Yesod /永久实体派生Show [英] Yesod/Persistent entity deriving Show

问题描述

在Yesod书的持久性章节中，给出了一个示例，其中该实体

In the Persistent chapter of the Yesod book, an example is given where this entity

{-# LANGUAGE QuasiQuotes, TypeFamilies, GeneralizedNewtypeDeriving, TemplateHaskell, OverloadedStrings, GADTs #-}
import Database.Persist
import Database.Persist.TH
import Database.Persist.Sqlite
import Control.Monad.IO.Class (liftIO)

mkPersist sqlSettings [persist|
Person
    name String
    age Int
    deriving Show
|]

生成代码

{-# LANGUAGE TypeFamilies, GeneralizedNewtypeDeriving, OverloadedStrings, GADTs #-}
import Database.Persist
import Database.Persist.Store
import Database.Persist.Sqlite
import Database.Persist.GenericSql.Raw (SqlBackend)
import Database.Persist.EntityDef
import Control.Monad.IO.Class (liftIO)
import Control.Applicative

data Person = Person
    { personName :: String
    , personAge :: Int
    }
  deriving (Show, Read, Eq)

type PersonId = Key Person

instance PersistEntity Person where
    -- A Generalized Algebraic Datatype (GADT).
    -- This gives us a type-safe approach to matching fields with
    -- their datatypes.
    data EntityField Person typ where
        PersonId   :: EntityField Person PersonId
        PersonName :: EntityField Person String
        PersonAge  :: EntityField Person Int

    type PersistEntityBackend Person = SqlBackend

    toPersistFields (Person name age) =
        [ SomePersistField name
        , SomePersistField age
        ]

    fromPersistValues [nameValue, ageValue] = Person
        <$> fromPersistValue nameValue
        <*> fromPersistValue ageValue
    fromPersistValues _ = Left "Invalid fromPersistValues input"

    -- Information on each field, used internally to generate SQL statements
    persistFieldDef PersonId = FieldDef
        (HaskellName "Id")
        (DBName "id")
        (FTTypeCon Nothing "PersonId")
        []
    persistFieldDef PersonName = FieldDef
        (HaskellName "name")
        (DBName "name")
        (FTTypeCon Nothing "String")
        []
    persistFieldDef PersonAge = FieldDef
        (HaskellName "age")
        (DBName "age")
        (FTTypeCon Nothing "Int")
        []

为什么将 derive Show 添加到Person实体会生成所有三个类型类（Show，Read，Eq）的派生？我是Haskell和Yesod的新手，因此很抱歉，不过我在任何地方都找不到答案！这只是文档中的错误吗？谢谢！

Why does adding deriving Show to the Person entity generate the derivation of all three typeclasses (Show, Read, Eq)? I'm very new to Haskell and Yesod, so I apologize if it's obvious, but I can't find the answer anywhere! Is it just an error in the documentation? Thanks!

推荐答案

简单：这是本书中的错字：)。如果查看实际生成的代码（带有 -ddump-splices ），您会看到它实际上只是派生 Show 实例。

Easy: it's a typo in the book :). If you look at the actual generated code (with -ddump-splices), you'll see that it's only actually deriving the Show instance.

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