将函数提升到IO monad中以解析从文件中读取的json字符串

Question

我有问题,readFile返回一个IO String但解析期望一个常规String(或在下面的例子中ByteString).我虽然只是使用,liftM但下面的例子仍然失败.问题是什么？

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

错误:

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

你错过了括号(parse json),所以liftM适用于parse