为什么此代码与Yesod.Persist的get404一起使用，但不是getBy404？

本文介绍了为什么此代码与Yesod.Persist的get404一起使用，但不是getBy404？的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！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 | ＃{dogName dog} ＃{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 | ＃{dogName dog} ＃{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 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 | 找到了狗！ | 解决方案正确。 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， ＃{dogName dog} with ＃{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 RepHtmlgetDogIdR dogId = do dog <- runDB $ get404 dogId defaultLayout $ do [whamlet| #{dogName dog} #{dogBreed dog}|]However, if I try creating the same page with the getDogNameR route,getDogNameR :: Text -> Handler RepHtmlgetDogNameR maybeDogName = do dog <- runDB $ getBy404 (UniqueDog maybeDogName) defaultLayout $ do [whamlet| #{dogName dog} #{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 RepHtmlgetDogNameR maybeDogName = do dog <- runDB $ getBy404 (UniqueDog maybeDogName) defaultLayout $ do [whamlet| 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 valgetBy404 :: (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#{dogName dog}with#{dogName (entityVal dog)}and similarly for the dogBreed, or, more conveniently, if you deconstruct the returned Entity on bindingEntity{entityVal = dog} <- runDB $ getBy404 ... 这篇关于为什么此代码与Yesod.Persist的get404一起使用，但不是getBy404？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！