为什么此代码与Yesod.Persist的get404一起使用，但不是getBy404？ [英] Why does this code work with Yesod.Persist's get404 but not getBy404?

问题描述

  share [mkPersist sqlSettings，mkMigratemigrateAll ] [坚持| 
 Dog 
名称Text 
品种Text 
 UniqueDog名称
 |] 
  

我有以下路线：

  mkYesodDogApp[parseRoutes | 
 / RootR GET 
 / dog / name /＃Text DogNameR GET 
 / dog / id /＃DogId DogIdR GET 
 |] 
  

我试着创建一个页面，返回给定名称的狗的品种。我可以通过getDogIdR路由在URL中使用Id来做到这一点：

  getDogIdR :: DogId  - > Handler RepHtml 
 getDogIdR dogId = do 
 dog<  -  runDB $ get404 dogId 
 defaultLayout $ do 
 [whamlet | 
< p> 
< b>＃{dogName dog} 
< p> 
＃{dogBreed dog} 
 |] 
  

但是，如果我尝试创建与getDogNameR路由相同的页面，

  getDogNameR :: Text  - > Handler RepHtml 
 getDogNameR maybeDogName = do 
 dog<  -  runDB $ getBy404（UniqueDog maybeDogName）
 defaultLayout $ do 
 [whamlet | 
< p> 
< b>＃{dogName dog} 
< p> 
＃{dogBreed dog} 
 |] 
  

出现以下错误：

  dog.hs：73：20：
无法与`DogGeneric'$ b匹配`Entity'类型$ b在`getBy404'调用的返回类型中
在`（$）'的第二个参数中，即
`getBy404（UniqueDog maybeDogName）'
在'做'块：
狗<  -  runDB $ getBy404（UniqueDog maybeDogName）
 
 dog.hs：73：20：
匹配类型时不兼容：
 backend0 ::（*  - > *） - > *  - > * 
 DogGeneric SqlPersist :: * 
在`getBy404'调用的返回类型中
在`（$）'的第二个参数中，即
`getBy404（UniqueDog maybeDogName ）'
在'do'块的标记中：
 dog < -  runDB $ getBy404（UniqueDog maybeDogName）
  

为什么会失败，我该如何解决它？

get404和getBy404函数似乎返回不同的类型。如果我按如下方式更改getDogNameR路由，它会运行得很好：

  getDogNameR :: Text  - > Handler RepHtml 
 getDogNameR maybeDogName = do 
 dog<  -  runDB $ getBy404（UniqueDog maybeDogName）
 defaultLayout $ do 
 [whamlet | 
< p> 
< b>找到了狗！ 
 | 
  

解决方案

<看起来get404和getBy404函数返回不同的类型。

正确。

  get404 ::（PersistStore（tm），PersistEntity val，Monad（tm），m〜GHandler子主，
 MonadTrans t，PersistMonadBackend（tm）〜PersistEntityBackend val ）
 =>键val  - > tm val 
 $ b getBy404 ::（PersistUnique（tm），PersistEntity val，m〜GHandler子主，
 Monad（tm），MonadTrans t，PersistEntityBackend val〜PersistMonadBackend（tm））
 =>独特的val  - > tm（实体val）
  

所以 getBy404 返回你的狗裹在 实体 ，您拥有 get404 中的纯狗，如果您更换了第二个代码，

 < b>＃{dogName dog} 
  

with

 < b>＃{dogName（entityVal dog）} 
  

以及类似的 dogBreed ，或者更多方便的话，如果你解析返回的实体绑定

 实体{entityVal = dog}<  -  runDB $ getBy404 ... 
  

Let's say I have a table of dog names and breeds as follows:

share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persist|
Dog
    name Text
    breed Text
    UniqueDog name
|]

I have the following routes:

mkYesod "DogApp" [parseRoutes|
/ RootR GET
/dog/name/#Text DogNameR GET
/dog/id/#DogId DogIdR GET
|]

And I'm trying to create a page that returns the breed of the dog given it's name. I can do this with the Id in the URL via the getDogIdR route just fine:

getDogIdR :: DogId -> Handler RepHtml
getDogIdR dogId = do 
    dog <- runDB $ get404 dogId
    defaultLayout $ do
        [whamlet|
<p>
    <b>#{dogName dog}
<p>
    #{dogBreed dog}
|]

However, if I try creating the same page with the getDogNameR route,

getDogNameR :: Text -> Handler RepHtml
getDogNameR maybeDogName = do 
    dog <- runDB $ getBy404 (UniqueDog maybeDogName)
    defaultLayout $ do
        [whamlet|
<p>
    <b>#{dogName dog}
<p>
    #{dogBreed dog}
|]

I get the following error:

dog.hs:73:20:
    Couldn't match type `Entity' with `DogGeneric'
    In the return type of a call of `getBy404'
    In the second argument of `($)', namely
      `getBy404 (UniqueDog maybeDogName)'
    In a stmt of a 'do' block:
      dog <- runDB $ getBy404 (UniqueDog maybeDogName)

dog.hs:73:20:
    Kind incompatibility when matching types:
      backend0 :: (* -> *) -> * -> *
      DogGeneric SqlPersist :: *
    In the return type of a call of `getBy404'
    In the second argument of `($)', namely
      `getBy404 (UniqueDog maybeDogName)'
    In a stmt of a 'do' block:
      dog <- runDB $ getBy404 (UniqueDog maybeDogName)

Why does this fail, and how can I fix it?

It appears that the get404 and getBy404 functions return different types. If I change the getDogNameR route as follows, it runs just fine:

getDogNameR :: Text -> Handler RepHtml
getDogNameR maybeDogName = do 
    dog <- runDB $ getBy404 (UniqueDog maybeDogName)
    defaultLayout $ do
        [whamlet|
<p>
    <b>Dog found!
|]

解决方案

It appears that the get404 and getBy404 functions return different types.

Right.

get404 :: (PersistStore (t m), PersistEntity val, Monad (t m), m ~ GHandler sub master,
           MonadTrans t, PersistMonadBackend (t m) ~ PersistEntityBackend val)
           => Key val -> t m val

getBy404 :: (PersistUnique (t m), PersistEntity val, m ~ GHandler sub master,
             Monad (t m), MonadTrans t, PersistEntityBackend val ~ PersistMonadBackend (t m))
             => Unique val -> t m (Entity val)

So getBy404 returns your dog wrapped in an Entity where you have the pure dog from get404, and your second codeshould work if you replace

<b>#{dogName dog}

with

<b>#{dogName (entityVal dog)}

and similarly for the dogBreed, or, more conveniently, if you deconstruct the returned Entity on binding

Entity{entityVal = dog} <- runDB $ getBy404 ...

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