Data.Iteratee.ListLike

Monadic Iteratees: incremental input parsers, processors and transformers

This module provides many basic iteratees from which more complicated iteratees can be built. In general these iteratees parallel those in Data.List, with some additions.

Iteratees

Iteratee Utilities

isFinished :: Nullable s => Iteratee s m Bool

Check if a stream has received EOF.

stream2list :: (Monad m, Nullable s, ListLike s el) => Iteratee s m [el]

Read a stream to the end and return all of its elements as a list. This iteratee returns all data from the stream *strictly*.

stream2stream :: (Monad m, Nullable s, Monoid s) => Iteratee s m s

Read a stream to the end and return all of its elements as a stream. This iteratee returns all data from the stream *strictly*.

Basic Iteratees

break :: ListLike s el => (el -> Bool) -> Iteratee s m s

Takes an element predicate and returns the (possibly empty) prefix of the stream. None of the characters in the string satisfy the character predicate. If the stream is not terminated, the first character of the remaining stream satisfies the predicate.

N.B. breakE should be used in preference to break. break will retain all data until the predicate is met, which may result in a space leak.

The analogue of List.break

dropWhile :: ListLike s el => (el -> Bool) -> Iteratee s m ()

Skip all elements while the predicate is true.

The analogue of List.dropWhile

drop :: (Nullable s, ListLike s el) => Int -> Iteratee s m ()

Drop n elements of the stream, if there are that many.

The analogue of List.drop

head :: ListLike s el => Iteratee s m el

Attempt to read the next element of the stream and return it Raise a (recoverable) error if the stream is terminated.

The analogue of List.head

Because head can raise an error, it shouldn't be used when constructing iteratees for convStream. Use tryHead instead.

tryHead :: ListLike s el => Iteratee s m (Maybe el)

Similar to head, except it returns Nothing if the stream is terminated.

last :: (ListLike s el, Nullable s) => Iteratee s m el

Attempt to read the last element of the stream and return it Raise a (recoverable) error if the stream is terminated

The analogue of List.last

heads :: (Monad m, Nullable s, ListLike s el, Eq el) => s -> Iteratee s m Int

Given a sequence of characters, attempt to match them against the characters on the stream. Return the count of how many characters matched. The matched characters are removed from the stream. For example, if the stream contains abd, then (heads abc) will remove the characters ab and return 2.

peek :: ListLike s el => Iteratee s m (Maybe el)

Look ahead at the next element of the stream, without removing it from the stream. Return Just c if successful, return Nothing if the stream is terminated by EOF.

roll

Arguments

:: (Monad m, Functor m, Nullable s, ListLike s el, ListLike s' s)

=> Int

length of chunk (t)

-> Int

amount to consume (d)

-> Iteratee s m s'

Return a chunk of t elements length while consuming d elements from the stream. Useful for creating a 'rolling average' with convStream.

length :: (Num a, ListLike s el) => Iteratee s m a

Return the total length of the remaining part of the stream.

This forces evaluation of the entire stream.

The analogue of List.length

chunkLength :: ListLike s el => Iteratee s m (Maybe Int)

Get the length of the current chunk, or Nothing if EOF.

This function consumes no input.

takeFromChunk :: (Nullable s, ListLike s el) => Int -> Iteratee s m s

Take n elements from the current chunk, or the whole chunk if n is greater.

Nested iteratee combinators

breakE :: (ListLike s el, NullPoint s) => (el -> Bool) -> Enumeratee s s m a

Takes an element predicate and an iteratee, running the iteratee on all elements of the stream until the predicate is met.

the following rule relates break to breakE break pred === joinI (breakE pred stream2stream)

breakE should be used in preference to break whenever possible.

take

Arguments

:: (Monad m, Nullable s, ListLike s el)

=> Int

number of elements to consume

-> Enumeratee s s m a

Read n elements from a stream and apply the given iteratee to the stream of the read elements. Unless the stream is terminated early, we read exactly n elements, even if the iteratee has accepted fewer.

The analogue of List.take

takeUpTo :: (Monad m, Nullable s, ListLike s el) => Int -> Enumeratee s s m a

Read n elements from a stream and apply the given iteratee to the stream of the read elements. If the given iteratee accepted fewer elements, we stop. This is the variation of take with the early termination of processing of the outer stream once the processing of the inner stream finished early.

Iteratees composed with takeUpTo will consume only enough elements to reach a done state. Any remaining data will be available in the outer stream.

