| 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
- 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 strwhereais the next element in the stream, andstris the rest of the streamEffect mstrwheremstris some action running inmwhich 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
| |
runStream :: forall (m :: Type -> Type) r' r. Applicative m => Stream m r' r -> StreamS m r' r Source #
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.