6

在Yesod book 的 Persistent 章节中，给出了一个示例，其中该实体

{-# 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")
        []

为什么添加deriving Show到 Person 实体会生成所有三个类型类的派生(Show, Read, Eq)？我对 Haskell 和 Yesod 很陌生，所以如果很明显，我很抱歉，但我在任何地方都找不到答案！这只是文档中的错误吗？谢谢！

4

1 回答 1

2

简单：这是书中的错字:)。如果您查看实际生成的代码（带有-ddump-splices），您会发现它实际上只是派生Show实例。

于 2012-12-31T06:55:38.503 回答