 > let iter = do
 h <- joinI $ takeUpTo 5 I.head
 t <- stream2list
 return (h,t)
 
 > enumPureNChunk [1..10::Int] 3 iter >>= run >>= print
 (1,[2,3,4,5,6,7,8,9,10])
 
 > enumPureNChunk [1..10::Int] 7 iter >>= run >>= print
 (1,[2,3,4,5,6,7,8,9,10])

in each case, I.head consumes only one element, returning the remaining 4 elements to the outer stream

takeWhile :: ListLike s el => (el -> Bool) -> Iteratee s m s

Takes an element predicate and returns the (possibly empty) prefix of the stream. All characters in the string will satisfy the character predicate. If the stream is not terminated, the first character of the remaining stream will not satisfy the predicate.

The analogue of List.takeWhile, see also break and takeWhileE

takeWhileE :: (ListLike s el, NullPoint s) => (el -> Bool) -> Enumeratee s s m a

Takes an element predicate and an iteratee, running the iteratee on all elements of the stream while the predicate is met.

This is preferred to takeWhile.

mapStream :: (ListLike (s el) el, ListLike (s el') el', NullPoint (s el), LooseMap s el el') => (el -> el') -> Enumeratee (s el) (s el') m a

Map the stream: another iteratee transformer Given the stream of elements of the type el and the function (el->el'), build a nested stream of elements of the type el' and apply the given iteratee to it.

The analog of List.map

rigidMapStream :: (ListLike s el, NullPoint s) => (el -> el) -> Enumeratee s s m a

Map the stream rigidly.

Like mapStream, but the element type cannot change. This function is necessary for ByteString and similar types that cannot have LooseMap instances, and may be more efficient.

filter :: (Monad m, Functor m, Nullable s, ListLike s el) => (el -> Bool) -> Enumeratee s s m a

Creates an enumeratee with only elements from the stream that satisfy the predicate function. The outer stream is completely consumed.

The analogue of List.filter

group

Arguments

:: (ListLike s el, Monad m, Nullable s)

=> Int

size of group

-> Enumeratee s [s] m a

Creates an Enumeratee in which elements from the stream are grouped into sz-sized blocks. The final block may be smaller than sz.

groupBy :: (ListLike s el, Monad m, Nullable s) => (el -> el -> Bool) -> Enumeratee s [s] m a

Creates an enumeratee in which elements are grouped into contiguous blocks that are equal according to a predicate.

The analogue of groupBy

merge :: (ListLike s1 el1, ListLike s2 el2, Nullable s1, Nullable s2, Monad m, Functor m) => (el1 -> el2 -> b) -> Enumeratee s2 b (Iteratee s1 m) a

merge offers another way to nest iteratees: as a monad stack. This allows for the possibility of interleaving data from multiple streams.

 -- print each element from a stream of lines.
 logger :: (MonadIO m) => Iteratee [ByteString] m ()
 logger = mapM_ (liftIO . putStrLn . B.unpack)

 -- combine alternating lines from two sources
 -- To see how this was derived, follow the types from
 -- 'ileaveLines logger' and work outwards.
 run =<< enumFile 10 "file1" (joinI $ enumLinesBS $
           ( enumFile 10 "file2" . joinI . enumLinesBS $ joinI
                 (ileaveLines logger)) >>= run)
 
 ileaveLines :: (Functor m, Monad m)
   => Enumeratee [ByteString] [ByteString] (Iteratee [ByteString] m)
        [ByteString]
 ileaveLines = merge (\l1 l2 ->
    [B.pack "f1:\n\t" ,l1 ,B.pack "f2:\n\t" ,l2 ]
 
 

mergeByChunks

Arguments

:: (Nullable c2, Nullable c1, NullPoint c2, NullPoint c1, ListLike c1 el1, ListLike c2 el2, Functor m, Monad m)

=> (c1 -> c2 -> c3)

merge function

-> (c1 -> c3)

-> (c2 -> c3)

-> Enumeratee c2 c3 (Iteratee c1 m) a

A version of merge which operates on chunks instead of elements.

mergeByChunks offers more control than merge. merge terminates when the first stream terminates, however mergeByChunks will continue until both streams are exhausted.

mergeByChunks guarantees that both chunks passed to the merge function will have the same number of elements, although that number may vary between calls.

Folds

foldl :: (ListLike s el, FoldableLL s el) => (a -> el -> a) -> a -> Iteratee s m a

Left-associative fold.

