Safe Haskell	None
Language	GHC2021

GHC.Data.Stream

Description

Monadic streams

Synopsis

newtype Stream (m :: Type -> Type) a b = Stream {
- runStreamInternal :: forall r' r. (a -> m r') -> (b -> StreamS m r' r) -> StreamS m r' r
}
data StreamS (m :: Type -> Type) a b
- = Yield a (StreamS m a b)
- | Done b
- | Effect (m (StreamS m a b))
runStream :: forall (m :: Type -> Type) r' r. Applicative m => Stream m r' r -> StreamS m r' r
yield :: forall (m :: Type -> Type) a. Monad m => a -> Stream m a ()
liftIO :: MonadIO m => IO a -> m a
liftEff :: Monad m => m b -> Stream m a b
hoistEff :: (Applicative m, Monad n) => (forall x. m x -> n x) -> Stream m a b -> Stream n a b
collect :: Monad m => Stream m a () -> m [a]
consume :: (Monad m, Monad n) => Stream m a b -> (forall a1. m a1 -> n a1) -> (a -> n ()) -> n b
fromList :: forall (m :: Type -> Type) a. Monad m => [a] -> Stream m a ()
map :: forall (m :: Type -> Type) a b x. Monad m => (a -> b) -> Stream m a x -> Stream m b x
mapM :: Monad m => (a -> m b) -> Stream m a x -> Stream m b x
mapAccumL_ :: forall m a b c r. Monad m => (c -> a -> m (c, b)) -> c -> Stream m a r -> Stream m b (c, r)

Documentation

newtype Stream (m :: Type -> Type) a b Source #

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 runStreamInternal in the Monad m, and it delivers either

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 existing monadic computations into streams.

The idea is that Stream is useful for making a monadic computation that produces values from time to time. This can be used for knitting together two complex monadic operations, so that the producer does not have to produce all its values before the consumer starts consuming them. We make the producer into a Stream, and the consumer pulls on the stream each time it wants a new value.

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

Constructors

Stream
Fields runStreamInternal :: forall r' r. (a -> m r') -> (b -> StreamS m r' r) -> StreamS m r' r

Instances

Instances details

Applicative (Stream m a) Source #
Instance details Defined in GHC.Data.Stream Methods pure :: a0 -> Stream m a a0 # (<>) :: Stream m a (a0 -> b) -> Stream m a a0 -> Stream m a b # liftA2 :: (a0 -> b -> c) -> Stream m a a0 -> Stream m a b -> Stream m a c # (>) :: Stream m a a0 -> Stream m a b -> Stream m a b # (<*) :: Stream m a a0 -> Stream m a b -> Stream m a a0 #
Functor (Stream f a) Source #
Instance details Defined in GHC.Data.Stream Methods fmap :: (a0 -> b) -> Stream f a a0 -> Stream f a b # (<$) :: a0 -> Stream f a b -> Stream f a a0 #
Monad (Stream m a) Source #
Instance details Defined in GHC.Data.Stream Methods (>>=) :: Stream m a a0 -> (a0 -> Stream m a b) -> Stream m a b # (>>) :: Stream m a a0 -> Stream m a b -> Stream m a b # return :: a0 -> Stream m a a0 #
MonadIO m => MonadIO (Stream m b) Source #
Instance details Defined in GHC.Data.Stream Methods liftIO :: IO a -> Stream m b a #

data StreamS (m :: Type -> Type) a b Source #

Constructors

Yield a (StreamS m a b)
Done b
Effect (m (StreamS m a b))

Instances

Instances details

Monad m => Applicative (StreamS m a) Source #
Instance details Defined in GHC.Data.Stream Methods pure :: a0 -> StreamS m a a0 # (<>) :: StreamS m a (a0 -> b) -> StreamS m a a0 -> StreamS m a b # liftA2 :: (a0 -> b -> c) -> StreamS m a a0 -> StreamS m a b -> StreamS m a c # (>) :: StreamS m a a0 -> StreamS m a b -> StreamS m a b # (<*) :: StreamS m a a0 -> StreamS m a b -> StreamS m a a0 #
Functor m => Functor (StreamS m a) Source #
Instance details Defined in GHC.Data.Stream Methods fmap :: (a0 -> b) -> StreamS m a a0 -> StreamS m a b # (<$) :: a0 -> StreamS m a b -> StreamS m a a0 #
Monad m => Monad (StreamS m a) Source #
Instance details Defined in GHC.Data.Stream Methods (>>=) :: StreamS m a a0 -> (a0 -> StreamS m a b) -> StreamS m a b # (>>) :: StreamS m a a0 -> StreamS m a b -> StreamS m a b # return :: a0 -> StreamS m a a0 #

runStream :: forall (m :: Type -> Type) r' r. Applicative m => Stream m r' r -> StreamS m r' r Source #

yield :: forall (m :: Type -> Type) a. Monad m => a -> Stream m a () Source #

liftIO :: MonadIO m => IO a -> m a #

liftEff :: Monad m => m b -> Stream m a b Source #

Lift an effect into the Stream

hoistEff :: (Applicative m, Monad n) => (forall x. m x -> n x) -> Stream m a b -> Stream n a b Source #

Hoist the underlying Stream effect Note this is not very efficience since, just like mapAccumL_, it also needs to traverse and rebuild the whole stream.

collect :: Monad m => Stream m a () -> m [a] Source #

Turn a Stream into an ordinary list, by demanding all the elements.

consume :: (Monad m, Monad n) => Stream m a b -> (forall a1. m a1 -> n a1) -> (a -> n ()) -> n b Source #

fromList :: forall (m :: Type -> Type) a. Monad m => [a] -> Stream m a () Source #

Turn a list into a Stream, by yielding each element in turn.

map :: forall (m :: Type -> Type) a b x. Monad m => (a -> b) -> Stream m a x -> Stream m b x Source #

Apply a function to each element of a Stream, lazily

mapM :: Monad m => (a -> m b) -> Stream m a x -> Stream m b x Source #

Apply a monadic operation to each element of a Stream, lazily

mapAccumL_ :: forall m a b c r. Monad m => (c -> a -> m (c, b)) -> c -> Stream m a r -> Stream m b (c, r) Source #

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 efficiency problems.