Streamly.Prelude

This module is designed to be imported qualified:

import qualified Streamly.Prelude as S

Functions with the suffix M are general functions that work on monadic arguments. The corresponding functions without the suffix M work on pure arguments and can in general be derived from their monadic versions but are provided for convenience and for consistency with other pure APIs in the base package.

Functions having a MonadAsync constraint work concurrently when used with appropriate stream type combinator. Please be careful to not use parallely with infinite streams.

Deconstruction and folds accept a SerialT type instead of a polymorphic type to ensure that streams always have a concrete monomorphic type by default, reducing type errors. In case you want to use any other type of stream you can use one of the type combinators provided in the Streamly module to convert the stream type.

Construction

Primitives to construct or inspect a stream.

nil :: IsStream t => t m a

An empty stream.

> toList nil
[]

Since: 0.1.0

consM :: (IsStream t, MonadAsync m) => m a -> t m a -> t m a infixr 5

Constructs a stream by adding a monadic action at the head of an existing stream. For example:

> toList $ getLine `consM` getLine `consM` nil
hello
world
["hello","world"]

Concurrent (do not use parallely to construct infinite streams)

Since: 0.2.0

(|:) :: (IsStream t, MonadAsync m) => m a -> t m a -> t m a infixr 5

Operator equivalent of consM. We can read it as "parallel colon" to remember that | comes before :.

> toList $ getLine |: getLine |: nil
hello
world
["hello","world"]

let delay = threadDelay 1000000 >> print 1
runStream $ serially  $ delay |: delay |: delay |: nil
runStream $ parallely $ delay |: delay |: delay |: nil

Concurrent (do not use parallely to construct infinite streams)

Since: 0.2.0

cons :: IsStream t => a -> t m a -> t m a infixr 5

Construct a stream by adding a pure value at the head of an existing stream. For pure values it can be faster than consM. For example:

> toList $ 1 `cons` 2 `cons` 3 `cons` nil
[1,2,3]

Since: 0.1.0

(.:) :: IsStream t => a -> t m a -> t m a infixr 5

Operator equivalent of cons.

> toList $ 1 .: 2 .: 3 .: nil
[1,2,3]

Since: 0.1.1

Generation by Unfolding

unfoldr :: IsStream t => (b -> Maybe (a, b)) -> b -> t m a

Build a stream by unfolding a pure step function starting from a seed. The step function returns the next element in the stream and the next seed value. When it is done it returns Nothing and the stream ends. For example,

let f b =
        if b > 3
        then Nothing
        else Just (b, b + 1)
in toList $ unfoldr f 0

[0,1,2,3]

Since: 0.1.0

unfoldrM :: (IsStream t, MonadAsync m) => (b -> m (Maybe (a, b))) -> b -> t m a

Build a stream by unfolding a monadic step function starting from a seed. The step function returns the next element in the stream and the next seed value. When it is done it returns Nothing and the stream ends. For example,

let f b =
        if b > 3
        then return Nothing
        else print b >> return (Just (b, b + 1))
in runStream $ unfoldrM f 0

 0
 1
 2
 3

When run concurrently, the next unfold step can run concurrently with the processing of the output of the previous step. Note that more than one step cannot run concurrently as the next step depends on the output of the previous step.

(asyncly $ S.unfoldrM (\n -> liftIO (threadDelay 1000000) >> return (Just (n, n + 1))) 0)
    & S.foldlM' (\_ a -> threadDelay 1000000 >> print a) ()

Concurrent

Since: 0.1.0

Special Generation

Generate a monadic stream from an input structure, a seed or a generation function.

once :: (IsStream t, Monad m) => m a -> t m a

Create a singleton stream by executing a monadic action once. Same as m `consM` nil but more efficient.

> toList $ once getLine
hello
["hello"]

Since: 0.2.0

replicateM :: (IsStream t, MonadAsync m) => Int -> m a -> t m a

Generate a stream by performing a monadic action n times.

runStream $ serially $ S.replicateM 10 $ (threadDelay 1000000 >> print 1)
runStream $ asyncly  $ S.replicateM 10 $ (threadDelay 1000000 >> print 1)

Concurrent

Since: 0.1.1

repeatM :: (IsStream t, MonadAsync m) => m a -> t m a

Generate a stream by repeatedly executing a monadic action forever.

runStream $ serially $ S.take 10 $ S.repeatM $ (threadDelay 1000000 >> print 1)
runStream $ asyncly  $ S.take 10 $ S.repeatM $ (threadDelay 1000000 >> print 1)

Concurrent, infinite (do not use with parallely)

Since: 0.2.0

iterate :: IsStream t => (a -> a) -> a -> t m a

Iterate a pure function from a seed value, streaming the results forever.

Since: 0.1.2

iterateM :: (IsStream t, MonadAsync m) => (a -> m a) -> a -> t m a

Iterate a monadic function from a seed value, streaming the results forever.

