CouchDB.Conduit:将视图映射到数据

Question

{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE DeriveGeneric, ScopedTypeVariables #-}

import Data.Generics (Data, Typeable)
import Data.Conduit
import qualified Data.Conduit.List as CL
import Database.CouchDB.Conduit.Generic
import Database.CouchDB.Conduit
import Database.CouchDB.Conduit.View
import Data.ByteString.Char8 (ByteString, empty)
import Control.Monad.IO.Class (liftIO)
import Data.Aeson
import Data.Aeson.Types
import GHC.Generics

data Page = Page { id_ :: ByteString, url :: ByteString }
            deriving (Show, Data, Typeable, Generic)

instance FromJSON Page

getPages :: IO ()
getPages = runCouch (def { couchHost = "192.168.0.103" }) $ do
    couchView_ "reader" "reader" "pages" [] $ CL.mapM_ (liftIO . print)

这是可行的，并提供了以下内容：

    *Main> getPages 
    fromList [("key",String "802e343945c7f8da2d8a71fdb80025a7"),("id",String "802e343945c7f8da2d8a71fdb80025a7"),("value",String "http://yandex.ru")]`

但我实际上想要一个函数getPages :: IO [Page]，所以我尝试了一下：

getPages = runCouch (def { couchHost = "192.168.0.103" }) $ do
    couchView_ "reader" "reader" "pages" [] $ toType =$ CL.consume`

这给了我类型错误：

`Reader/Couch.hs:24:47:
    Couldn't match expected type `Object' with actual type `Value'
    Expected type: Conduit Object m1 b0
      Actual type: Conduit Value m0 a0
    In the first argument of `(=$)', namely `toType'
    In the second argument of `($)', namely `toType =$ CL.consume'
Failed, modules loaded: none.

这并不奇怪，因为couchView需要Sink Object m a作为参数。

问题是:如何实现getPages :：IO页面

Answer 1

我不熟悉conduit、aeson或couchDB，但这至少是类型检查：

getPages :: IO [Result Page]
getPages = runCouch (def { couchHost = "192.168.0.103" }) $ do
    couchView_ "reader" "reader" "pages" [] $ CL.map (fromJSON . Object) =$ CL.consume