptState gst) st
          case r of
              Yield x s -> return $ Skip $ SplitSepByCheck s x fs
              Skip s -> return $ Skip (SplitSepByInitFold1 s fs)
              Stop -> final fs >> return Stop
  
      step _ (SplitSepByCheck st x fs) = do
          if predicate x
          then do
              b <- final fs
              return $ Skip $ SplitSepByYield b (SplitSepByInitFold0 st)
          else do
              fres <- fstep fs x
              return
                  $ Skip
                  $ case fres of
                        FL.Done b -> SplitSepByYield b (SplitSepByInitFold0 st)
                        FL.Partial fs1 -> SplitSepByNext st fs1
  
      step gst (SplitSepByNext st fs) = do
          r <- step1 (adaptState gst) st
          case r of
              Yield x s -> return $ Skip $ SplitSepByCheck s x fs
              Skip s -> return $ Skip (SplitSepByNext s fs)
              Stop -> do
                  b <- final fs
                  return $ Skip $ SplitSepByYield b SplitSepByDone
  
      step _ (SplitSepByYield b next) = return $ Yield b next
      step _ SplitSepByDone = return Stop
  
  {-# DEPRECATED splitOn "Please use splitSepBy_ instead. Note the difference in behavior on splitting empty stream." #-}
  {-# INLINE splitOn #-}
  splitOn :: Monad m => (a -> Bool) -> Fold m a b -> Stream m a -> Stream m b
  splitOn predicate f =
      foldManyPost (FL.takeEndBy_ predicate f)
  