Package Info

ghc-streaming-bytestring


Effectful byte steams, or: bytestring io done right


Development/Libraries/Haskell

This is an implementation of effectful, memory-constrained bytestrings (byte streams) and functions for streaming bytestring manipulation, adequate for non-lazy-io.

The implementation follows the details of 'Data.ByteString.Lazy' and 'Data.ByteString.Lazy.Char8' in unrelenting detail, omitting only transparently non-streaming operations like 'reverse'. It is just a question of replacing the lazy bytestring type:

> data ByteString = Empty | Chunk Strict.ByteString ByteString

with the /minimal/ effectful variant:

> data ByteString m r = Empty r | Chunk Strict.ByteString (ByteString m r) | Go (m (ByteString m r))

(Constructors are necessarily hidden in internal modules in both the 'Lazy' and the 'Streaming'.)

That's it. As a lazy bytestring is implemented internally by a sort of list of strict bytestring chunks, a streaming bytestring is simply implemented as a /producer/ or /generator/ of strict bytestring chunks. Most operations are defined by simply adding a line to what we find in 'Data.ByteString.Lazy'.

Something like this alteration of type is of course obvious and mechanical, once the idea of an effectful bytestring type is contemplated and lazy io is rejected. Indeed it seems that this is the proper expression of what was intended by lazy bytestrings to begin with. The documentation, after all, reads

  • "A key feature of lazy ByteStrings is the means to manipulate large or unbounded streams of data without requiring the entire sequence to be resident in memory. To take advantage of this you have to write your functions in a lazy streaming style, e.g. classic pipeline composition. The default I/O chunk size is 32k, which should be good in most circumstances."

... which is very much the idea of this library: the default chunk size for 'hGetContents' and the like follows 'Data.ByteString.Lazy'; operations like 'lines' and 'append' and so on are tailored not to increase chunk size.

The present library is thus nothing but /lazy bytestring done right/. The authors of 'Data.ByteString.Lazy' must have supposed that the directly monadic formulation of such their type would necessarily make things slower. This appears to be a prejudice. For example, passing a large file of short lines through this benchmark transformation

> Lazy.unlines . map (bs -> "!" <> Lazy.drop 5 bs) . Lazy.lines > Streaming.unlines . S.maps (bs -> chunk "!" >> Streaming.drop 5 bs) . Streaming.lines

gives pleasing results like these

> $ time ./benchlines lazy >> /dev/null > real 0m2.097s > ... > $ time ./benchlines streaming >> /dev/null > real 0m1.930s

For a more sophisticated operation like

> Lazy.intercalate "!n" . Lazy.lines > Streaming.intercalate "!n" . Streaming.lines

we get results like these:

> time ./benchlines lazy >> /dev/null > real 0m1.250s > ... > time ./benchlines streaming >> /dev/null > real 0m1.531s

The pipes environment would express the latter as

> Pipes.intercalates (Pipes.yield "!n") . view Pipes.lines

meaning almost exactly what we mean above, but with results like this

> time ./benchlines pipes >> /dev/null > real 0m6.353s

The difference, however, is emphatically not intrinsic to pipes; it is just that this library depends the 'streaming' library, which is used in place of 'free' to express the <http://www.haskellforall.com/2013/09/perfect-streaming-using-pipes-bytestring.html "perfectly streaming"> splitting and iterated division or "chunking" of byte streams.

These concepts belong to the ABCs of streaming; 'lines' is just a textbook example, and it is of course handled correctly in 'Data.ByteString.Lazy'. But the concepts are /catastrophically mishandled/ in /all/ streaming io libraries other than pipes. Already the 'enumerator' and 'iteratee' libraries were completely defeated by 'lines': see e.g. the 'enumerator' implementation of <http://hackage.haskell.org/package/enumerator-0.4.20/docs/Data-Enumerator-Text.html#v:splitWhen splitWhen and lines>. This will concatenate strict text forever, if that's what is coming in. The rot spreads from there. It is just a fact that in all of the general streaming io frameworks other than pipes,it becomes torture to express elementary distinctions that are transparently and immediately contained in any idea of streaming whatsoever.

Though, as was said above, we barely alter signatures in 'Data.ByteString.Lazy' more than is required by the types, the point of view that emerges is very much that of 'pipes-bytestring' and 'pipes-group'. In particular we have these correspondences:

> Lazy.splitAt :: Int -> ByteString -> (ByteString, ByteString) > Streaming.splitAt :: Int -> ByteString m r -> ByteString m (ByteString m r) > Pipes.splitAt :: Int -> Producer ByteString m r -> Producer ByteString m (Producer ByteString m r)

and

> Lazy.lines :: ByteString -> [ByteString] > Streaming.lines :: ByteString m r -> Stream (ByteString m) m r > Pipes.lines :: Producer ByteString m r -> FreeT (Producer ByteString m) m r

where the 'Stream' type expresses the sequencing of 'ByteString m _' layers with the usual 'free monad' sequencing.

Interoperation with 'pipes-bytestring' uses this isomorphism:

> Streaming.ByteString.unfoldrChunks Pipes.next :: Monad m => Producer ByteString m r -> ByteString m r > Pipes.unfoldr Streaming.ByteString.nextChunk :: Monad m => ByteString m r -> Producer ByteString m r

Interoperation with 'io-streams' is thus:

> IOStreams.unfoldM Streaming.ByteString.unconsChunk :: ByteString IO () -> IO (InputStream ByteString) > Streaming.ByteString.reread IOStreams.read :: InputStream ByteString -> ByteString IO ()

and similarly for other rational streaming io libraries.

Problems and questions about the library can be put as issues on the github page, or mailed to the <https://groups.google.com/forum/#!forum/haskell-pipes pipes list>.

A tutorial module is in the works; <https://gist.github.com/michaelt/6c6843e6dd8030e95d58 here>, for the moment, is a sequence of simplified implementations of familiar shell utilities. The same programs are implemented at the end of the excellent <http://hackage.haskell.org/package/io-streams-1.3.2.0/docs/System-IO-Streams-Tutorial.html io-streams tutorial>. It is generally much simpler; in some case simpler than what you would write with lazy bytestrings. <https://gist.github.com/michaelt/2dcea1ba32562c091357 Here> is a simple GET request that returns a byte stream. .


License: BSD-3-Clause
URL: https://hackage.haskell.org/package/streaming-bytestring

Categories

Releases

Package Version Update ID Released Package Hub Version Platforms Subpackages
0.1.4.6-bp150.1.3 info GA Release 2018-07-30 15
  • AArch64
  • ppc64le
  • x86-64
  • ghc-streaming-bytestring
  • ghc-streaming-bytestring-devel