从 yesod 处理程序返回 JSON [英] Return JSON from yesod handler

问题描述

我正在尝试从 Yesod 的处理程序中编写一个最简单的 JSON 响应，但有一些非常愚蠢的错误(显然).我的处理程序代码是这样的:

I'm trying to write a simplest JSON response from Yesod's handler, but have some really stupid error (apparently). My handler code is this:

-- HelloYesod/Handler/Echo.hs
module Handler.Echo where

import           Data.Aeson      (object, (.=))
import qualified Data.Aeson      as J
import           Data.Text       (pack)
import           Import
import           Yesod.Core.Json (returnJson)

getEchoR :: String -> Handler RepJson
getEchoR theText = do
  let json = object $ ["data" .= "val"]
  return json

错误是这样的:

Handler/Echo.hs:12:10:
    Couldn't match expected type `RepJson' with actual type `Value'
    In the first argument of `return', namely `json'
    In a stmt of a 'do' block: return json
    In the expression:
      do { let json = object $ ...;
           return json }
Build failure, pausing...

推荐答案

我也遇到了这个问题:你只需要改变你的类型签名，它就会起作用:

I got caught by this one too: you just have to change your type signature and it will work:

getEchoR :: String -> Handler Value

我的理解是整个 Rep 系统在 Yesod 1.2 中已弃用，因此处理程序现在返回 Html 和 Value 而不是 RepHtml 和 RepJson.

My understanding is that the whole Rep system is deprecated in Yesod 1.2, so Handler's now return Html and Value rather than RepHtml and RepJson.

希望这会有所帮助！

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