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 :: Resource m => [a] -> Source m a

Convert a list into a source.

Sinks

Pure

fold :: Resource m => (b -> a -> b) -> b -> Sink a m b

A strict left fold.

take :: Resource m => Int -> Sink 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

drop :: Resource m => Int -> Sink 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.

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

Apply a transformation to all values in a stream.

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

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

head :: Resource m => Sink a m (Maybe a)

Take a single value from the stream, if available.

peek :: Resource m => Sink a m (Maybe a)

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

consume :: Resource m => Sink 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.

sinkNull :: Resource m => Sink a m ()

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

Monadic

foldM :: Resource m => (b -> a -> m b) -> b -> Sink a m b

A monadic strict left fold.

mapM_ :: Resource m => (a -> m ()) -> Sink a m ()

Apply the action to all values in the stream.

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

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

Pure

isolate :: Resource 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
     ...

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

Keep only values in the stream passing a given predicate.

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_.