我有一个 JSON 文档,其中一些值可以为 null。在 json4s 中使用 for 表达式,我如何才能产生 None,而不是什么都没有?
当任一字段的值 FormattedID
时,以下将无法产生或 PlanEstimate
是 null
.
val j: json4s.JValue = ...
for {
JObject(list) <- j
JField("FormattedID", JString(id)) <- list
JField("PlanEstimate", JDouble(points)) <- list
} yield (id, points)
例如:
import org.json4s._
import org.json4s.jackson.JsonMethods._
scala> parse("""{
| "FormattedID" : "the id",
| "PlanEstimate" : null
| }""")
res1: org.json4s.JValue = JObject(List((FormattedID,JString(the id)),
(PlanEstimate,JNull)))
scala> for {
| JObject(thing) <- res1
| JField("FormattedID", JString(id)) <- thing
| } yield id
res2: List[String] = List(the id)
scala> for {
| JObject(thing) <- res1
| JField("PlanEstimate", JDouble(points)) <- thing
| } yield points
res3: List[Double] = List()
// Ideally res3 should be List[Option[Double]] = List(None)
最佳答案
scala> object OptExtractors {
|
| // Define a custom extractor for using in for-comprehension.
| // It returns Some[Option[Double]], instead of Option[Double].
| object JDoubleOpt {
| def unapply(e: Any) = e match {
| case d: JDouble => Some(JDouble.unapply(d))
| case _ => Some(None)
| }
| }
| }
defined object OptExtractors
scala>
scala> val j = parse("""{
| "FormattedID" : "the id",
| "PlanEstimate" : null
| }""")
j: org.json4s.JValue = JObject(List((FormattedID,JString(the id)), (PlanEstimate,JNull)))
scala>
scala> import OptExtractors._
import OptExtractors._
scala>
scala> for {
| JObject(list) <- j
| JField("FormattedID", JString(id)) <- list
| JField("PlanEstimate", JDoubleOpt(points)) <- list
| } yield (id, points)
res1: List[(String, Option[Double])] = List((the id,None))
关于json - 使用 for 表达式从可能为空的 JSON 值中提取选项,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/25200005/