haskell couchdb-conduit 示例

Question

{-# LANGUAGE DeriveDataTypeable, OverloadedStrings, ScopedTypeVariables #-}
module Db (
    couchTest
) where

import Control.Monad.IO.Class (liftIO)
import Data.ByteString (ByteString)
import Data.Generics (Data, Typeable)
import Database.CouchDB.Conduit
import Database.CouchDB.Conduit.Generic

conn :: CouchConnection
conn = def {couchLogin = "admin", couchPass = "admin"}

data D = D { f1 :: Int, f2 :: String } deriving (Show, Data, Typeable)

couchTest = runCouch conn $ do
    rev1 <- couchPut "mydb" "my-doc1" "" [] $ D 123 "str"
    rev2 <- couchPut "mydb" "my-doc1" rev1 [] $ D 1234 "another"
    (rev3, d1 :: D) <- couchGet "mydb" "my-doc1" []
    liftIO $ print d1
    couchPut' "mydb" "my-doc1" [] $ D 12345 "third"    -- notice - no rev
    rev3 <- couchRev "mydb" "my-doc1"
    couchDelete "mydb" "my-doc1" rev3

错误

No instance for (monad-control-0.3.1.3:Control.Monad.Trans.Control.MonadBaseControl
                   IO m0)
  arising from a use of `couchPut'
Possible fix:
  add an instance declaration for
  (monad-control-0.3.1.3:Control.Monad.Trans.Control.MonadBaseControl
     IO m0)
In the expression: couchPut "mydb" "my-doc1" "" []
In a stmt of a 'do' block:
  rev1 <- couchPut "mydb" "my-doc1" "" [] $ D 123 "str"
In the second argument of `($)', namely
  `do { rev1 <- couchPut "mydb" "my-doc1" "" [] $ D 123 "str";
        rev2 <- couchPut "mydb" "my-doc1" rev1 [] $ D 1234 "another";
        (rev3, d1 :: D) <- couchGet "mydb" "my-doc1" [];
        liftIO $ print d1;
        .... }'

1) 如何为 monad-control-0.3.1.3:Control.Monad.Trans.Control.MonadBaseControl IO m0 添加实例声明？

2)是否可以使用 couchdb-conduit 创建附件？

3)是否有一个直接使用couchdb与http包的示例，主要是为了看看它需要多少代码？

Answer 1

最好将您的问题分开，因为您问的是三个不同的问题。就 #1 而言，MonadBaseControl 不需要新实例。添加类型签名以消除歧义并立即编译:

{-# LANGUAGE DeriveDataTypeable, FlexibleContexts,
             OverloadedStrings, ScopedTypeVariables #-}
module Db (
    couchTest
) where

import Control.Monad.IO.Class (MonadIO, liftIO)
import Control.Monad.Trans.Resource (MonadThrow, MonadUnsafeIO)
import Control.Monad.Trans.Control (MonadBaseControl)
import Data.ByteString (ByteString)
import Data.Generics (Data, Typeable)
import Database.CouchDB.Conduit
import Database.CouchDB.Conduit.Generic

conn :: CouchConnection
conn = def {couchLogin = "admin", couchPass = "admin"}

data D = D { f1 :: Int, f2 :: String } deriving (Show, Data, Typeable)

couchTest
  :: (MonadIO m, MonadUnsafeIO m, MonadThrow m, MonadBaseControl IO m)
  => m ()
-- couchTest :: IO () -- restricting it to IO is also an option
couchTest = runCouch conn $ do
    rev1 <- couchPut "mydb" "my-doc1" "" [] $ D 123 "str"
    rev2 <- couchPut "mydb" "my-doc1" rev1 [] $ D 1234 "another"
    (rev3, d1 :: D) <- couchGet "mydb" "my-doc1" []
    liftIO $ print d1
    couchPut' "mydb" "my-doc1" [] $ D 12345 "third"    -- notice - no rev
    rev3 <- couchRev "mydb" "my-doc1"
    couchDelete "mydb" "my-doc1" rev3

我通过启用 NoMonomorphismRestriction 语言编译指示并在 GHCi 中运行以下命令，让 GHC 为我提供了约束列表。在许多情况下，您还可以保留 NMR 标志设置，而不是添加显式签名，但我更喜欢显式签名。

*Db> :info couchTest couchTest ::  (MonadIO m, Control.Monad.Trans.Resource.MonadUnsafeIO m,   Control.Monad.Trans.Resource.MonadThrow m,   MonadBaseControl IO m) =>  m ()    -- Defined at /tmp/cdb.hs:21:5