json - 为什么 Scala 编译器会区别对待这两个代码块？

Question

这会编译。 (请注意，implicit val subscriptionReads 由 emailIdReads 和 intervalReads 声明在隐式之外，而 subscriptionWrites 由隐式内部声明的Writes组成。)

import play.api.libs.json.Reads._
import play.api.libs.json.Writes._
import play.api.libs.json._
import play.api.libs.functional.syntax._

case class Subscription(emailId: String, interval: String)

object Subscription {

  val emailIdReads: Reads[String]  = (JsPath \ "emailId").read[String]

  val intervalReads: Reads[String] = (JsPath \ "interval").read[String]

  implicit val subscriptionReads: Reads[Subscription] = (
      emailIdReads and
      intervalReads
    )(Subscription.apply _)

  implicit val subscriptionWrites: Writes[Subscription] = (
    (JsPath \ "emailId").write[String] and
    (JsPath \ "interval").write[String]
    )(unlift(Subscription.unapply))
}

然而下面的代码——其中两个 Writes 值被声明在隐式之外——没有编译，产生错误消息:“值 and 不是 play.api.libs.json.Writes[String] 的成员。”

import play.api.libs.json.Reads._
import play.api.libs.json.Writes._
import play.api.libs.json._
import play.api.libs.functional.syntax._

case class Subscription(emailId: String, interval: String)

object Subscription {

  val emailIdReads: Reads[String]  = (JsPath \ "emailId").read[String]

  val intervalReads: Reads[String] = (JsPath \ "interval").read[String]

  implicit val subscriptionReads: Reads[Subscription] = (
      emailIdReads and
      intervalReads
    )(Subscription.apply _)

  val emailIdWrites: Writes[String]  = (JsPath \ "emailId").write[String]

  val intervalWrites: Writes[String] = (JsPath \ "interval").write[String]

  implicit val subscriptionWrites: Writes[Subscription] = (
      emailIdWrites and
      intervalWrites
    )(unlift(Subscription.unapply))

}

我错过了什么？为什么我不能用 equals 代替 equals？此外，为什么我可以使用Reads 而不是Writes。我想理解为什么这行不通。

和的相关定义见play.api.libs.functional。

package play.api.libs.functional

import scala.language.higherKinds

case class ~[A, B](_1: A, _2: B)

trait FunctionalCanBuild[M[_]] {

  def apply[A, B](ma: M[A], mb: M[B]): M[A ~ B]

}

class FunctionalBuilderOps[M[_], A](ma: M[A])(implicit fcb: FunctionalCanBuild[M]) {

  def ~[B](mb: M[B]): FunctionalBuilder[M]#CanBuild2[A, B] = {
    val b = new FunctionalBuilder(fcb)
    new b.CanBuild2[A, B](ma, mb)
  }

  def and[B](mb: M[B]): FunctionalBuilder[M]#CanBuild2[A, B] = this.~(mb)
}

/* Additional arities elided */

Answer 1

我不得不深入挖掘一下，原因实际上是在这个文件中，其中定义了所需的隐式:

https://github.com/playframework/playframework/blob/26a9c7625d6c482abf1c6ecedacff34d670bee01/framework/src/play-json/src/main/scala/play/api/libs/json/Writes.scala

它是为类型 OWrites 而不是 Writes 定义的。你从 .write[String] 得到的实际上是 OWrites[String] 类型。它不起作用的原因是，您明确指定了更宽的 Writes 类型。