Haskell Aeson JSON库ByteString问题

Question

我无法找到一个函数或变通方法来转换一个字符串到Data.ByteString.Lazy.Internal.ByteString

Aeson Json库中的一个功能是decode并具有以下描述:

decode :: FromJSON a => bytestring-0.10.0.2:Data.ByteString.Lazy.Internal.ByteString -> Maybe a

我已尝试在Data.ByteString.Lazy.Char8中使用pack函数,但返回不同的ByteString.任何人都知道如何解决这个问题？

以下是我正在研究的例子:

import Data.Aeson
import Data.Text
import Control.Applicative
import Control.Monad (mzero)
import qualified Data.ByteString.Lazy.Internal as BLI
import qualified Data.ByteString.Lazy.Char8 as BSL

data Person = Person 
    { name :: Text
    , age :: Int 
    } deriving Show

instance FromJSON Person where 
    parseJSON (Object v) = Person <$>
                   v .: (pack "name") <*>
                   v .: (pack "age")
    parseJSON _          = mzero

我尝试使用decode (BSL.pack "{\"name\":\"Joe\",\"age\":12}") :: Maybe Person 并收到以下错误消息:

Couldn't match expected type `bytestring-0.10.0.2:Data.ByteString.Lazy.Internal.ByteString'
                with actual type `BSL.ByteString'
    In the return type of a call of `BSL.pack'
    In the first argument of `decode', namely
      `(BSL.pack "{\"name\":\"Joe\",\"age\":12}")'
    In the expression:
        decode (BSL.pack "{\"name\":\"Joe\",\"age\":12}") :: Maybe Person

救命!

Answer 1

您需要使用c2w将Char转换为Word8(在Data.ByteString.Internal中)

Data.ByteString.Lazy.pack $ map c2w "abcd"

我写了包的完全限定名称也保证使用正确的名称,但你可以在导入部分清理它.我跑的时候

> :t Data.ByteString.Lazy.pack $ map c2w "abcd"

我得到":: Data.ByteString.Lazy.Internal.ByteString"

请记住,Data.ByteString.Lazy表示数字值的字符串(你甚至不能在字符串上运行它的包,你需要提供一个数字数组"pack [1,2,3,4]"),所以你可能实际上想要使用char等效的Data.ByteString.Lazy.Char8.