没有同伴的顶级类只能扩展为同名类或由同名同伴组成的块。

Question

我试图按以下方式使用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的内容

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

我已经将新类型的宏移动到package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)

}

但是编译器仍然抱怨：

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的README.md说：

这将扩展为类型和伴生对象定义，因此必须在对象或包对象中定义新类型。

宏可以将类扩展到具有相同名称和伴生对象的其他类中，但据我所知，newtype注释将您的case class转换为同名的对象(以及类似type DbUrl = DbUrl.Type的类型别名)。这种行为(将顶级注释者转换为其他类型的树)是不允许的。但是，如果注释生成了一个类DbUrl，或者是一个同名的对象，那么它就没有问题，但是其他的东西几乎都不能工作。

要解决您的问题，只需将其移动到包对象(或其他范围，只要它不是顶级的)。

编辑:正如Dmytro所指出的，创建的类型不是DbUrl类型，而是类似于大写"T“的type DbUrl = DbUrl.Type，其中DbUrl.Type的定义类似于这样(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