R – Many types of String (ByteString)

bytestringhaskell

I wish to compress my application's network traffic.

According to the (latest?) "Haskell Popularity Rankings", zlib seems to be a pretty popular solution. zlib's interface uses ByteStrings:

compress :: ByteString -> ByteString
decompress :: ByteString -> ByteString

I am using regular Strings, which are also the data types used by read, show, and Network.Socket:

sendTo :: Socket -> String -> SockAddr -> IO Int
recvFrom :: Socket -> Int -> IO (String, Int, SockAddr)

So to compress my strings, I need some way to convert a String to a ByteString and vice-versa.
With hoogle's help, I found:

Data.ByteString.Char8 pack :: String -> ByteString

Trying to use it:

Prelude Codec.Compression.Zlib Data.ByteString.Char8> compress (pack "boo")

<interactive>:1:10:
    Couldn't match expected type `Data.ByteString.Lazy.Internal.ByteString'
           against inferred type `ByteString'
    In the first argument of `compress', namely `(pack "boo")'
    In the expression: compress (pack "boo")
In the definition of `it': it = compress (pack "boo")

Fails, because (?) there are different types of ByteString ?

So basically:

Are there several types of ByteString? What types, and why?
What's "the" way to convert Strings to ByteStrings?

Btw, I found that it does work with Data.ByteString.Lazy.Char8's ByteString, but I'm still intrigued.

Best Answer

There are two kinds of bytestrings: strict (defined in Data.Bytestring.Internal) and lazy (defined in Data.Bytestring.Lazy.Internal). zlib uses lazy bytestrings, as you've discovered.

Related Solutions

R – Casting an mmapped ByteString to other types

I don't think it is safe to do this. UArr are Haskell heap allocated unpinned memory, the GC will move it. ByteStrings (and mmapped ones) are ForeignPtrs to pinned memory. They're different objects in the runtime system.

You will need to copy for this to be safe, if you're changing the underlying type from ForeignPtr to a Haskell value 'a'.

Haskell – the best way to convert String to ByteString

Here is my cheat sheet for Haskell String/Text/ByteString strict/lazy conversion assuming the desired encoding is UTF-8. The Data.Text.Encoding library has other encodings available.

Please make sure to not write (using OverloadedStrings):

lazyByteString :: BL.ByteString
lazyByteString = "lazyByteString ä ß" -- BAD!

This will get encoded in an unexpected way. Try

lazyByteString = BLU.fromString "lazyByteString ä ß" -- good

instead.

String literals of type 'Text' work fine with regard to encoding.

Cheat sheet:

import Data.ByteString.Lazy as BL
import Data.ByteString as BS
import Data.Text as TS
import Data.Text.Lazy as TL
import Data.ByteString.Lazy.UTF8 as BLU -- from utf8-string
import Data.ByteString.UTF8 as BSU      -- from utf8-string
import Data.Text.Encoding as TSE
import Data.Text.Lazy.Encoding as TLE

-- String <-> ByteString

BLU.toString   :: BL.ByteString -> String
BLU.fromString :: String -> BL.ByteString
BSU.toString   :: BS.ByteString -> String
BSU.fromString :: String -> BS.ByteString

-- String <-> Text

TL.unpack :: TL.Text -> String
TL.pack   :: String -> TL.Text
TS.unpack :: TS.Text -> String
TS.pack   :: String -> TS.Text

-- ByteString <-> Text

TLE.encodeUtf8 :: TL.Text -> BL.ByteString
TLE.decodeUtf8 :: BL.ByteString -> TL.Text
TSE.encodeUtf8 :: TS.Text -> BS.ByteString
TSE.decodeUtf8 :: BS.ByteString -> TS.Text

-- Lazy <-> Strict

BL.fromStrict :: BS.ByteString -> BL.ByteString
BL.toStrict   :: BL.ByteString -> BS.ByteString
TL.fromStrict :: TS.Text -> TL.Text
TL.toStrict   :: TL.Text -> TS.Text

Please +1 Peaker's answer, because he correctly deals with encoding.

Related Topic

Haskell – Couldn’t match expected type `Data.ByteString.Internal.ByteString’ with actual type `ByteString’