| Safe Haskell | Safe-Inferred | 
|---|---|
| Language | Haskell2010 | 
GHC.Data.Stream
Description
Monadic streams
Synopsis
- newtype Stream m a b = Stream {- runStreamInternal :: forall r' r. (a -> m r') -> (b -> StreamS m r' r) -> StreamS m r' r
 
- data StreamS m a b
- runStream :: Applicative m => Stream m r' r -> StreamS m r' r
- yield :: Monad m => a -> Stream m a ()
- liftIO :: MonadIO m => IO a -> m a
- collect :: Monad m => Stream m a () -> m [a]
- consume :: (Monad m, Monad n) => Stream m a b -> (forall a. m a -> n a) -> (a -> n ()) -> n b
- 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_ :: forall m a b c r. Monad m => (c -> a -> m (c, b)) -> c -> Stream m a r -> Stream m b (c, r)
Documentation
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 strwhere- ais the next element in the stream, and- stris the rest of the stream
- Effect mstrwhere- mstris some action running in- mwhich 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 
 | |
Instances
| MonadIO m => MonadIO (Stream m b) Source # | |
| Applicative (Stream m a) Source # | |
| Defined in GHC.Data.Stream Methods pure :: a0 -> Stream m a a0 Source # (<*>) :: Stream m a (a0 -> b) -> Stream m a a0 -> Stream m a b Source # liftA2 :: (a0 -> b -> c) -> Stream m a a0 -> Stream m a b -> Stream m a c Source # (*>) :: Stream m a a0 -> Stream m a b -> Stream m a b Source # (<*) :: Stream m a a0 -> Stream m a b -> Stream m a a0 Source # | |
| Functor (Stream f a) Source # | |
| Monad (Stream m a) Source # | |
Instances
| Monad m => Applicative (StreamS m a) Source # | |
| Defined in GHC.Data.Stream Methods pure :: a0 -> StreamS m a a0 Source # (<*>) :: StreamS m a (a0 -> b) -> StreamS m a a0 -> StreamS m a b Source # liftA2 :: (a0 -> b -> c) -> StreamS m a a0 -> StreamS m a b -> StreamS m a c Source # (*>) :: StreamS m a a0 -> StreamS m a b -> StreamS m a b Source # (<*) :: StreamS m a a0 -> StreamS m a b -> StreamS m a a0 Source # | |
| Monad m => Functor (StreamS m a) Source # | |
| Monad m => Monad (StreamS m a) Source # | |
liftIO :: MonadIO m => IO a -> m a Source #
Lift a computation from the IO monad.
 This allows us to run IO computations in any monadic stack, so long as it supports these kinds of operations
 (i.e. IO is the base monad for the stack).
Example
import Control.Monad.Trans.State -- from the "transformers" library printState :: Show s => StateT s IO () printState = do state <- get liftIO $ print state
Had we omitted liftIO
• Couldn't match type ‘IO’ with ‘StateT s IO’ Expected type: StateT s IO () Actual type: IO ()
The important part here is the mismatch between StateT s IO () and IO ()
Luckily, we know of a function that takes an IO a(m a): liftIO
> evalStateT printState "hello" "hello" > evalStateT printState 3 3
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 a. m a -> n a) -> (a -> n ()) -> n b Source #
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_ :: 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 effiency problems.