When run concurrently, the next iteration can run concurrently with the processing of the previous iteration. Note that more than one iteration cannot run concurrently as the next iteration depends on the output of the previous iteration.

runStream $ serially $ S.take 10 $ S.iterateM
     (\x -> threadDelay 1000000 >> print x >> return (x + 1)) 0

runStream $ asyncly  $ S.take 10 $ S.iterateM
     (\x -> threadDelay 1000000 >> print x >> return (x + 1)) 0

Concurrent

Since: 0.1.2

fromFoldable :: (IsStream t, Foldable f) => f a -> t m a

Construct a stream from a Foldable containing pure values.

Since: 0.2.0

fromFoldableM :: (IsStream t, MonadAsync m, Foldable f) => f (m a) -> t m a

Construct a stream from a Foldable containing monadic actions.

runStream $ serially $ S.fromFoldableM $ replicate 10 (threadDelay 1000000 >> print 1)
runStream $ asyncly  $ S.fromFoldableM $ replicate 10 (threadDelay 1000000 >> print 1)

Concurrent (do not use with parallely on infinite containers)

Since: 0.3.0

Deconstruction

uncons :: (IsStream t, Monad m) => SerialT m a -> m (Maybe (a, t m a))

Decompose a stream into its head and tail. If the stream is empty, returns Nothing. If the stream is non-empty, returns Just (a, ma), where a is the head of the stream and ma its tail.

Since: 0.1.0

Elimination by Folding

General Folds

foldr :: Monad m => (a -> b -> b) -> b -> SerialT m a -> m b

Lazy right associative fold. For example, to fold a stream into a list:

>> runIdentity $ foldr (:) [] (serially $ fromFoldable [1,2,3])
[1,2,3]

Since: 0.1.0

foldrM :: Monad m => (a -> b -> m b) -> b -> SerialT m a -> m b

Lazy right fold with a monadic step function. For example, to fold a stream into a list:

>> runIdentity $ foldrM (\x xs -> return (x : xs)) [] (serially $ fromFoldable [1,2,3])
[1,2,3]

Since: 0.2.0

foldl' :: Monad m => (b -> a -> b) -> b -> SerialT m a -> m b

Strict left associative fold.

Since: 0.2.0

foldlM' :: Monad m => (b -> a -> m b) -> b -> SerialT m a -> m b

Like foldl' but with a monadic step function.

Since: 0.2.0

foldx :: Monad m => (x -> a -> x) -> x -> (x -> b) -> SerialT m a -> m b

Strict left fold with an extraction function. Like the standard strict left fold, but applies a user supplied extraction function (the third argument) to the folded value at the end. This is designed to work with the foldl library. The suffix x is a mnemonic for extraction.

Since: 0.2.0

foldxM :: Monad m => (x -> a -> m x) -> m x -> (x -> m b) -> SerialT m a -> m b

Like foldx, but with a monadic step function.

Since: 0.2.0

Special Folds

mapM_ :: Monad m => (a -> m b) -> SerialT m a -> m ()

Apply a monadic action to each element of the stream and discard the output of the action.

Since: 0.1.0

toList :: Monad m => SerialT m a -> m [a]

Convert a stream into a list in the underlying monad.

Since: 0.1.0

all :: Monad m => (a -> Bool) -> SerialT m a -> m Bool

Determine whether all elements of a stream satisfy a predicate.

Since: 0.1.0

any :: Monad m => (a -> Bool) -> SerialT m a -> m Bool

Determine whether any of the elements of a stream satisfy a predicate.

Since: 0.1.0

head :: Monad m => SerialT m a -> m (Maybe a)

Extract the first element of the stream, if any.

Since: 0.1.0

tail :: (IsStream t, Monad m) => SerialT m a -> m (Maybe (t m a))

Extract all but the first element of the stream, if any.

Since: 0.1.1

last :: Monad m => SerialT m a -> m (Maybe a)

Extract the last element of the stream, if any.

Since: 0.1.1

null :: Monad m => SerialT m a -> m Bool

Determine whether the stream is empty.

Since: 0.1.1

length :: Monad m => SerialT m a -> m Int

Determine the length of the stream.

Since: 0.1.0

elem :: (Monad m, Eq a) => a -> SerialT m a -> m Bool

Determine whether an element is present in the stream.

Since: 0.1.0

notElem :: (Monad m, Eq a) => a -> SerialT m a -> m Bool

Determine whether an element is not present in the stream.

Since: 0.1.0

maximum :: (Monad m, Ord a) => SerialT m a -> m (Maybe a)

Determine the maximum element in a stream.

Since: 0.1.0

minimum :: (Monad m, Ord a) => SerialT m a -> m (Maybe a)

Determine the minimum element in a stream.

