scala - 重新分配给 Scala 中的 val

Question

我正在 Scala 中进行培训练习并收到此 val 重新分配错误。我看不到在哪里将新值重新分配给 val

class personTest
{
  val alf = Person("Alf", 30, List(EmailAddress("alf.kristian@gmail.com")))
  val fredrik = Person("Fredrik", 33, List(EmailAddress("fredrik@vraalsen.no"), EmailAddress("fvr@knowit.no")))
  val johannes = Person("Johannes", 0, Nil)

  val persons = List(alf, fredrik, johannes)

   @Test
  def testNameToEmailAddress
  {
    // Create a map from each persons name to their e-mail addresses,
    // filtering out persons without e-mail addresses
    // Hint: First filter list, then use foldLeft to accumulate...
    val emptyMap: Map[String, List[EmailAddress]] = Map()

    val nameToEmail = persons.filter(_.emailAddresses.length>0).foldLeft(emptyMap)((b,p)=> b+=p.name->p.emailAddresses)

    assertEquals(Map(alf.name -> alf.emailAddresses, fredrik.name -> fredrik.emailAddresses), nameToEmail)
  }

}

我收到这个错误

error: reassignment to val
val nameToEmail = persons.filter(_.emailAddresses.length>0).foldLeft(emptyMap)((b,p)=> b+=p.name->p.emailAddresses)

Answer 1

b这是闭包参数的名称本身就是 val ，不能重新分配。
foldLeft通过将一次闭包调用的返回值作为参数传递来工作 b到下一个，所以你需要做的就是返回 b + (p.name->p.emailAddresses) . (不要忘记括号的优先级。)