关于Haskell：如何在scotty和selda中使用monad堆栈？

How to use a monad stack with scotty and selda?

我一直在尝试使运行scotty的Web服务器可以使用selda与我的数据库进行通信。我认为使用monad变压器堆栈将是完成类似任务的方式。我一直在尝试解决问题，但遇到了一些死胡同，这些类型似乎根本无法工作。

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{-# LANGUAGE DeriveGeneric, OverloadedStrings, OverloadedLabels #-}

module Server where

import Web.Scotty
import Data.Monoid (mconcat)
import Data.Aeson (ToJSON)
import GHC.Generics
import Web.Scotty
import Database.Selda
import Database.Selda.SQLite
import Control.Monad.Trans.Class

import Models

type App = SeldaT SQLite ScottyM

-- withPersist:: (MonadIO m, MonadMask m) => SeldaT SQLite m a -> m a
server = scotty 4200 (withPersist router)

router :: App ()
router = do
lift $ get"/book/:id" searchBook

searchBook:: ActionM ()
searchBook = do
books <- query selectBookQuery
json books
where
selectBookQuery = do
book <- select goodreadsBooks
restrict (book ! #goodreadsId .=="20")
return book

我试图根据此处的答案来宽松地建立它，但是我想package路由器而不是单独的路线。我不希望我打开的连接数与我拥有的路由数成正比，如果可以避免的话，我也不希望每个路由都必须有一个withPersist调用。铅>

所以我有一个App类型的SeldaT Sqlite ScottyM，然后使用withPersist(它是== withSQLite"mydb.db")，我会将那个SeldaT Sqlite ScottyM变成一个ScottyM 。不过还是有很多问题，这是我对它们的理解：

SeldaT m a受(MonadIO m, MonadMask m)约束，并且ScottyM没有MonadIO的实例
Scotty.get返回一个ScottyM ()，我觉得这是我要使用lift将其变成SeldaT Sqlite ScottyM的地方，但是我遇到了一个错误，可能与ScottyM不是一个实例有关从上方MonadIO的。
由于searchBook仍然是ActionM，因此我无法在其中运行查询。不确定如何获取get来接受我的转换器堆栈而不是ActionM

以下是错误：

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/home/marcus/Documents/projects/nowwhatdoiread/nwdir-server/app/Server.hs:19:23: error:
a€￠ No instance for (MonadIO ScottyM)
arising from a use of a€?withPersista€?
a€￠ In the second argument of a€?scottya€?, namely a€?(withPersist router)a€?
In the expression: scotty 4200 (withPersist router)
In an equation for a€?servera€?:
server = scotty 4200 (withPersist router)
|
19 | server = scotty 4200 (withPersist router)
| ^^^^^^^^^^^^^^^^^^

/home/marcus/Documents/projects/nowwhatdoiread/nwdir-server/app/Server.hs:23:3: error:
a€￠ No instance for (MonadTrans (SeldaT SQLite))
arising from a use of a€?lifta€?
a€￠ In a stmt of a 'do' block: lift $ get"/book/:id" searchBook
In the expression: do lift $ get"/book/:id" searchBook
In an equation for a€?routera€?:
router = do lift $ get"/book/:id" searchBook
|
23 | lift $ get"/book/:id" searchBook
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

/home/marcus/Documents/projects/nowwhatdoiread/nwdir-server/app/Server.hs:27:12: error:
a€￠ No instance for (MonadSelda
(Web.Scotty.Internal.Types.ActionT
Data.Text.Internal.Lazy.Text IO))
arising from a use of a€?querya€?
a€￠ In a stmt of a 'do' block: books <- query selectBookQuery
In the expression:
do books <- query selectBookQuery
json books
In an equation for a€?searchBooka€?:
searchBook
= do books <- query selectBookQuery
json books
where
selectBookQuery
= do book <- select goodreadsBooks
....
|
27 | books <- query selectBookQuery
| ^^^^^^^^^^^^^^^^^^^^^

更新：在看了更多类似的问题之后，我可能需要使用ScottyT。但是不确定如何将SeldaT嵌套在ScottyT转换器中。

环顾了很多其他答案并了解了有关monad变压器的更多信息后，我才知道了这一点，我想到的解决方案是：

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{-# LANGUAGE DeriveGeneric, OverloadedStrings, OverloadedLabels #-}

module Server where

import Web.Scotty.Trans
import Database.Selda
import Database.Selda.SQLite
import Control.Monad.Trans.Class
import Control.Monad.Identity
import qualified Data.Text.Lazy as TL

import Models

server :: IO ()
server = scottyT 4200 withPersist router

router :: ScottyT TL.Text (SeldaT SQLite IO) ()
router = do
get"/book/:id" searchBook

searchBook:: ActionT TL.Text (SeldaT SQLite IO) ()
searchBook = do
books <- lift $ query selectBookQuery
json books
where
selectBookQuery = do
book <- select goodreadsBooks
restrict (book ! #goodreadsId .=="20")
return book

有几个键：

Scotty有自己的monad变压器ScottyT，它必须是最外面的变压器
需要使用Scott.Trans来使变压器堆栈与scotty一起使用，因为类型更为通用
需要使用ActionT转换器进行操作，因此他们也可以访问transformer堆栈