Opaleye newtype

Question

我的PostgreSQL数据库中的表的数据类型中的一个字段是一个名为UUID的{​​{1}}换行的新类型。

ItemId

然而，当我编译我的项目时，GHC抛出：

import Data.Profunctor.Product.TH (makeAdaptorAndInstance)
import Data.DateTime (DateTime)
import Data.UUID
import GHC.Generics
import qualified Opaleye as O
import Data.Text (pack, Text)

newtype ItemId = ItemId UUID
  deriving (Show, Eq, Generic)

toItemId :: UUID -> ItemId
toItemId = ItemId

fromItemId :: ItemId -> UUID
fromItemId (ItemId x) = x

data Item' id name desc num most
  = Item {
    _itemId          :: id,
    _itemName        :: name,
    _itemDesc        :: desc,
    _numTimesOrdered :: num,
    _mostRecentOrder :: most
 }

type ItemRead = Item' ItemId Text Text Int DateTime
type ItemWrite = Item' (Maybe ItemId) Text Text (Maybe Int) (Maybe DateTime)
type ItemColRead = Item' (O.Column O.PGUuid)
                         (O.Column O.PGText)
                         (O.Column O.PGText)
                         (O.Column O.PGInt4)
                         (O.Column O.PGTimestamptz)
type ItemColWrite = Item' (Maybe (O.Column O.PGUuid))
                          (O.Column O.PGText)
                          (O.Column O.PGText)
                          (Maybe (O.Column O.PGInt4))
                          (Maybe (O.Column O.PGTimestamptz))

$(makeAdaptorAndInstance "pItem" ''Item')

itemTable :: O.Table ItemColWrite ItemColRead
itemTable = O.Table "items" (pItem Item { _itemId          = O.optional "id"
                                        , _itemName        = O.required "name"
                                        , _itemDesc        = O.required "desc"
                                        , _numTimesOrdered = O.optional "numTimesOrdered"
                                        , _mostRecentOrder = O.optional "mostRecentOrder"
                                        })

itemToPG :: ItemWrite -> ItemColWrite
itemToPG = pItem Item { _itemId          = const Nothing
                      , _itemName        = O.pgStrictText
                      , _itemDesc        = O.pgStrictText
                      , _numTimesOrdered = const Nothing
                      , _mostRecentOrder = const Nothing
                      }

其中/home/gigavinyl/Projects/ordermage/src/Api/Item.hs:34:3: error: • No instance for (O.QueryRunnerColumnDefault O.PGUuid ItemId) arising from a use of ‘O.runInsertManyReturning’ • In the second argument of ‘(<$>)’, namely ‘O.runInsertManyReturning con itemTable [itemToPG item] _itemId’ In the second argument of ‘($)’, namely ‘listToMaybe <$> O.runInsertManyReturning con itemTable [itemToPG item] _itemId’ In the expression: liftIO $ listToMaybe <$> O.runInsertManyReturning con itemTable [itemToPG item] _itemId 是：

src/Api/Item.hs

我对Haskell仍然相当新，但问题似乎是Opaleye不知道如何将{-# LANGUAGE DataKinds #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE OverloadedStrings #-} module Api.Item where import Control.Monad.IO.Class (liftIO) import Data.Maybe (listToMaybe) import Database.PostgreSQL.Simple (Connection) import Models.Item import Queries.Item import Servant import qualified Opaleye as O type ItemApi = Get '[JSON] [ItemRead] :<|> Capture "itemId" ItemId :> Get '[JSON] (Maybe ItemRead) :<|> ReqBody '[JSON] ItemWrite :> Post '[JSON] (Maybe ItemId) itemServer :: Connection -> Server ItemApi itemServer con = getItems con :<|> getItemById con :<|> postItem con getItems :: Connection -> Handler [ItemRead] getItems con = liftIO $ O.runQuery con itemsQuery getItemById :: Connection -> ItemId -> Handler (Maybe ItemRead) getItemById con itemID = liftIO $ listToMaybe <$> O.runQuery con (itemByIdQuery itemID) postItem :: Connection -> ItemWrite -> Handler (Maybe ItemId) postItem con item = liftIO $ listToMaybe <$> O.runInsertManyReturning con itemTable [itemToPG item] _itemId 转换为ItemId，但我知道它可以转换{{ 1}}到PGUuid。我如何编写实例以允许Opaleye进行此转换？

Answer 1

  

问题似乎是Opaleye不知道如何将ItemId转换为PGUuid，但我知道它可以将UUID转换为PGUuid

反过来说。它正在尝试将Column PGUuid转换为ItemId，并且只知道如何将其转换为UUID。一种方法是自己添加实例：

instance O.QueryRunnerColumnDefault O.PGUuid ItemId where
  queryRunnerColumnDefault =
         O.queryRunnerColumn id ItemId queryRunnerColumnDefault

另一种方法是使ItemId多态：

newtype ItemId' a = ItemId a
$(makeAdaptorAndInstance "pItemId" ''ItemId')

然后您可以在Haskell端和Opaleye端使用它，而无需编写额外的实例。