Scala/Play 的语法和含义！代码示例 [英] Syntax and meaning of a Scala/Play! code sample

问题描述

我有以下 Scala/Play! 代码:

case class Tweet(from: String, text: String)

implicit val tweetReads = (
 (JsPath  "from_user_name").read[String] ~
 (JsPath  "text").read[String]) (Tweet.apply _)

我有几个关于上述代码的语法和含义的问题:

• 在哪个类/对象上调用 ~ 方法?
• Tweet.apply 的参数的类/类型是什么?

• On what class/object is the ~ method invoked on?
• What is the class/type of the argument to Tweet.apply?

编辑 1:完整源代码:

package models

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

case class Tweet(from: String, text: String)

object Tweet {

  implicit val tweetReads = (
      (JsPath  "from_user_name").read[String] ~
      (JsPath  "text").read[String])(Tweet.apply _)

  implicit val tweetWrites = (
    (JsPath  "from").write[String] ~
    (JsPath  "text").write[String])(unlift(Tweet.unapply))
}

推荐答案

循序渐进:

对象 JsPath 上的方法

Method on object JsPath

val path1: JsPath = JsPath  "from_user_name"
val path2: JsPath = JsPath  "text"

方法 read 在 JsPath

val reads1: Reads[String] = path1.read[String]
val reads2: Reads[String] = path2.read[String]

Reads中没有方法~，但是FunctionalBuilderOps中有这样的方法，并且有一个来自M的隐式转换[T] 到 FunctionalBuilderOps[M[_], T] 在 play.api.libs.functional.syntax - toFunctionalBuilderOps.

There is no method ~ in Reads, but there is such method in FunctionalBuilderOps and there is an implicit conversion from M[T] to FunctionalBuilderOps[M[_], T] in play.api.libs.functional.syntax - toFunctionalBuilderOps.

val reads1FunctionalBuilderOps: FunctionalBuilderOps[Reads, String] =
  toFunctionalBuilderOps(reads1)

val canBuild2: CanBuild2[String, String] = reads1FunctionalBuilderOps.~(reads2)

Tweet.apply _ 是使用带有 N 参数的方法创建 FunctionN 的 Scala 语法:

Tweet.apply _ is a scala syntax to create a FunctionN using method with N arguments:

val func: (String, String) => Tweet = Tweet.apply _

CanBuild2[A, B] 中有一个 apply 方法.它接受 (A, B) =>C 并返回 Reads[C](在本例中):

There is an apply method in CanBuild2[A, B]. It accepts (A, B) => C and returns Reads[C] (in this case):

implicit val tweetReads: Reads[Tweet] = canBuild2.apply(func)

其实JsPath#read、toFunctionalBuilderOps 和CanBuild2#apply 方法中也有隐式参数.使用该参数:

Actually there are also implicit parameters in JsPath#read, toFunctionalBuilderOps and CanBuild2#apply methods. With that parameters:

val reads1: Reads[String] = path1.read[String](Reads.StringReads)

...
val reads1FunctionalBuilderOps: FunctionalBuilderOps[Reads, String] =
  toFunctionalBuilderOps(reads1)(
    functionalCanBuildApplicative(
      Reads.applicative(JsResult.applicativeJsResult)))

...
implicit val tweetReads: Reads[Tweet] =
  canBuild2.apply(func)(functorReads)

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