Question

我遇到的问题是readFile会返回IO String，但解析需要定期String（或者在ByteString下面的示例中）。我虽然只使用liftM，但下面的示例仍然失败。有什么问题？

import Data.Aeson
import Data.Attoparsec
import Data.ByteString (ByteString, pack)
import Data.Maybe
import Network.HTTP
import Network.URI
import qualified Data.ByteString.Char8 as C
import Control.Monad

main = do
    myres  <-  liftM parse json (C.readFile "dummy.json")
    print myres

ERROR：

Couldn't match expected type `t0 -> Parser a0'
            with actual type `Parser Value'
In the second argument of `liftM', namely `json'
In a stmt of a 'do' block:
  myres <- liftM parse json (C.readFile "dummy.json")
In the expression:
  do { myres <- liftM parse json (C.readFile "dummy.json");
       print myres }

Answer 1

您错过(parse json)周围的括号，因此liftM已应用于parse

将函数提升到IO monad中以解析从文件中读取的json字符串

1 个答案: