Scala 类类型参数上的未成形:透镜编译错误
我为所有case类定义了一个基本特征,并打算更改fieldse.g。createTime,updateTime在基本特性中通过不成形镜头 基于以下代码Scala 类类型参数上的未成形:透镜编译错误,scala,shapeless,lenses,Scala,Shapeless,Lenses,我为所有case类定义了一个基本特征,并打算更改fieldse.g。createTime,updateTime在基本特性中通过不成形镜头 基于以下代码 // LensTest.scala package demo import java.sql.Timestamp import shapeless._ trait BaseEntity { def id: Int def createTime: Timestamp def updateTime: Timestamp } ca
// LensTest.scala
package demo
import java.sql.Timestamp
import shapeless._
trait BaseEntity {
def id: Int
def createTime: Timestamp
def updateTime: Timestamp
}
case class User(id: Int, name: String, age: Int, createTime: Timestamp, updateTime: Timestamp) extends BaseEntity
object Util {
val witCreateTime = Witness('createTime)
type tpeCreateTime = witCreateTime.T
def updateCreateTime[A](a: A, time: Timestamp)(implicit mkLens: MkFieldLens.Aux[A, tpeCreateTime, Timestamp]): A = {
val lenCreateTime = mkLens()
lenCreateTime.set(a)(time)
}
}
/**
* LensTest
*
*/
object LensTest extends App {
val user = User(1, "Tom", 18, null, null)
val now = new Timestamp(System.currentTimeMillis())
def normal_method_call_is_ok(): Unit = {
println(Util.updateCreateTime(user, now))
}
normal_method_call_is_ok()
// However, in the below `BaseRepo` definition,
// Compiler gives an compile error:
//
// could not find implicit value for parameter mkLens: shapeless.MkFieldLens.Aux[E,demo.Util.tpeCreateTime,java.sql.Timestamp]
//
//
// Why? And how to fixed it?
}
class BaseRepo[E <: BaseEntity] {
def beforeUpdate(e: E): E = {
val now = new Timestamp(System.currentTimeMillis())
Util.updateCreateTime(e, now)
}
}
正如我在代码中被注释的那样。我写了一个updateCreateTime函数来实现我的目标。当我在普通函数中调用它时,它就工作了。然而,当我把它放在有类型参数的BaseRepo类中时,它编译失败了。
我不知道为什么会发生这种情况,以及如何修复它?任何帮助都是感激的 为编译器提供它所需要的:
class BaseRepo[E <: BaseEntity] {
def beforeUpdate(e: E)(implicit in: MkFieldLens.Aux[E, tpeCreateTime, Timestamp]): E = {
val now = new Timestamp(System.currentTimeMillis())
Util.updateCreateTime(e, now)
}
}
object LensTest extends App {
val user = User(1, "Tom", 18, null, null)
val now = new Timestamp(System.currentTimeMillis())
def normal_method_call_is_ok(): Unit = {
println(Util.updateCreateTime(user, now))
}
//normal_method_call_is_ok()
val _user = new BaseRepo[User]
println(_user.beforeUpdate(user))
// However, in the below `BaseRepo` definition,
// Compiler gives an compile error:
//
// could not find implicit value for parameter mkLens: shapeless.MkFieldLens.Aux[E,demo.Util.tpeCreateTime,java.sql.Timestamp]
//
//
// Why? And how to fixed it?
}
给出:
用户1,汤姆,182018-05-10 11:39:50.287,空!!非常感谢你!!