Streamly.Internal.Data.Producer

A Producer is an Unfold with an extract function added to extract the state. It is more powerful but less general than an Unfold.

A Producer represents steps of a loop generating a sequence of elements. While unfolds are closed representation of imperative loops with some opaque internal state, producers are open loops with the state being accessible to the user.

Unlike an unfold, which runs a loop till completion, a producer can be stopped in the middle, its state can be extracted, examined, changed, and then it can be resumed later from the stopped state.

A producer can be used in places where a CPS stream would otherwise be needed, because the state of the loop can be passed around. However, it can be much more efficient than CPS because it allows stream fusion and unecessary function calls can be avoided.

Documentation

data Producer m a b

A Producer m a b is a generator of a stream of values of type b from a seed of type a in Monad m.

Pre-release

Constructors

forall s. Producer (s -> m (Step s b)) (a -> m s) (s -> m a)

Producer step inject extract

Instances

Functor m => Functor (Producer m a)

Maps a function on the output of the producer (the type b).

Defined in Streamly.Internal.Data.Producer.Type

Methods

fmap :: (a0 -> b) -> Producer m a a0 -> Producer m a b

(<$) :: a0 -> Producer m a b -> Producer m a a0

Converting

simplify :: Producer m a b -> Unfold m a b

Simplify a producer to an unfold.

Pre-release

Producers

nil :: Monad m => Producer m a b

nilM :: Monad m => (a -> m c) -> Producer m a b

unfoldrM :: Monad m => (a -> m (Maybe (b, a))) -> Producer m a b

fromStreamD :: Monad m => Producer m (Stream m a) a

Convert a StreamD stream into a producer.

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fromList :: Monad m => Producer m [a] a

Convert a list of pure values to a Stream

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Combinators

data NestedLoop s1 s2

State representing a nested loop.

Constructors

OuterLoop s1

InnerLoop s1 s2

concat :: Monad m => Producer m a b -> Producer m b c -> Producer m (NestedLoop a b) c

Apply the second unfold to each output element of the first unfold and flatten the output in a single stream.

Pre-release