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我是一个scala新手,一直在试图为我的应用程序开发一个DAO层。以下代码片段概述了模型对象结构及其关联的数据访问对象。又一个scala类型不匹配错误
// Model object base class
abstract class Model[M <: Model[M]] {
val dao: Dao[M]
}
// DAO for each model object, with find, delete, update
abstract class Dao[M <: Model[M]] {
// meta data describing the model object
case class Column(val name:String, val get: M => _)
val columns : Map[String,Column]
}
下面是模型及其关联的DAO的具体使用。
// example simple model object with it's DAO
case class ItemModel (val name:String) extends Model[ItemModel] {
val dao = ItemDao
}
object ItemDao extends Dao[ItemModel] {
val columns = Map("name" -> Column("name", { v:ItemModel => v.name}))
}
现在,当我使用模型对象,它直接关联DAO,生活是美好的。
object Works {
// normal access pattern
def good1(value: ItemModel) = value.name
// even through the DAO
def good2(value: ItemModel) = value.dao.columns("name").get(value)
}
问题是当我试图访问一个对象一般。如果没有编译器抱怨,我无法获得用于传递Model值的方法签名。
// Trouble trying to manipulate base model objects
object Trouble {
// type mismatch; found : value.type (with underlying type test.Model[_]) required: _$2 where type _$2 Test.scala
def bad1(value: Model[_]) = value.dao.columns("name").get(value)
// type mismatch; found : value.type (with underlying type test.Model[_ <: test.Model[_]]) required: _$3 where type _$3 <: test.Model[_]
def bad2(value: Model[_ <: Model[_]]) = value.dao.columns("name").get(value)
// type mismatch; found : value.type (with underlying type X forSome { type X <: models.Model[X] }) required: X where type X <: models.Model[X]
def bad3(value: X forSome {type X <: Model[X]}) = value.dao.columns("name").get(value)
}
任何帮助或指针将不胜感激。
谢谢你的工作。你这个人! – 2012-03-06 18:26:03