The analogue of List.foldl

foldl' :: (ListLike s el, FoldableLL s el) => (a -> el -> a) -> a -> Iteratee s m a

Left-associative fold that is strict in the accumulator. This function should be used in preference to foldl whenever possible.

The analogue of List.foldl'.

foldl1 :: (ListLike s el, FoldableLL s el) => (el -> el -> el) -> Iteratee s m el

Variant of foldl with no base case. Requires at least one element in the stream.

The analogue of List.foldl1.

foldl1' :: (ListLike s el, FoldableLL s el) => (el -> el -> el) -> Iteratee s m el

Strict variant of foldl1.

Special Folds

sum :: (ListLike s el, Num el) => Iteratee s m el

Sum of a stream.

product :: (ListLike s el, Num el) => Iteratee s m el

Product of a stream.

Enumerators

Basic enumerators

enumPureNChunk :: (Monad m, ListLike s el) => s -> Int -> Enumerator s m a

The pure n-chunk enumerator It passes a given stream of elements to the iteratee in n-sized chunks.

Enumerator Combinators

enumPair :: (Monad m, Nullable s, ListLike s el) => Iteratee s m a -> Iteratee s m b -> Iteratee s m (a, b)

Deprecated: use Data.Iteratee.ListLike.zip

Enumerate two iteratees over a single stream simultaneously. Deprecated, use zip instead.

Compare to zip.

enumWith :: (Monad m, Nullable s, ListLike s el) => Iteratee s m a -> Iteratee s m b -> Iteratee s m (a, b)

Enumerate over two iteratees in parallel as long as the first iteratee is still consuming input. The second iteratee will be terminated with EOF when the first iteratee has completed. An example use is to determine how many elements an iteratee has consumed:

 snd <$> enumWith (dropWhile (<5)) length

Compare to zip

zip :: (Monad m, Nullable s, ListLike s el) => Iteratee s m a -> Iteratee s m b -> Iteratee s m (a, b)

Enumerate two iteratees over a single stream simultaneously.

Compare to List.zip.

zip3 :: (Monad m, Nullable s, ListLike s el) => Iteratee s m a -> Iteratee s m b -> Iteratee s m c -> Iteratee s m (a, b, c)

zip4 :: (Monad m, Nullable s, ListLike s el) => Iteratee s m a -> Iteratee s m b -> Iteratee s m c -> Iteratee s m d -> Iteratee s m (a, b, c, d)

zip5 :: (Monad m, Nullable s, ListLike s el) => Iteratee s m a -> Iteratee s m b -> Iteratee s m c -> Iteratee s m d -> Iteratee s m e -> Iteratee s m (a, b, c, d, e)

sequence_ :: (Monad m, ListLike s el, Nullable s) => [Iteratee s m a] -> Iteratee s m ()

Enumerate a list of iteratees over a single stream simultaneously and discard the results. This is a different behavior than Prelude's sequence_ which runs iteratees in the list one after the other.

Compare to Prelude.sequence_.

countConsumed :: forall a s el m n. (Monad m, ListLike s el, Nullable s, Integral n) => Iteratee s m a -> Iteratee s m (a, n)

Transform an iteratee into one that keeps track of how much data it consumes.

greedy :: (Monad m, Functor m, ListLike s el', Monoid a) => Iteratee s m a -> Iteratee s m a

Convert an iteratee to a "greedy" version.

When a chunk is received, repeatedly run the input iteratee until the entire chunk is consumed, then the outputs are combined (via mconcat).

 > let l = [1..5::Int]
 > run =<< enumPure1Chunk l (joinI (take 2 stream2list))
 [1,2]
 > run =<< enumPure1Chunk l (greedy $ joinI (I.take 2 stream2list))
 [1,2,3,4,5]

Note that a greedy iteratee will consume the entire input chunk and force the next chunk before returning a value. A portion of the second chunk may be consumed.

greedy may be useful on the first parameter of convStream, e.g.

 convStream (greedy someIter)

to create more efficient converters.

Monadic functions

mapM_ :: (Monad m, ListLike s el, Nullable s) => (el -> m b) -> Iteratee s m ()

Map a monadic function over the elements of the stream and ignore the result.

foldM :: (Monad m, ListLike s b, Nullable s) => (a -> b -> m a) -> a -> Iteratee s m a

The analogue of Control.Monad.foldM

Re-exported modules

module Data.Iteratee.Iteratee