Data.Conduit.List

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

For many purposes, it's recommended to use the conduit-combinators library, which provides a more complete set of 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

Yield the values from the list.

Subject to fusion

sourceNull :: Monad m => Producer m a

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

Subject to fusion

Since 0.3.0

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

Generate a source from a seed value.

Subject to fusion

Since 0.4.2

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

A monadic unfold.

Subject to fusion

Since 1.1.2

enumFromTo :: (Enum a, Ord 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.

Subject to fusion

Since 0.4.2

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

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

Subject to fusion

replicate :: Monad m => Int -> a -> Producer m a

Replicate a single value the given number of times.

Subject to fusion

Since 1.2.0

replicateM :: Monad m => Int -> m a -> Producer m a

Replicate a monadic value the given number of times.

Subject to fusion

Since 1.2.0

Sinks

Pure

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

A strict left fold.

Subject to fusion

Since 0.3.0

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

A monoidal strict left fold.

Subject to fusion

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

Subject to fusion

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.

Subject to fusion

Since 0.3.0

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

Take a single value from the stream, if available.

Subject to fusion

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.

Subject to fusion

Since 0.3.0

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

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

Subject to fusion

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.

Subject to fusion

Since 0.3.0

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

Apply the action to all values in the stream.

Subject to fusion

Since 0.3.0

Conduits

Pure

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

Apply a transformation to all values in a stream.

Subject to fusion

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.

Subject to fusion

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.

Subject to fusion

Since 1.0.6

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

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

Subject to fusion

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.

Subject to fusion

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.

Subject to fusion

Since 0.3.0

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

concatMap with a strict accumulator.

Subject to fusion

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.

Subject to fusion

Since 1.1.1

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

Analog of mapAccumL for lists. Note that in contrast to mapAccumL, the function argument takes the accumulator as its second argument, not its first argument, and the accumulated value is strict.

Subject to fusion

Since 1.1.1

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

Grouping input according to an equality function.

Subject to fusion

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 (:|))

Subject to fusion

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
    ...

Subject to fusion

Since 0.3.0

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

Keep only values in the stream passing a given predicate.

Subject to fusion

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_.

Subject to fusion

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)

Subject to fusion

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.

Subject to fusion

Since 1.1.1

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

Monadic mapAccum.

Subject to fusion

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.

Subject to fusion

Since 0.5.1

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

Monadic variant of mapFoldable.

Subject to fusion

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.

Subject to fusion

Since 0.3.0

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

concatMapM with a strict accumulator.

Subject to fusion

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