Since: 0.1.0

sum :: (Monad m, Num a) => SerialT m a -> m a

Determine the sum of all elements of a stream of numbers

Since: 0.1.0

product :: (Monad m, Num a) => SerialT m a -> m a

Determine the product of all elements of a stream of numbers

Since: 0.1.1

Scans

scanl' :: IsStream t => (b -> a -> b) -> b -> t m a -> t m b

Strict left scan. Like foldl', but returns the folded value at each step, generating a stream of all intermediate fold results. The first element of the stream is the user supplied initial value, and the last element of the stream is the same as the result of foldl'.

Since: 0.2.0

scanx :: IsStream t => (x -> a -> x) -> x -> (x -> b) -> t m a -> t m b

Strict left scan with an extraction function. Like scanl', but applies a user supplied extraction function (the third argument) at each step. This is designed to work with the foldl library. The suffix x is a mnemonic for extraction.

Since: 0.2.0

Filtering

filter :: IsStream t => (a -> Bool) -> t m a -> t m a

Include only those elements that pass a predicate.

Since: 0.1.0

take :: IsStream t => Int -> t m a -> t m a

Take first n elements from the stream and discard the rest.

Since: 0.1.0

takeWhile :: IsStream t => (a -> Bool) -> t m a -> t m a

End the stream as soon as the predicate fails on an element.

Since: 0.1.0

drop :: IsStream t => Int -> t m a -> t m a

Discard first n elements from the stream and take the rest.

Since: 0.1.0

dropWhile :: IsStream t => (a -> Bool) -> t m a -> t m a

Drop elements in the stream as long as the predicate succeeds and then take the rest of the stream.

Since: 0.1.0

Reordering

reverse :: IsStream t => t m a -> t m a

Returns the elements of the stream in reverse order. The stream must be finite.

Since: 0.1.1

Mapping

mapM :: (IsStream t, MonadAsync m) => (a -> m b) -> t m a -> t m b

Replace each element of the stream with the result of a monadic action applied on the element.

runStream $ S.replicateM 10 (return 1)
          & (serially . S.mapM (\x -> threadDelay 1000000 >> print x))

runStream $ S.replicateM 10 (return 1)
          & (asyncly . S.mapM (\x -> threadDelay 1000000 >> print x))

Concurrent (do not use with parallely on infinite streams)

Since: 0.1.0

mapMaybe :: IsStream t => (a -> Maybe b) -> t m a -> t m b

Map a Maybe returning function to a stream, filter out the Nothing elements, and return a stream of values extracted from Just.

Since: 0.3.0

mapMaybeM :: (IsStream t, MonadAsync m, Functor (t m)) => (a -> m (Maybe b)) -> t m a -> t m b

Like mapMaybe but maps a monadic function.

Concurrent (do not use with parallely on infinite streams)

Since: 0.3.0

sequence :: (IsStream t, MonadAsync m) => t m (m a) -> t m a

Reduce a stream of monadic actions to a stream of the output of those actions.

runStream $ S.replicateM 10 (return $ threadDelay 1000000 >> print 1)
          & (serially . S.sequence)

runStream $ S.replicateM 10 (return $ threadDelay 1000000 >> print 1)
          & (asyncly . S.sequence)

Concurrent (do not use with parallely on infinite streams)

Since: 0.1.0

Zipping

zipWith :: IsStream t => (a -> b -> c) -> t m a -> t m b -> t m c

Zip two streams serially using a pure zipping function.

Since: 0.1.0

zipWithM :: IsStream t => (a -> b -> t m c) -> t m a -> t m b -> t m c

Zip two streams serially using a monadic zipping function.

Since: 0.1.0

zipAsyncWith :: (IsStream t, MonadAsync m) => (a -> b -> c) -> t m a -> t m b -> t m c

Zip two streams concurrently (i.e. both the elements being zipped are generated concurrently) using a pure zipping function.

Since: 0.1.0

zipAsyncWithM :: (IsStream t, MonadAsync m) => (a -> b -> t m c) -> t m a -> t m b -> t m c

Zip two streams asyncly (i.e. both the elements being zipped are generated concurrently) using a monadic zipping function.

Since: 0.1.0

IO

fromHandle :: (IsStream t, MonadIO m) => Handle -> t m String

Read lines from an IO Handle into a stream of Strings.

Since: 0.1.0

toHandle :: MonadIO m => Handle -> SerialT m String -> m ()

Write a stream of Strings to an IO Handle.

Since: 0.1.0

Deprecated

each :: (IsStream t, Foldable f) => f a -> t m a

Deprecated: Please use fromFoldable instead.

Same as fromFoldable.

Since: 0.1.0

scan :: IsStream t => (x -> a -> x) -> x -> (x -> b) -> t m a -> t m b

Deprecated: Please use scanx instead.

Since: 0.1.1

foldl :: Monad m => (x -> a -> x) -> x -> (x -> b) -> SerialT m a -> m b

Deprecated: Please use foldx instead.

Since: 0.1.0

foldlM :: Monad m => (x -> a -> m x) -> m x -> (x -> m b) -> SerialT m a -> m b

Deprecated: Please use foldxM instead.

Since: 0.1.0