haskell - 无法将标头附加到请求

Question

我需要添加一个标头并发送一个请求：

 import Network.HTTP.Conduit
 import qualified Data.ByteString.Char8 as C8

 --..........
 res <- withManager $ httpLbs $ createReq request
  return ()
    where
      createReq r = r {
        --...........
        requestHeaders = ("content-type", "application/json") : requestHeaders r
      }

我有两个错误：

   Couldn't match type `[Char]'
                  with `case-insensitive-1.0.0.1:Data.CaseInsensitive.CI
                          C8.ByteString'
    Expected type: HeaderName
      Actual type: [Char]
    In the expression: "content-type"
    In the first argument of `(:)', namely
      `("content-type", "application/json")'
    In the `requestHeaders' field of a record


    Couldn't match expected type `C8.ByteString'
                with actual type `[Char]'
    In the expression: "application/json"
    In the first argument of `(:)', namely
      `("content-type", "application/json")'

我该如何解决？

更新： C8.pack不会，它会导致其他错误。

Answer 1

正如错误所暗示的那样，编译器需要一个 type Data.CaseInsensitive.CI C8.ByteString，而您为它提供[Char]（aka String）。

我怀疑您的问题是由缺少OverloadedStrings扩展名引起的，这使得可以从字符串文字构造任意类型。要解决此问题，请在模块开头添加以下行：

{-# LANGUAGE OverloadedStrings #-}