Haskell Riak驱动程序:进行简单的"放置"操作

Question

我试图用Haskell驱动程序向Riak介绍自己,并且我遇到了一个简单的put操作.我对put功能的签名感到困惑.那里的任何地方都没有一个例子.

所以有这个签名:

put :: (FromJSON c, ToJSON c, Resolvable c) => Connection -> Bucket 
       -> Key -> Maybe VClock -> c -> W -> DW -> IO (c, VClock)

我有一些问题.

什么是Maybe VClock？我是否必须以某种方式生成它或者只是在那里指定Nothing就足够了？为什么我在返回的元组中得到这个VClock？

我是否必须为每个简单的值编写FromJSON和ToJSON实例,即使它是一个简单的字符串值？就像我想用"名称"键来设置值"Stitch"一样,我该怎么做？

什么是Resolvable实例？如何使Text或String值可解析？我知道我必须定义resolve函数,但我不太了解它的含义以及如何做到这一点.

Answer 1

我希望以下简单的 put 和 get 操作代码可以帮助您：

{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE UndecidableInstances #-}

module Main where
import Data.Aeson
import qualified Network.Riak as Riak 
import Network.Riak.Types
import Data.ByteString.Char8 hiding (putStrLn)
import Data.ByteString.Lazy as L hiding (pack,putStrLn)
import GHC.Generics hiding (R)

-- convert String to lazy ByteString
toBS :: String -> L.ByteString
toBS str = L.fromChunks (pack (str):[])

putBucket :: Bucket
putBucket = toBS "Bucket"

putKey :: Key
putKey = toBS "key"

r :: R
r = Default

w :: W
w = Default

dw :: DW
dw = Default

rw :: RW
rw = Default 

-- declare a data and create its instances
data Coord = Coord { x :: Double, y :: Double } deriving (Generic, Show)

instance FromJSON Coord

instance ToJSON Coord

instance (Show Coord) => Riak.Resolvable Coord where
    resolve a b = a

value :: Coord
value = Coord {
  x = 2.2,
  y = 3.3
}


main :: IO ()
main = do
    -- establish a connection
    let client = Riak.defaultClient
    con <- Riak.connect client
    -- simple put operation
    put_ret <- (Riak.put con putBucket putKey Nothing value w dw)
    putStrLn (show put_ret)
    buckets <- Riak.listBuckets con
    print buckets
    -- simple get operation
    get_ret <- (Riak.get con putBucket putKey r) :: IO ( Maybe (Coord, VClock))
    putStrLn (show get_ret)
    -- delete a value
    Riak.delete con putBucket putKey rw
    -- if try to get that value we will find "Nothing"
    get_ret <- (Riak.get con putBucket putKey r) :: IO ( Maybe (Coord, VClock))
    putStrLn (show get_ret)
    -- print the final bucket list
    buckets <- Riak.listBuckets con
    print buckets