json - Haskell Servant 自定义 JSON 解析错误

Question

给定以下 Servant 服务器:

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeOperators #-}

module ServantSample (main) where

import Data.Aeson
import Data.Aeson.TH
import Network.Wai
import Network.Wai.Handler.Warp
import Servant

data Spec = Spec
  { schema :: Object
  } deriving (Eq, Show)
$(deriveJSON defaultOptions ''Spec)

type Api = ReqBody '[JSON] Spec :> Post '[JSON] NoContent

server :: Server Api
server = postDoc

postDoc :: Spec -> Handler NoContent
postDoc _ = return NoContent

api :: Proxy Api
api = Proxy

app :: Application
app = serve api server

main :: IO ()
main = run 8080 app

...以及上述服务器正在运行的实例的以下 curl:

curl localhost:8080 -H 'Content-Type: application/json' --data '{"schema": "I am not an object but I should be!"}'

我回来了:

Error in $.schema: expected HashMap ~Text v, encountered String

有没有办法拦截 Aeson 错误并将其替换为不会向客户端泄露实现细节的东西？据我所知，这一切都发生在 Servant 机器的幕后，我找不到任何关于如何连接它的文档。

例如，我很乐意返回如下内容:

Expected a JSON Object under the key "schema", but got the String "I am not an object but I should be!"

谢谢!

Answer 1

手动编写 FromJSON 实例至少可以解决一半的问题。

instance FromJSON Spec where
  parseJSON (Object o) = do
    schema <- o .: "schema"
    case schema of
      (Object s) -> pure $ Spec s
      (String s) -> fail $ "Expected a JSON Object under the key \"schema\", but got the String \"" ++ unpack s ++ "\"\n"
      _          -> fail $ "Expected a JSON Object under the key \"schema\", but got the other type"
  parseJSON wat = typeMismatch "Spec" wat

然后您的 curl 命令返回:

Error in $: Expected a JSON Object under the key "schema", but got the String "I am not an object but I should be!"

显然，您可以检查 Aeson 的不同 Value 类型构造函数，并将其分解到一个单独的函数中。

通过查看 Data.Aeson.Types.typeMismatch 的实现获得代码