Data.Conduit.List

Higher-level functions to interact with the elements of a stream. Most of these are based on list functions.

Note that these functions all deal with individual elements of a stream as a sort of "black box", where there is no introspection of the contained elements. Values such as ByteString and Text will likely need to be treated specially to deal with their contents properly (Word8 and Char, respectively). See the Data.Conduit.Binary and Data.Conduit.Text modules.

Sources

sourceList :: Monad m => [a] -> Producer m a

sourceNull :: Monad m => Producer m a

A source that outputs no values. Note that this is just a type-restricted synonym for mempty.

Since 0.3.0

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

Generate a source from a seed value.

Since 0.4.2

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

A monadic unfold.

Since 1.1.2

enumFromTo :: (Enum a, Eq a, Monad m) => a -> a -> Producer m a

Enumerate from a value to a final value, inclusive, via succ.

This is generally more efficient than using Prelude's enumFromTo and combining with sourceList since this avoids any intermediate data structures.

Since 0.4.2

iterate :: Monad m => (a -> a) -> a -> Producer m a

Produces an infinite stream of repeated applications of f to x.

Sinks

Pure

fold :: Monad m => (b -> a -> b) -> b -> Consumer a m b

A strict left fold.

Since 0.3.0

foldMap :: (Monad m, Monoid b) => (a -> b) -> Consumer a m b

A monoidal strict left fold.

Since 0.5.3

take :: Monad m => Int -> Consumer a m [a]

Take some values from the stream and return as a list. If you want to instead create a conduit that pipes data to another sink, see isolate. This function is semantically equivalent to:

 take i = isolate i =$ consume

Since 0.3.0

drop :: Monad m => Int -> Consumer a m ()

Ignore a certain number of values in the stream. This function is semantically equivalent to:

 drop i = take i >> return ()

However, drop is more efficient as it does not need to hold values in memory.

Since 0.3.0

head :: Monad m => Consumer a m (Maybe a)

Take a single value from the stream, if available.

Since 0.3.0

peek :: Monad m => Consumer a m (Maybe a)

Look at the next value in the stream, if available. This function will not change the state of the stream.

Since 0.3.0

consume :: Monad m => Consumer a m [a]

Consume all values from the stream and return as a list. Note that this will pull all values into memory. For a lazy variant, see Data.Conduit.Lazy.

Since 0.3.0

sinkNull :: Monad m => Consumer a m ()

Ignore the remainder of values in the source. Particularly useful when combined with isolate.

Since 0.3.0

Monadic

foldMapM :: (Monad m, Monoid b) => (a -> m b) -> Consumer a m b

A monoidal strict left fold in a Monad.

Since 1.0.8

foldM :: Monad m => (b -> a -> m b) -> b -> Consumer a m b

A monadic strict left fold.

Since 0.3.0

mapM_ :: Monad m => (a -> m ()) -> Consumer a m ()

Apply the action to all values in the stream.

Since 0.3.0

Conduits

Pure

map :: Monad m => (a -> b) -> Conduit a m b

Apply a transformation to all values in a stream.

Since 0.3.0

mapMaybe :: Monad m => (a -> Maybe b) -> Conduit a m b

Apply a transformation that may fail to all values in a stream, discarding the failures.

Since 0.5.1

mapFoldable :: (Monad m, Foldable f) => (a -> f b) -> Conduit a m b

Generalization of mapMaybe and concatMap. It applies function to all values in a stream and send values inside resulting Foldable downstream.

Since 1.0.6

catMaybes :: Monad m => Conduit (Maybe a) m a

Filter the Just values from a stream, discarding the Nothing values.

Since 0.5.1

concat :: (Monad m, Foldable f) => Conduit (f a) m a

Generalization of catMaybes. It puts all values from Foldable into stream.

Since 1.0.6

concatMap :: Monad m => (a -> [b]) -> Conduit a m b

Apply a transformation to all values in a stream, concatenating the output values.

