没有同伴的顶级 class 只能扩展为同名 class 或由同名同伴组成的块

Question

我正在尝试按如下方式使用 https://github.com/estatico/scala-newtype：

import io.estatico.newtype.macros.newtype
import cats._
import io.databaker.env._

@newtype case class DbUrl(v: String)

@newtype case class DbUser(v: String)

@newtype case class DbPw(v: String)

final case class DbParams(url: DbUrl, user: DbUser, pw: DbPw)

trait DbConnector[F[_]] {
  def read(url: DbUrl, user: DbUser, pw: DbPw): F[DbParams]
}


object DbConnector {

  def impl[F[_] : MonadError[*[_], Throwable]](env: Environment[F])
  : DbConnector[F] =
    new LiveDbConnector[F](env)

}

编译器抱怨：

[error] ../db/DbConnector.scala:7:2: top-level class without companion can only expand either into an eponymous class or into a block consisting in eponymous companions
[error] @newtype case class DbUrl(v: String)
[error]  ^
[error] ../db/DbConnector.scala:9:2: top-level class without companion can only expand either into an eponymous class or into a block consisting in eponymous companions
[error] @newtype case class DbUser(v: String)
[error]  ^
[error] ../db/DbConnector.scala:11:2: top-level class without companion can only expand either into an eponymous class or into a block consisting in eponymous companions
[error] @newtype case class DbPw(v: String)
[error]  ^
[error] ../env/Environment.scala:8:2: top-level class without companion can only expand either into an eponymous class or into a block consisting in eponymous companions
[error] @newtype case class EnvValue(v: String)
[error]  ^
[error] ../env/Environment.scala:6:2: top-level class without companion can only expand either into an eponymous class or into a block consisting in eponymous companions
[error] @newtype case class EnvVariable(v: String)

build.sbt内容：

lazy val root = (project in file("."))
  .enablePlugins(JettyPlugin)
  .settings(
    organization := "io.example",
    name := "user-svc",
    version := "0.0.1-SNAPSHOT",
    scalaVersion := "2.13.2",
    mainClass := Some("io.example.Main"),
    containerPort := 9090,
    libraryDependencies ++= Seq(
      "org.http4s" %% "http4s-servlet" % Http4sVersion,
      "org.http4s" %% "http4s-circe" % Http4sVersion,
      "org.http4s" %% "http4s-dsl" % Http4sVersion,
      "io.circe" %% "circe-generic" % CirceVersion,
      "org.scalameta" %% "munit" % MunitVersion % "test",
      "ch.qos.logback" % "logback-classic" % LogbackVersion,
      "io.estatico" %% "newtype" % NewTypeVersion,
      "javax.servlet" % "javax.servlet-api" % ServletVersion % "provided"
    ),
    addCompilerPlugin("org.typelevel" %% "kind-projector" % "0.11.0" cross CrossVersion.full),
    addCompilerPlugin("com.olegpy" %% "better-monadic-for" % "0.3.1"),
  )

scalacOptions ++= Seq(
  "-deprecation",
  "-encoding", "UTF-8",
  "-language:higherKinds",
  "-language:postfixOps",
  "-feature",
  "-Xfatal-warnings",
  "-Ymacro-annotations"
)

我做错了什么？

更新

我已将 newtype 宏移动到包对象中，如下所示：

package object db {
  
  @newtype case class DbUrl(v: String)

  @newtype case class DbUser(v: String)

  @newtype case class DbPw(v: String)

}

但编译器仍然报错：

implicit conversion method opsThis should be enabled
[error] by making the implicit value scala.language.implicitConversions visible.
[error]   @newtype case class DbUser(v: String)

Answer 1

scala-newtype 的 README.md 说：

This expands into a type and companion object definition, so newtypes must be defined in an object or package object.

宏可以扩展 classes 到其他具有相同名称和伴随对象的 classes，但据我所知，newtype 注释将你的 case class 到一个同名的对象中（连同类型别名，如 type DbUrl = DbUrl.Type）。这种行为（将顶级注释者变成某种其他类型的树）是不允许的。如果注解生成了一个 class DbUrl，也许还有一个同名的对象，那么它就没问题了，但几乎所有其他东西都不起作用。

要解决您的问题，您需要做的就是将其移动到一个包对象中（或其他范围，只要它不是顶级的）。

编辑：正如 Dmytro Mitin 指出的那样，创建的类型不是 DbUrl 的类型，而是类似 type DbUrl = DbUrl.Type 的大写“T”，其中 [=17= 的定义] 看起来像这样（我只是从自述文件中复制它）：

type Base = Any { type DbUrl$newtype }
trait Tag extends Any
type Type <: Base with Tag

没有同伴的顶级 class 只能扩展为同名 class 或由同名同伴组成的块

top-level class without companion can only expand either into an eponymous class or into a block consisting in eponymous companions

scala

scala-macros

scala-cats