ghc-7.8.2: The GHC API

Safe Haskell | Safe-Inferred
Language | Haskell98

Stream

Synopsis

- newtype Stream m a b = Stream {
  - runStream :: m (Either b (a, Stream m a b))
  }
- yield :: Monad m => a -> Stream m a ()
- liftIO :: IO a -> Stream IO b a
- collect :: Monad m => Stream m a () -> m [a]
- fromList :: Monad m => [a] -> Stream m a ()
- map :: 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 :: Monad m => (c -> a -> m (c, b)) -> c -> Stream m a () -> Stream m b c

# Documentation

newtype Stream m a b

`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` 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.

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.

Constructors

Stream |
Fields runStream :: m (Either b (a, Stream m a b))

Instances

Monad m => Monad (Stream m a) |
Monad f => Functor (Stream f a) |
Monad m => Applicative (Stream m a) |

yield :: Monad m => a -> Stream m a ()

liftIO :: IO a -> Stream IO b a

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

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

fromList :: Monad m => [a] -> Stream m a ()

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

map :: Monad m => (a -> b) -> Stream m a x -> Stream m b x

Apply a function to each element of a `Stream`, lazily

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

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

mapAccumL :: Monad m => (c -> a -> m (c, b)) -> c -> Stream m a () -> Stream m b c

analog of the list-based `mapAccumL` on Streams. This is a simple way to map over a Stream while carrying some state around.