关于隐式解析的 NullPointerException

Question

此代码导致 NullPointerException：

import anotherpackage.MyContext
import anotherpackage.builders.aMyContext

case class Context(id: String)
object Context {
  implicit def `ContextHolder to Context`(implicit holder: ContextHolder): Context = holder.context
}
trait ContextHolder {
  def context: Context
}

object anotherpackage {
  case class MyContext(name: String, context: Context) extends ContextHolder

  object builders {
    def aMyContext(name: String)(implicit context: Context = Context("test")): MyContext =
      MyContext(name, context)
  }
}

object SimpleDemo extends App {
  implicit val myContext: MyContext = aMyContext("name")
}

堆栈跟踪：

Exception in thread "main" java.lang.NullPointerException
    at Context$.ContextHolder$u0020to$u0020Context(SimpleDemo.scala:8)
    at SimpleDemo$.delayedEndpoint$SimpleDemo(SimpleDemo.scala:24)
    at SimpleDemo$delayedInit$body.apply(SimpleDemo.scala:23)
    at scala.Function0.apply$mcV$sp(Function0.scala:39)
    at scala.Function0.apply$mcV$sp$(Function0.scala:39)
    at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:17)
    at scala.App.$anonfun$main$adapted(App.scala:80)
    at scala.collection.immutable.List.foreach(List.scala:392)
    at scala.App.main(App.scala:80)
    at scala.App.main$(App.scala:78)
    at SimpleDemo$.main(SimpleDemo.scala:23)
    at SimpleDemo.main(SimpleDemo.scala)

为什么 Scala 将函数 aMyContext 的隐式参数解析为自身：

implicit val myContext: MyContext = aMyContext("name")

是的，aMyContext 获取隐式 Context，对于 MyContext，存在到 Context 的隐式转换。但是它如何将其解析为字段本身？

Answer 1

尝试

implicit val myContext: MyContext = {
  val myContext = null
  aMyContext("name")
}

隐藏隐式名称。

Caching the circe implicitly resolved Encoder/Decoder instances

How can an implicit be unimported from the Scala repl?

[视频] Some Mistakes We Made When Designing Implicits (11:18)

关于隐式解析的 NullPointerException

NullPointerException on implicit resolution

scala

implicit