e46d341 · streamly · git.iko.soy

git.iko.soy/streamly

add mapMaybe and mapMaybeM

commite46d341819e05367af7083ae5a9ff0d3c67259a3 authorXiaokui Shu <subbyte@gmail.com> dateSat 26 May 2018, 15:58 (8 years ago) parente603579

3 files changed+34

compare with master

unifiedsplit

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

Changelog.md+1

@@ -26,6 +26,7 @@

2626 concurrently when used at appropriate stream types.

2727* Added concurrent function application operators to run stages of a

2828 stream processing function application pipeline concurrently.

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

2930

3031## 0.2.0

3132

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,

@@ -570,6 +573,24 @@ dropWhile p m = fromStream $ go (toStream m)

570573 | otherwise = yld a r

571574 in (S.runStream m1) ctx stp single yield

572575

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

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

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

579+--

580+-- @since 0.2.1

581+{-# INLINE mapMaybe #-}

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

583+mapMaybe f m = go (toStream m)

584+ where

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

586+ let single a = case f a of

587+ Just b -> sng b

588+ Nothing -> stp

589+ yield a r = case f a of

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

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

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

593+

573594-- | Determine whether all elements of a stream satisfy a predicate.

574595--

575596-- @since 0.1.0

@@ -750,6 +771,14 @@ mapM f m = go (toStream m)

750771 yield a r = S.runStream (toStream (f a |: (go r))) svr stp sng yld

751772 in (S.runStream m1) Nothing stp single yield

752773

774+-- | Monadic version mapMaybe.

775+--

776+-- @since 0.2.1

777+{-# INLINE mapMaybeM #-}

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

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

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

781+

753782-- XXX this can utilize parallel mapping if we implement it as runStream . mapM

754783-- | Apply a monadic action to each element of the stream and discard the

755784-- 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)