Haskell - 无法将类型“PersistEntityBackend record0"与“SqlBackend"匹配 [英] Haskell - Couldn't match type ‘PersistEntityBackend record0’ with ‘SqlBackend’

问题描述

我正在尝试通过 Yesod 中的 id 获取记录.我的代码是:

I am trying to get a record by id in Yesod. My code is:

getEditActorR :: Handler Html
getEditActorR = do
    actorId <- runInputGet $ ireq intField "id"
    actor <- runDB $ get $ Key $ PersistInt64 (fromIntegral actorId)
    defaultLayout $ do
        $(widgetFile "actor-edit")

我得到的错误是:

• Couldn't match type ‘PersistEntityBackend record0’
                 with ‘SqlBackend’
    arising from a use of ‘get’
  The type variable ‘record0’ is ambiguous
• In the second argument of ‘($)’, namely
    ‘get $ Key $ PersistInt64 (fromIntegral actorId)’
  In a stmt of a 'do' block:
    actor <- runDB $ get $ Key $ PersistInt64 (fromIntegral actorId)
  In the expression:
    do { actorId <- runInputGet $ ireq intField "id";
         actor <- runDB $ get $ Key $ PersistInt64 (fromIntegral actorId);
         defaultLayout $ do { (do { ... }) } }

我该如何解决?

推荐答案

我做的第一件事是运行 stack ghci.

First thing I did was to run stack ghci.

然后我运行 :info Actor，其中 Actor 是我的 PersistEntity 的名称.

Then I run :info Actor, where Actor is the name of my PersistEntity.

除此之外，还有:

newtype instance Key Actor = ActorKey {unActorKey :: Int}

所以我写道:

maybeActor <- runDB $ get $ ActorKey actorId
case maybeActor of
    Just actor -> defaultLayout $ do
        $(widgetFile "actor-edit")
    Nothing -> defaultLayout $ do
        $(widgetFile "actor-new")

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