Yesod持久性类型错误

问题描述

我尝试在Yesod应用程序中持久化。我的模型文件包含

 工作
 issuer MemberId 
 addDate UTCTime 
 lastDate UTCTime 
 title文本
描述文本
派生显示读取
  

和我的Handler：

  getProfileR :: Handler RepHtml 
 getProfileR = do 
 jobs<  -  runDB $ selectList [] [Desc JobAddDate] 
 defaultLayout $ do 
 setTitletitle
 $（widgetFileprofile）
  

在profile.hamlet中，我循环对象

  $ forall作业发布者addDate lastDate标题说明< -  jobs 
< p>＃{issuer} 
  

但是，我得到以下错误

  Handler / Profile.hs：36：18：
无法使用JobGeneric匹配类型实体 
在`selectList'调用的返回类型中
在`（$）'的第二个参数中，即
`selectList [] [Desc JobAddDate]'
在'do'块的结构：
 jobs<  -  runDB $ selectList [] [Desc JobAddDate] 
 
 Handler / Profile.hs：36：18：
种类不兼容匹配类型：
 t0 ::（*  - > *） - > *  - > * 
 JobGeneric Database.Persist.GenericSql.Raw.SqlPersist :: * 
在`selectList'调用的返回类型中
在`（$）'的第二个参数中，即
`selectList [] [Desc JobAddDate]'
在'do'块的标记中：
作业<  -  runDB $ selectList [] [Desc JobAddDate] 
构建失败，暂停... 
  

第36行是runDB行。

作为Haskell的新手，我无法弄清楚什么是错的。我遵循Yesod Book。他们不幸的是避免了Scaffolded站点，所以我不能完全模仿他们的代码。

selectList 不会返回 [作业] ，它实际上是 [实体作业] ，它包含 Job 及其键 *

方法来重构这个来处理它，其中一个是：

  $ forall实体jobId工作<  - 工作
< ; p>＃{jobIssuer job} 
  

在您的模板中。

另外，您可以在任意点使用 map entityVal 来打开 [Entity Job] - > [作业] 如果你喜欢使用它。

实体 >和 Key 类型实际上有点复杂，但我觉得用这种方式考虑它们会更容易。如果您有兴趣，请阅读文档。

I am trying out persistent in a Yesod application. My model file contains

Job
 issuer MemberId
 addDate UTCTime
 lastDate UTCTime
 title Text
 description Text
 deriving Show Read

And my Handler:

getProfileR :: Handler RepHtml
getProfileR = do
 jobs <- runDB $ selectList [] [Desc JobAddDate]
 defaultLayout $ do
  setTitle "title"
  $(widgetFile "profile")

In profile.hamlet I loop trough the objects

$forall Job issuer addDate lastDate title description <- jobs
 <p>#{issuer}

However, I get the following error

Handler/Profile.hs:36:18:
    Couldn't match type `Entity' with `JobGeneric'
    In the return type of a call of `selectList'
    In the second argument of `($)', namely
      `selectList [] [Desc JobAddDate]'
    In a stmt of a 'do' block:
      jobs <- runDB $ selectList [] [Desc JobAddDate]

Handler/Profile.hs:36:18:
    Kind incompatibility when matching types:
      t0 :: (* -> *) -> * -> *
      JobGeneric Database.Persist.GenericSql.Raw.SqlPersist :: *
    In the return type of a call of `selectList'
    In the second argument of `($)', namely
      `selectList [] [Desc JobAddDate]'
    In a stmt of a 'do' block:
      jobs <- runDB $ selectList [] [Desc JobAddDate]
Build failure, pausing...

Where line 36 is the runDB line.

Being new to Haskell, I can't figure out whats wrong. I am following the Yesod Book. They are unfortunately avoiding the Scaffolded Site so I can't completely mimic their code.

解决方案

selectList does not return [Job], it's actually [Entity Job] which contains both the Job and its Key*

There are a number of ways to refactor this to handle it, one would be:

$forall Entity jobId job <- jobs
    <p>#{jobIssuer job}

In your template.

Alternatively, you can use map entityVal at any point to turn [Entity Job] -> [Job] if you'd prefer to work with that.

*The Entity and Key types are actually a bit more complex, but I find it's easier to think about them this way. Please read the docs if you're interested.

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