SAM-convertible type and Function type

build.properties

sbt.version=1.1.0

build.sbt

organization := "com.github.kmizu"
name := "play-json-example"
version := "0.1-SNAPSHOT"
scalaVersion := "2.12.4"
libraryDependencies += "com.typesafe.play" %% "play-json" % "2.6.7"

Main.scala

package com.github.kmizu.play_json_example
import play.api.libs.json._

object Main {
  implicit class RichWrites[A](w: Writes[A]) {
    def ignore(ignoreFields: String*): Writes[A] = {
      w.transform{
        case JsObject(fields) => JsObject(fields.filterNot {
          case (k, _) => ignoreFields.contains(k)
        })
        case json => json
      }
    }
  }
}