Question

我尝试使用servant库创建到Web API的客户端绑定。我希望能够发送任何JSON对象。

import           Control.Monad.Trans.Except (ExceptT, runExceptT)
import           Data.Proxy
import           Network.HTTP.Client (Manager)
import           Servant.API
import           Servant.Client
import           Data.Aeson

-- | This methods accepts any instance of 'ToJSON'
--   I would like to have only this method exported from the module
send :: ToJSON a => a -> Manager -> IO (Either ServantError Result)
send x manager = runExceptT $ send_ x manager baseUrl

type MyAPI a = "acceptAnyJson" 
    :> ReqBody '[JSON] a
    :> Post '[JSON] Result

api :: ToJSON a => Proxy (MyAPI a) 
api = Proxy 

send_ :: ToJSON a => a -> Manager -> BaseUrl -> ExceptT ServantError IO Result
send_ = client api

现在当我尝试编译它时，我有错误消息：

Couldn't match type ‘a0’ with ‘a’
      because type variable ‘a’ would escape its scope
    This (rigid, skolem) type variable is bound by
      the inferred type for ‘send_’:
      ...

如何将MyAPI，client和Proxy参数化以接受类型变量？

Answer 1

您需要将api的类型与您要发送的内容的类型联系起来：

{-# LANGUAGE ScopedTypeVariables #-}
send_ :: forall a. (FromJSON a) => a -> Manager -> BaseUrl -> ExceptT ServantError IO Result
send_ = client (api :: Proxy (MyAPI a))

或者为什么在这一点上甚至打扰api：

send_ = client (Proxy :: Proxy (MyAPI a))

具有FromJSON约束的类型变量的Servant客户端

1 个答案: