Data.ByteString与Data.ByteString.Char8的区别

Question

我看到Char8只支持ASCII字符，如果您使用其他Unicode字符，使用它将是危险的。

{-# LANGUAGE OverloadedStrings #-}

--import qualified Data.ByteString as B
import qualified Data.ByteString.Char8 as BC
import qualified Data.Text.IO as TIO
import qualified Data.Text.Encoding as E
import qualified Data.Text as T

name :: T.Text
name = "{ \"name\": \"哈时刻\" }"

nameB :: BC.ByteString
nameB = E.encodeUtf8 name

main :: IO ()
main = do
  BC.writeFile "test.json" nameB
  putStrLn "done"

产生的结果与

{-# LANGUAGE OverloadedStrings #-}

import qualified Data.ByteString as B
--import qualified Data.ByteString.Char8 as BC
import qualified Data.Text.IO as TIO
import qualified Data.Text.Encoding as E
import qualified Data.Text as T

name :: T.Text
name = "{ \"name\": \"哈时刻\" }"

nameB :: B.ByteString
nameB = E.encodeUtf8 name

main :: IO ()
main = do
  B.writeFile "test.json" nameB
  putStrLn "done"

那么使用Data.ByteString.Char8和Data.ByteString有什么区别呢？

Answer 1

如果您比较Data.ByteString和Data.ByteString.Char8，您会注意到在前者中引用Word8的一组函数在后者中引用了Char。

-- Data.ByteString
map :: (Word8 -> Word8) -> ByteString -> ByteString
cons :: Word8 -> ByteString -> ByteString
snoc :: ByteString -> Word8 -> ByteString
head :: ByteString -> Word8
uncons :: ByteString -> Maybe (Word8, ByteString) 
{- and so on... -}


-- Data.ByteString.Char8
map :: (Char -> Char) -> ByteString -> ByteString
cons :: Char -> ByteString -> ByteString
snoc :: ByteString -> Char -> ByteString
head :: ByteString -> Char
uncons :: ByteString -> Maybe (Char, ByteString) 
{- and so on... -}

对于这些函数，以及仅针对这些函数，Data.ByteString.Char8提供了不必不断地将Word8值转换为Char值和从Char值中转换出来的方便。writeFile 在两个模块中都做了完全相同的事情。

下面是查看Text、ByteString和ByteString.Char8中类似函数的不同行为的好方法

{-# LANGUAGE OverloadedStrings #-}

import Data.Text.Encoding

import qualified Data.Text as T
import qualified Data.ByteString as B
import qualified Data.ByteString.Char8 as BC

nameText :: T.Text
nameText = "哈时刻"

nameByteString :: B.ByteString
nameByteString = encodeUtf8 nameText

main :: IO ()
main = do
  print $ T.head nameText               -- '\21704'     actual first character
  print $ B.head nameByteString         -- 229          first byte
  print $ BC.head nameByteString        -- '\299'       first byte as character

  putStrLn [ T.head nameText ]          -- 哈           actual first character
  putStrLn [ BC.head nameByteString ]   -- å            first byte as character