这将编译。 (请注意，implicit val subscriptionReads由外部隐式声明为的emailIdReads和intervalReads组成，而subscriptionWrites由隐含的内部声明为的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))
}
但是下面的代码-在之外将两个Writes值声明为的隐式对象-不会编译，从而产生错误消息:“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))
}
我想念什么？为什么我不能用平等代替平等？此外，为什么我可以使用Reads而不是Writes来做到这一点。我想了解为什么这不起作用。and的相关定义在play.api.libs.functional中列出。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 */
最佳答案
我必须进行一点挖掘，原因实际上是在此文件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OWrites Not Writes类型定义的。从.write [String]中得到的实际上是OWrites [String]类型。它不起作用的原因是，您正在显式指定更宽的类型Writes。