Scala编译错误:"没有隐式视图可用"和"分散隐式扩展"

Question

 def MyFun(result: ListBuffer[(String, DateTime, List[(String, Int)])]):
 String = {
 val json =
  (result.map {
    item => (
      ("subject" -> item._1) ~
        ("time" -> item._2) ~
        ("student" -> item._3.map {
          student_description=> (
            ("name" -> lb_result._1) ~
              ("age" -> lb_result._2) 
            )
        })
      )
    }
    )
    val resultFormat = compact(render(json))
    resultFormat  
}

错误1:org.joda.time.DateTime => org.json4s.JsonAST.JValue没有隐式视图.("subject" - > item._1)〜

错误2:类型Nothing => org.json4s.JsonAST.JValue的分散隐式扩展从特征中的方法seq2jvalue开始JSONDSL val resultFormat = compact(render(json))

Answer 1

我暗示了json4s-ext的joda-time支持,但只导入这个子模块不能解决你的问题.

JsonDSL用于创建JValues,序列化器仅用于转换JValues为JSON,反之亦然(序列化和反序列化).

如果我们尝试用以下代码创建一个更简单的json对象DateTime:

val jvalue = ("subj" -> "foo") ~ ("time" -> DateTime.now)

我们得到了同样的错误:

error: No implicit view available from org.joda.time.DateTime => org.json4s.JsonAST.JValue.

就像我说的,当我们使用JsonDSL创建时,不使用DateTime来自json4s-ext的序列化器JValues.

您可以创建隐式函数DateTime => JValue或执行类似DateTime.now.getMillis或DateTime.now.toString分别创建a JInt或a的操作JString,但如果joda时间序列化器已经存在,为什么我们会重新发明轮子？

我们可以引入一些case类来保存数据result,然后json4s可以为我们序列化它们:

import scala.collection.mutable.ListBuffer

import com.github.nscala_time.time.Imports._

import org.json4s._
import org.json4s.JsonDSL._
import org.json4s.native.JsonMethods._
import org.json4s.native.Serialization
import org.json4s.native.Serialization.{read, write}

implicit val formats = 
  Serialization.formats(NoTypeHints) ++ org.json4s.ext.JodaTimeSerializers.all

case class Lesson(subject: String, time: org.joda.time.DateTime, students: List[Student])
case class Student(name: String, age: Int)

val result = ListBuffer(("subj", DateTime.now, ("Alice", 20) :: Nil))
val lessons =  result.map { case (subj, time, students) => 
  Lesson(subj, time, students.map(Student.tupled))
}

write(lessons)
// String = [{"subject":"subj","time":"2015-09-09T11:22:59.762Z","students":[{"name":"Alice","age":20}]}]

请注意,你仍然需要像Andreas Neumann解释的那样添加json4s-ext.