快照，IO和酸态

Question

试图在 Snap 中使用酸态，我遇到了障碍。

这是我到目前为止得到的。

首先是我的酸态相关对象（这是一本带有 isbn 编号的虚拟书）：

{-# LANGUAGE DeriveDataTypeable         #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses      #-}
{-# LANGUAGE TemplateHaskell            #-}
{-# LANGUAGE TypeFamilies               #-}
{-# LANGUAGE OverloadedStrings          #-}
module Models where
import Prelude hiding ((.), id)
import Control.Category ((.))
import Control.Monad.Reader (asks)
import Data.ByteString (ByteString)
import Data.SafeCopy (base, deriveSafeCopy)
import qualified Data.Text as T
import Data.Typeable (Typeable)
import Data.Acid (Update, Query, makeAcidic)
import Control.Monad.Reader (ask)
import Control.Applicative ((<$>))
import Data.Data (Data)
data Book = Book { isbn :: String }
     deriving (Eq, Ord, Read, Data, Show, Typeable)
$(deriveSafeCopy 0 'base ''Book)
-- Retrieve the book's isbn
queryIsbn :: Query Book String
queryIsbn = isbn <$> ask
$(makeAcidic ''Book ['queryIsbn])

然后我实际尝试将其与 Snap 集成。如您所见，我在定义 __ doQuery__函数时遇到问题，该函数应返回一个字符串 isbn：

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE OverloadedStrings #-}
module Application where
import           Control.Monad.Trans.Class (lift)
import           Data.Text.Encoding (decodeUtf8)
import           Text.XmlHtml (Node(TextNode),Node (Element), 
                 getAttribute, setAttribute, nodeText)
import           Data.ByteString (ByteString)
import           Data.Maybe
import           Snap.Core
import           Snap.Snaplet
import           Snap.Snaplet.Heist (Heist, HasHeist(heistLens), heistInit,
                        addSplices, liftHeist, render)
import           Snap.Util.FileServe
import           Text.Templating.Heist (HeistT, Template, getParamNode)
import           Data.Lens.Template
import Models
import Data.Acid.Advanced (query')
import Data.Acid (AcidState, openLocalState, closeAcidState, IsAcidic, query)
import Data.Text (pack)
import Control.Monad.IO.Class (liftIO, MonadIO)
import Snap (snapletValue)
import Data.Lens.Common (getL, (^$), (^.), Lens) 
import Control.Monad.Reader (ask, asks)
import Control.Applicative ((<$>))
import Data.Typeable (typeOf)

import Prelude hiding ((.), id)
import Control.Category ((.), id)
------------------------------------------------------------------------------
type AppHandler = Handler App App
--------------
-- Acid
---------------
-- Used for holding data for the snapplet
data Acid st = Acid { _state ::  AcidState st }
-- Initializer function for the snapplet
seedBook =  Book "9213-23123-2311"
acidInit ::  SnapletInit b (Acid Book)
acidInit = makeSnaplet "storage" "Snaplet providing storage functionality" Nothing initializer
--The 'm' is the type variable of the MonadSnaplet type class. 'b' is the base state, and 'v' is the state of the current "view" snaplet (or simply, current state).
initializer :: Initializer b v (Acid Book)
initializer = do
      st <- liftIO (openLocalState seedBook)
      --onUnload (closeAcidState st)
      return $ Acid st
-----------------------
-- Snap Global State
--------------------
data App = App
    { _heist :: Snaplet (Heist App),
      _acid  :: Snaplet (Acid Book)
    }
makeLens ''App
----------------------------------------------------------------------------------
instance HasHeist App where
    heistLens = subSnaplet heist
-----------------------------------------------
-- | Initialize app
-----------------------------------------------
appInit :: SnapletInit App App
appInit = makeSnaplet "app" "Website" Nothing $ do
    h <- nestSnaplet "" heist $ heistInit "templates"
    a <- nestSnaplet "isbn"  acid (acidInit)
    addRoutes routes --see below
    addSplices [ ("menuEntry", liftHeist menuEntrySplice) ]
    return $ App h a

------------------------------------------------
-- | The application's routes.
------------------------------------------------
routes :: [(ByteString, Handler App App ())]
routes = [ ("/books",    handleBooks)
         , ("/contact",  render "contact")
         , ("/isbn",     liftIO doQuery >>= writeBS )
         , ("",          serveDirectory "static")
         ]
-- Is this Function signature possible? Or must it run inside Snap or other monad?
doQuery :: IO ByteString
doQuery = do    -- ???????????
        --somehow retrieve acid store from snaplet
        --run queryIsbn on it
        --return isbn string
        return "BLAH"

handleBooks :: Handler App App ()
handleBooks = render "books"

对我所缺少的任何帮助将不胜感激。如果有什么不清楚的地方，请告诉我，我会更新问题。

Answer 1

MathematicalOrchid 是正确的，最简单的答案是在 openLocalState 调用中使用 liftIO。

但从更广泛的角度来看，你在这里做的事情已经通过小鱼酸态包为你做了，所以我建议你使用它。 存储库还包括一个演示如何使用它的示例应用程序。

Answer 2

我不知道

你正在使用的包，但看起来问题只是openLocalState是一个IO操作，但你的类型签名要求它是一个Initializer操作。

修复它可能就像将一个电话塞进去liftIO一样简单。不过我不太确定...我不知道这些类型中的每一个来自哪个模块。