streaming-bytestring
Fast, effectful byte streams.
https://github.com/haskell-streaming/streaming-bytestring
| LTS Haskell 23.21: | 0.3.4 |
| Stackage Nightly 2025-05-06: | 0.3.4 |
| Latest on Hackage: | 0.3.4 |
streaming-bytestring-0.3.4@sha256:ec00ff8c00a3e27dcaa3923bb80984ffd71c37c96dc7d6f01db30ccbf5148f59,3084Module documentation for 0.3.4
streaming-bytestring
This library enables fast and safe streaming of byte data, in either Word8 or
Char form. It is a core addition to the streaming
ecosystem and avoids the usual pitfalls
of combinbing lazy ByteStrings with lazy IO.
This library is used by
streaming-attoparsec
to enable vanilla Attoparsec
parsers to work with streaming “for free”.
Usage
Importing and Types
Modules from this library are intended to be imported qualified. To avoid
conflicts with both the bytestring library and streaming, we recommended Q
as the qualified name:
import qualified Streaming.ByteString.Char8 as Q
Like the bytestring library, leaving off the Char8 will expose an API based
on Word8. Following the philosophy of streaming that “the best API is the
one you already know”, these APIs are based closely on bytestring. The core
type is ByteStream m r, where:
m: The Monad used to fetch further chunks from the “source”, usuallyIO.r: The final return value after all streaming has concluded, usually()as instreaming.
You can imagine this type to represent an infinitely-sized collection of bytes,
although internally it references a strict ByteString no larger than 32kb,
followed by monadic instructions to fetch further chunks.
Examples
File Input
To open a file of any size and count its characters:
import Control.Monad.Trans.Resource (runResourceT)
import qualified Streaming.Streaming.Char8 as Q
-- | Represents a potentially-infinite stream of `Char`.
chars :: ByteStream IO ()
chars = Q.readFile "huge-file.txt"
main :: IO ()
main = runResourceT (Q.length_ chars) >>= print
Note that file IO specifically requires the
resourcet library.
Line splitting and Stream interop
In the example above you may have noticed a lack of Of that we usually see
with Stream. Our old friend lines hints at this too:
lines :: Monad m => ByteStream m r -> Stream (ByteStream m) m r
A stream-of-streams, yet no Of here either. The return type can’t naively be
Stream (Of ByteString) m r, since the first line break might be at the very
end of a large file. Forcing that into a single strict ByteString would crash
your program.
To count the number of lines whose first letter is i:
countOfI :: IO Int
countOfI = runResourceT
. S.length_ -- IO Int
. S.filter (== 'i') -- Stream (Of Char) IO ()
. S.concat -- Stream (Of Char) IO ()
. S.mapped Q.head -- Stream (Of (Maybe Char)) IO ()
. Q.lines -- Stream (ByteStream IO) IO ()
$ Q.readFile "huge-file.txt" -- ByteStream IO ()
Critically, there are several functions which when combined with mapped can
bring us back into Of-land:
head :: Monad m => ByteStream m r -> m (Of (Maybe Char) r)
last :: Monad m => ByteStream m r -> m (Of (Maybe Char) r)
null :: Monad m => ByteStream m r -> m (Of Bool) r)
count :: Monad m => ByteStream m r -> m (Of Int) r)
toLazy :: Monad m => ByteStream m r -> m (Of ByteString r) -- Be careful with this.
toStrict :: Monad m => ByteStream m r -> m (Of ByteString r) -- Be even *more* careful with this.
When moving in the opposite direction API-wise, consider:
fromChunks :: Stream (Of ByteString) m r -> ByteStream m r