src/Streamly/Data/Fold/Prelude.hs · streamly

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1-- |

2-- Module : Streamly.Data.Fold.Prelude

4-- License : BSD-3-Clause

5-- Maintainer : streamly@composewell.com

6-- Stability : released

7-- Portability : GHC

8--

9module Streamly.Data.Fold.Prelude

10 ( module Streamly.Data.Fold

11 , toHashMapIO

12 )

13where

15import Control.Monad.IO.Class (MonadIO)

16import Data.HashMap.Strict (HashMap)

17import Data.Hashable (Hashable)

18import Streamly.Data.Fold

19import Streamly.Internal.Data.Fold.Container (toContainerIO)

20import Streamly.Internal.Data.IsMap.HashMap ()

22-- | Split the input stream based on a hashable component of the key field and

23-- fold each split using the given fold. Useful for map/reduce, bucketizing

24-- the input in different bins or for generating histograms.

26-- Example:

28-- >>> import Data.HashMap.Strict (HashMap, fromList)

29-- >>> import qualified Streamly.Data.Fold.Prelude as Fold

30-- >>> import qualified Streamly.Data.Stream as Stream

31-- >>> import Streamly.Data.Fold.Prelude (toHashMapIO)

32-- >>> :{

33-- do

34-- let input = Stream.fromList [("ONE",1),("ONE",1.1),("TWO",2), ("TWO",2.2)]

35-- classify = toHashMapIO fst (Fold.lmap snd Fold.toList)

36-- x <- Stream.fold classify input :: IO (HashMap String [Double])

37-- let y = fromList [("ONE",[1.0,1.1]),("TWO",[2.0,2.2])]

38-- return (x == y)

39-- :}

40-- True

42-- /Pre-release/

44{-# INLINE toHashMapIO #-}

45toHashMapIO :: (MonadIO m, Hashable k, Ord k) =>

46 (a -> k) -> Fold m a b -> Fold m a (HashMap k b)

47toHashMapIO = toContainerIO