The streaming-bytestring package
This is an implementation of effectful, memory-constrained
bytestrings (byte streams) and functions for streaming
bytestring manipulation, adequate for non-lazy-io.
Some examples of the use of byte streams to implement simple
shell progams can be found
See also the illustrations of use with e.g.
zlib etc. in the
library. Usage is as close as possible to that of
ByteString IO () is the most natural representation of
an effectful stream of bytes arising chunkwise from a handle.
Indeed, the implementation follows the
in unrelenting detail, omitting only transparently non-streaming
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
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. The only possible simplification would
involve specializing to
IO, throughout - but this would e.g. block
the use of
ResourceT to manage handles and the like, and a number
of other convenient operations like
copy, which permits one to
apply two operations simultaneously over the length of the byte stream.
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
append and so on are tailored not to increase chunk size.
The present library is thus if you like 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
free to express the
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
But the concepts are catastrophically mishandled in all streaming io libraries
other than pipes. Already the
were completely defeated by
see e.g. the
enumerator implementation of
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
more than is required by the types, the point of view that emerges
is very much that of
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)
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
Stream type expresses the sequencing of
ByteString m _ layers
with the usual 'free monad' sequencing.
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
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 pipes list.
A tutorial module is in the works; 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 io-streams tutorial. It is generally much simpler; in some case simpler than what you would write with lazy bytestrings. Here is a simple GET request that returns a byte stream.
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|Versions||0.1.0.0, 0.1.0.1, 0.1.0.2, 0.1.0.3, 0.1.0.4, 0.1.0.5, 0.1.0.6, 0.1.0.7, 0.1.0.8, 0.1.1.0, 0.1.2.0, 0.1.2.2, 0.1.3.0, 0.1.4.0, 0.1.4.2, 0.1.4.3, 0.1.4.4, 0.1.4.5, 0.1.4.6, 0.1.5 (info)|
|Dependencies||base (<5.0), bytestring, bytestring-builder, deepseq, exceptions, mmorph (>=1.0 && <1.2), mtl (>=2.1 && <2.3), resourcet, streaming (>=0.1.4.0 && <0.3), transformers (>=0.3 && <0.6), transformers-base [details]|
|Category||Data, Pipes, Streaming|
|Source repo||head: git clone https://github.com/michaelt/streaming-bytestring|
|Uploaded||Wed Dec 13 13:33:24 UTC 2017 by andrewthad|
|Distributions||LTSHaskell:0.1.5, NixOS:0.1.5, Stackage:0.1.5, openSUSE:0.1.5|
|Downloads||5158 total (139 in the last 30 days)|
|Rating||2.0 (votes: 1) [estimated by rule of succession]|
|Status||Docs available [build log]
Last success reported on 2017-12-13 [all 1 reports]
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