Since 0.3.0

concatMapAccum :: Monad m => (a -> accum -> (accum, [b])) -> accum -> Conduit a m b

concatMap with an accumulator.

Since 0.3.0

scanl :: Monad m => (a -> s -> (s, b)) -> s -> Conduit a m b

Deprecated: Use mapAccum instead

Deprecated synonym for mapAccum

Since 1.0.6

scan :: Monad m => (a -> b -> b) -> b -> ConduitM a b m b

Analog of scanl for lists.

Since 1.1.1

mapAccum :: Monad m => (a -> s -> (s, b)) -> s -> ConduitM a b m s

Analog of mapAccumL for lists.

Since 1.1.1

groupBy :: Monad m => (a -> a -> Bool) -> Conduit a m [a]

Grouping input according to an equality function.

Since 0.3.0

groupOn1 :: (Monad m, Eq b) => (a -> b) -> Conduit a m (a, [a])

groupOn1 is similar to groupBy id

returns a pair, indicating there are always 1 or more items in the grouping. This is designed to be converted into a NonEmpty structure but it avoids a dependency on another package

 import Data.List.NonEmpty

 groupOn1 :: (Monad m, Eq b) => (a -> b) -> Conduit a m (NonEmpty a)
 groupOn1 f = CL.groupOn1 f =$= CL.map (uncurry (:|))

Since 1.1.7

isolate :: Monad m => Int -> Conduit a m a

Ensure that the inner sink consumes no more than the given number of values. Note this this does not ensure that the sink consumes all of those values. To get the latter behavior, combine with sinkNull, e.g.:

 src $$ do
     x <- isolate count =$ do
         x <- someSink
         sinkNull
         return x
     someOtherSink
     ...

Since 0.3.0

filter :: Monad m => (a -> Bool) -> Conduit a m a

Keep only values in the stream passing a given predicate.

Since 0.3.0

Monadic

mapM :: Monad m => (a -> m b) -> Conduit a m b

Apply a monadic transformation to all values in a stream.

If you do not need the transformed values, and instead just want the monadic side-effects of running the action, see mapM_.

Since 0.3.0

iterM :: Monad m => (a -> m ()) -> Conduit a m a

Apply a monadic action on all values in a stream.

This Conduit can be used to perform a monadic side-effect for every value, whilst passing the value through the Conduit as-is.

 iterM f = mapM (\a -> f a >>= \() -> return a)

Since 0.5.6

scanlM :: Monad m => (a -> s -> m (s, b)) -> s -> Conduit a m b

Deprecated: Use mapAccumM instead

Deprecated synonym for mapAccumM

Since 1.0.6

scanM :: Monad m => (a -> b -> m b) -> b -> ConduitM a b m b

Monadic scanl.

Since 1.1.1

mapAccumM :: Monad m => (a -> s -> m (s, b)) -> s -> ConduitM a b m s

Monadic mapAccum.

Since 1.1.1

mapMaybeM :: Monad m => (a -> m (Maybe b)) -> Conduit a m b

Apply a monadic transformation that may fail to all values in a stream, discarding the failures.

Since 0.5.1

mapFoldableM :: (Monad m, Foldable f) => (a -> m (f b)) -> Conduit a m b

Monadic variant of mapFoldable.

Since 1.0.6

concatMapM :: Monad m => (a -> m [b]) -> Conduit a m b

Apply a monadic transformation to all values in a stream, concatenating the output values.

Since 0.3.0

concatMapAccumM :: Monad m => (a -> accum -> m (accum, [b])) -> accum -> Conduit a m b

concatMapM with an accumulator.

Since 0.3.0

Misc

sequence

Arguments

:: Monad m

=> Consumer i m o

Pipe to run repeatedly

-> Conduit i m o

Run a Pipe repeatedly, and output its result value downstream. Stops when no more input is available from upstream.

Since 0.5.0