无法向请求追加标头

Question

我需要添加一个标题并发送一个请求:

 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")'

我如何解决它们?

:C8.pack没有，它会导致其他错误

Answer 1

正如错误提示的那样，编译器期望的类型是Data.CaseInsensitive.CI C8.ByteString，而您提供的类型是[Char](又名String)。

我怀疑您的问题是由缺少OverloadedStrings扩展引起的，该扩展允许从字符串字面量构造任意类型。要解决这个问题，在模块的开头添加以下行:

{-# LANGUAGE OverloadedStrings #-}