如何提升到Servant Server类型？答案

【问题标题】：How to lift to Servant Server type?如何提升到Servant Server类型？
【发布时间】：2021-05-28 00:31:25
【问题描述】：

以下代码有问题：

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

module API where

import           Data.Text
import           Servant.API
import           Servant
import           Repository 
import           FileOperation
import           Util
import           Config
import qualified Data.Map                   as M
import           Control.Monad.State.Strict (liftIO)
import           Network.Wai.Handler.Warp

type RepositoryAPI = "repository" :> "all" :> Get '[JSON] [Repository] 
                :<|> "all" :> "repository" :> Get '[JSON] [Repository]

server :: Server RepositoryAPI
server = do
  repositoriesMap <- liftIO $ loadFromFile repositoryMapFile
  let repositories = M.elems repositoriesMap in do
    return repositories 

repositoryAPI :: Proxy RepositoryAPI 
repositoryAPI = Proxy

app :: Application
app = serve repositoryAPI server

main :: IO ()
main = run 8081 app

我得到的错误是：

API.hs:22:3: error:
    • Couldn't match type ‘[a0]’ with ‘Handler [Repository]’
      Expected type: Server RepositoryAPI
        Actual type: Handler [Repository] :<|> [a0]
    • In a stmt of a 'do' block:
        repositoriesMap <- liftIO $ loadFromFile repositoryMapFile
      In the expression:
        do repositoriesMap <- liftIO $ loadFromFile repositoryMapFile
           let repositories = ... in do return repositories
      In an equation for ‘server’:
          server
            = do repositoriesMap <- liftIO $ loadFromFile repositoryMapFile
                 let ... in do ...
   |
22 |   repositoriesMap <- liftIO $ loadFromFile repositoryMapFile
   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

问题是如何正确升降，让升降返回Servant Server类型？

【问题讨论】：

标签： haskell servant

【解决方案1】：

这个问题与举重无关。真正的问题是您只指定了一个Handler，但您的服务器需要两个Handlers，一个用于"repository" :> "all" :> Get '[JSON] [Repository]，一个用于"all" :> "repository" :> Get '[JSON] [Repository]。这是你的第二个骨架（用这个替换你现有的server）：

getRepositoryAll :: Handler [Repository]
getRepositoryAll = do
  repositoriesMap <- liftIO $ loadFromFile repositoryMapFile
  let repositories = M.elems repositoriesMap in do
    return repositories 

getAllRepository :: Handler [Repository]
getAllRepository = undefined -- fill this in

server :: Server RepositoryAPI
server = getRepositoryAll :<|> getAllRepository

【讨论】：