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-rw-r--r--compiler/GHC/Data/Stream.hs165
1 files changed, 87 insertions, 78 deletions
diff --git a/compiler/GHC/Data/Stream.hs b/compiler/GHC/Data/Stream.hs
index 7996ee7343..4e2bee4311 100644
--- a/compiler/GHC/Data/Stream.hs
+++ b/compiler/GHC/Data/Stream.hs
@@ -1,3 +1,6 @@
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE TupleSections #-}
-- -----------------------------------------------------------------------------
--
-- (c) The University of Glasgow 2012
@@ -6,25 +9,28 @@
-- | Monadic streams
module GHC.Data.Stream (
- Stream(..), yield, liftIO,
- collect, collect_, consume, fromList,
- map, mapM, mapAccumL, mapAccumL_
+ Stream(..), StreamS(..), runStream, yield, liftIO,
+ collect, consume, fromList,
+ map, mapM, mapAccumL_
) where
import GHC.Prelude hiding (map,mapM)
import Control.Monad hiding (mapM)
+import Control.Monad.IO.Class
-- |
-- @Stream m a b@ is a computation in some Monad @m@ that delivers a sequence
-- of elements of type @a@ followed by a result of type @b@.
--
--- More concretely, a value of type @Stream m a b@ can be run using @runStream@
+-- More concretely, a value of type @Stream m a b@ can be run using @runStreamInternal@
-- in the Monad @m@, and it delivers either
--
--- * the final result: @Left b@, or
--- * @Right (a,str)@, where @a@ is the next element in the stream, and @str@
--- is a computation to get the rest of the stream.
+-- * the final result: @Done b@, or
+-- * @Yield a str@ where @a@ is the next element in the stream, and @str@
+-- is the rest of the stream
+-- * @Effect mstr@ where @mstr@ is some action running in @m@ which
+-- generates the rest of the stream.
--
-- Stream is itself a Monad, and provides an operation 'yield' that
-- produces a new element of the stream. This makes it convenient to turn
@@ -38,57 +44,73 @@ import Control.Monad hiding (mapM)
-- Stream, and the consumer pulls on the stream each time it wants a
-- new value.
--
-newtype Stream m a b = Stream { runStream :: m (Either b (a, Stream m a b)) }
+-- 'Stream' is implemented in the "yoneda" style for efficiency. By
+-- representing a stream in this manner 'fmap' and '>>=' operations are
+-- accumulated in the function parameters before being applied once when
+-- the stream is destroyed. In the old implementation each usage of 'mapM'
+-- and '>>=' would traverse the entire stream in order to apply the
+-- substitution at the leaves.
+--
+-- The >>= operation for 'Stream' was a hot-spot in the ticky profile for
+-- the "ManyConstructors" test which called the 'cg' function many times in
+-- @StgToCmm.hs@
+--
+newtype Stream m a b =
+ Stream { runStreamInternal :: forall r' r .
+ (a -> m r') -- For fusing calls to `map` and `mapM`
+ -> (b -> StreamS m r' r) -- For fusing `>>=`
+ -> StreamS m r' r }
+
+runStream :: Applicative m => Stream m r' r -> StreamS m r' r
+runStream st = runStreamInternal st pure Done
+
+data StreamS m a b = Yield a (StreamS m a b)
+ | Done b
+ | Effect (m (StreamS m a b))
-instance Monad f => Functor (Stream f a) where
+instance Monad m => Functor (StreamS m a) where
fmap = liftM
-instance Monad m => Applicative (Stream m a) where
- pure a = Stream (return (Left a))
+instance Monad m => Applicative (StreamS m a) where
+ pure = Done
(<*>) = ap
-instance Monad m => Monad (Stream m a) where
+instance Monad m => Monad (StreamS m a) where
+ a >>= k = case a of
+ Done r -> k r
+ Yield a s -> Yield a (s >>= k)
+ Effect m -> Effect (fmap (>>= k) m)
- Stream m >>= k = Stream $ do
- r <- m
- case r of
- Left b -> runStream (k b)
- Right (a,str) -> return (Right (a, str >>= k))
+instance Functor (Stream f a) where
+ fmap = liftM
-yield :: Monad m => a -> Stream m a ()
-yield a = Stream (return (Right (a, return ())))
+instance Applicative (Stream m a) where
+ pure a = Stream $ \_f g -> g a
+ (<*>) = ap
-liftIO :: IO a -> Stream IO b a
-liftIO io = Stream $ io >>= return . Left
+instance Monad (Stream m a) where
+ Stream m >>= k = Stream $ \f h -> m f (\a -> runStreamInternal (k a) f h)
+
+instance MonadIO m => MonadIO (Stream m b) where
+ liftIO io = Stream $ \_f g -> Effect (g <$> liftIO io)
+
+yield :: Monad m => a -> Stream m a ()
+yield a = Stream $ \f rest -> Effect (flip Yield (rest ()) <$> f a)
-- | Turn a Stream into an ordinary list, by demanding all the elements.
collect :: Monad m => Stream m a () -> m [a]
-collect str = go str []
+collect str = go [] (runStream str)
where
- go str acc = do
- r <- runStream str
- case r of
- Left () -> return (reverse acc)
- Right (a, str') -> go str' (a:acc)
+ go acc (Done ()) = return (reverse acc)
+ go acc (Effect m) = m >>= go acc
+ go acc (Yield a k) = go (a:acc) k
--- | Turn a Stream into an ordinary list, by demanding all the elements.
-collect_ :: Monad m => Stream m a r -> m ([a], r)
-collect_ str = go str []
- where
- go str acc = do
- r <- runStream str
- case r of
- Left r -> return (reverse acc, r)
- Right (a, str') -> go str' (a:acc)
-
-consume :: Monad m => Stream m a b -> (a -> m ()) -> m b
-consume str f = do
- r <- runStream str
- case r of
- Left ret -> return ret
- Right (a, str') -> do
- f a
- consume str' f
+consume :: (Monad m, Monad n) => Stream m a b -> (forall a . m a -> n a) -> (a -> n ()) -> n b
+consume str l f = go (runStream str)
+ where
+ go (Done r) = return r
+ go (Yield a p) = f a >> go p
+ go (Effect m) = l m >>= go
-- | Turn a list into a 'Stream', by yielding each element in turn.
fromList :: Monad m => [a] -> Stream m a ()
@@ -96,40 +118,27 @@ fromList = mapM_ yield
-- | Apply a function to each element of a 'Stream', lazily
map :: Monad m => (a -> b) -> Stream m a x -> Stream m b x
-map f str = Stream $ do
- r <- runStream str
- case r of
- Left x -> return (Left x)
- Right (a, str') -> return (Right (f a, map f str'))
+map f str = Stream $ \g h -> runStreamInternal str (g . f) h
-- | Apply a monadic operation to each element of a 'Stream', lazily
mapM :: Monad m => (a -> m b) -> Stream m a x -> Stream m b x
-mapM f str = Stream $ do
- r <- runStream str
- case r of
- Left x -> return (Left x)
- Right (a, str') -> do
- b <- f a
- return (Right (b, mapM f str'))
-
--- | analog of the list-based 'mapAccumL' on Streams. This is a simple
--- way to map over a Stream while carrying some state around.
-mapAccumL :: Monad m => (c -> a -> m (c,b)) -> c -> Stream m a ()
- -> Stream m b c
-mapAccumL f c str = Stream $ do
- r <- runStream str
- case r of
- Left () -> return (Left c)
- Right (a, str') -> do
- (c',b) <- f c a
- return (Right (b, mapAccumL f c' str'))
-
-mapAccumL_ :: Monad m => (c -> a -> m (c,b)) -> c -> Stream m a r
+mapM f str = Stream $ \g h -> runStreamInternal str (g <=< f) h
+
+-- | Note this is not very efficient because it traverses the whole stream
+-- before rebuilding it, avoid using it if you can. mapAccumL used to
+-- implemented but it wasn't used anywhere in the compiler and has similar
+-- effiency problems.
+mapAccumL_ :: forall m a b c r . Monad m => (c -> a -> m (c,b)) -> c -> Stream m a r
-> Stream m b (c, r)
-mapAccumL_ f c str = Stream $ do
- r <- runStream str
- case r of
- Left r -> return (Left (c, r))
- Right (a, str') -> do
- (c',b) <- f c a
- return (Right (b, mapAccumL_ f c' str'))
+mapAccumL_ f c str = Stream $ \f h -> go c f h (runStream str)
+
+ where
+ go :: c
+ -> (b -> m r')
+ -> ((c, r) -> StreamS m r' r1)
+ -> StreamS m a r
+ -> StreamS m r' r1
+ go c _f1 h1 (Done r) = h1 (c, r)
+ go c f1 h1 (Yield a p) = Effect (f c a >>= (\(c', b) -> f1 b
+ >>= \r' -> return $ Yield r' (go c' f1 h1 p)))
+ go c f1 h1 (Effect m) = Effect (go c f1 h1 <$> m)
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