4c00da9 · streamly · git.iko.soy

git.iko.soy/streamly

Merge pull request #76 from subbyte/master

mapMaybe and mapMaybeM Implemented

commit4c00da9ab4604e2e26c6e4a81b87b32e660baaa5 authorHarendra Kumar <harendra.kumar@gmail.com> dateWed 30 May 2018, 05:24 (8 years ago) parent54419aa e46d341

3 files changed+34

compare with master

unifiedsplit

Changelog.md+1 src/Streamly/Prelude.hs+29 test/Prop.hs+4

Changelog.md+1

@@ -24,6 +24,7 @@

2424 concurrently when used at appropriate stream types.

2525* Added concurrent function application operators to run stages of a

2626 stream processing function application pipeline concurrently.

27+* Added two functions `mapMaybe` and `mapMaybeM`.

2728

2829## 0.2.0

2930

src/Streamly/Prelude.hs+29

@@ -100,12 +100,14 @@ module Streamly.Prelude

100100 , takeWhile

101101 , drop

102102 , dropWhile

103+ , mapMaybe

103104

104105 -- * Reordering

105106 , reverse

106107

107108 -- * Mapping

108109 , mapM

110+ , mapMaybeM

109111 , sequence

110112

111113 -- * Zipping

@@ -129,6 +131,7 @@ where

129131import Control.Monad (void)

130132import Control.Monad.IO.Class (MonadIO(..))

131133import Data.Semigroup (Semigroup(..))

134+import Data.Maybe (isJust, fromJust)

132135import Prelude hiding (filter, drop, dropWhile, take,

133136 takeWhile, zipWith, foldr, foldl,

134137 mapM, mapM_, sequence, all, any,

@@ -577,6 +580,24 @@ dropWhile p m = fromStream $ go (toStream m)

577580 | otherwise = yld a r

578581 in (S.runStream m1) ctx stp single yield

579582

583+-- | Map + filter: compute a stream of b from a stream of a and filter out

584+-- elements that are mapped to Nothing. It is similar to 'mapMaybe' in

585+-- 'Data.Maybe' but applies to streams instead of list.

586+--

587+-- @since 0.2.1

588+{-# INLINE mapMaybe #-}

589+mapMaybe :: (IsStream t) => (a -> Maybe b) -> t m a -> t m b

590+mapMaybe f m = go (toStream m)

591+ where

592+ go m1 = fromStream $ Stream $ \_ stp sng yld ->

593+ let single a = case f a of

594+ Just b -> sng b

595+ Nothing -> stp

596+ yield a r = case f a of

597+ Just b -> yld b (toStream $ go r)

598+ Nothing -> (S.runStream r) Nothing stp single yield

599+ in (S.runStream m1) Nothing stp single yield

600+

580601-- | Determine whether all elements of a stream satisfy a predicate.

581602--

582603-- @since 0.1.0

@@ -757,6 +778,14 @@ mapM f m = go (toStream m)

757778 yield a r = S.runStream (toStream (f a |: (go r))) svr stp sng yld

758779 in (S.runStream m1) Nothing stp single yield

759780

781+-- | Monadic version mapMaybe.

782+--

783+-- @since 0.2.1

784+{-# INLINE mapMaybeM #-}

785+mapMaybeM :: (IsStream t, MonadAsync m, Functor (t m))

786+ => (a -> m (Maybe b)) -> t m a -> t m b

787+mapMaybeM f = fmap fromJust . filter isJust . mapM f

788+

760789-- XXX this can utilize parallel mapping if we implement it as runStream . mapM

761790-- | Apply a monadic action to each element of the stream and discard the

762791-- output of the action.

test/Prop.hs+4

@@ -9,6 +9,7 @@ import Control.Applicative (ZipList(..))

99import Control.Concurrent (MVar, takeMVar, putMVar, newEmptyMVar)

1010import Control.Monad (replicateM, replicateM_)

1111import Data.List (sort, foldl', scanl')

12+import Data.Maybe (mapMaybe)

1213import GHC.Word (Word8)

1314

1415import Test.Hspec.QuickCheck (prop)

@@ -295,6 +296,9 @@ transformOps constr desc t eq = do

295296 prop (desc ++ " takeWhile > 0") $

296297 transform (takeWhile (> 0)) $ t . (A.takeWhile (> 0))

297298

299+ let f x = if odd x then Just (x + 100) else Nothing

300+ prop (desc ++ " mapMaybe") $ transform (mapMaybe f) $ t . (A.mapMaybe f)

301+

298302 prop (desc ++ " drop maxBound") $

299303 transform (drop maxBound) $ t . (A.drop maxBound)

300304 prop (desc ++ " drop 0") $ transform (drop 0) $ t . (A.drop 0)