2248ae7 · streamly · git.iko.soy

git.iko.soy/streamly

Add "scan" for left scans

commit2248ae7d6be2dfb485683759cd29b3f8f7086327 authorAbhiroop Sarkar <asiamgenius@gmail.com> dateSat 10 Mar 2018, 03:21 (9 years ago) parent70a56d1

2 files changed+16

unifiedsplit

src/Streamly/Prelude.hs+14

@@ -29,6 +29,7 @@ module Streamly.Prelude

2929 -- ** General Folds

3030 , foldr

3131 , foldrM

32+ , scan

3233 , foldl

3334 , foldlM

3435 , uncons

@@ -155,6 +156,19 @@ foldrM step acc m = go (toStream m)

155156 yield a (Just x) = step a (go x)

156157 in (runStream m1) Nothing stop yield

157158

159+-- | Scan left. A strict left fold which accumulates the result of its reduction steps inside a stream, from left.

160+{-# INLINE scan #-}

161+scan :: Streaming t => (x -> a -> x) -> x -> (x -> b) -> t m a -> t m b

162+scan step begin done m = cons (done begin) $ fromStream $ go (toStream m) begin

163+ where

164+ go m1 !acc = Stream $ \_ stp yld ->

165+ let stop = stp

166+ yield a Nothing = yld (done $ step acc a) Nothing

167+ yield a (Just x) =

168+ let s = step acc a

169+ in yld (done s) (Just (go x s))

170+ in runStream m1 Nothing stop yield

171+

158172-- XXX This more straightforward implementation of fold is almost twice as

159173-- costly as the implementation we are using! Need to figure out what's wrong

160174-- with it. Is it GHC that is not able to optimize this somehow? Or is it

test/Main.hs+2

@@ -666,6 +666,8 @@ streamOperations (stream, list, len) = do

666666 it "dropWhile < some" $ transform (A.dropWhile (< (len `div` 2)))

667667 (dropWhile (< (len `div` 2)))

668668

669+ it "scan left" $ transform (A.scan (+) 0 id) (scanl (+) 0)

670+

669671 -- Elimination

670672 it "foldl" $ elimination (A.foldl (+) 0 id) (foldl (+) 0)

671673 it "all" $ elimination (A.all even) (all even)