ghc-7.10.3: The GHC API

Safe HaskellSafe




newtype Stream m 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 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.




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


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 () Source

liftIO :: IO a -> Stream IO b a Source

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

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

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

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

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