使用for表达式从可能为空的JSON值中提取选项 [英] Extract Options from potentially null JSON values using for expression

问题描述

我有一个JSON文档，其中某些值可以为null.在json4s中使用表达式时，如何产生None(无)而不是什么?

I have a JSON document where some values can be null. Using for expressions in json4s, how I can yield None, instead of nothing?

当字段FormattedID或PlanEstimate的值是null时，以下内容将无法产生.

The following will fail to yield when the value for either of the fields FormattedID or PlanEstimate is 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)

推荐答案

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))

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