Scala、cats、http4s - 无法识别来自 Http4s 的 <+> 符号

Question

我创建了两条Http4s 路由：

class FirstRoutes[F[_] : Async](service: FirstService[F]) extends Http4sDsl[F] {        
  def routes: HttpRoutes[F] = HttpRoutes.of[F] {
        //... some code
  }
}   

class SecondRoutes[F[_] : Async] extends Http4sDsl[F] {            
    def routes: HttpRoutes[F] = HttpRoutes.of[F] {
    //... some code
    }
}

现在在我的main 方法中，我想这样称呼这条路线：

override def run(args: List[String]): IO[ExitCode] =
    for {
      _ <- {
        val app = {
          //...
          val firstRoutes = new FirstRoutes[F](someService)
          val secondRoutes = new SecondRoutes[F]
          (firstRoutes.routes <+> secondRoutes.routes).orNotFound
        }

但是当我编译这段代码时，我得到了一个错误：

Error:(26, 33) value <+> is not a member of org.http4s.HttpRoutes[Server.F]
          (firstRoutes.routes <+> secondRoutes.routes).orNotFound

这对我来说很奇怪，因为当我在路由类上使用 ctrl+space 时，我通常可以使用这个 <+> 符号，而且我有很好的导入：

import cats.effect._
import cats.data._
import org.http4s.server.blaze.BlazeServerBuilder
import cats.effect._
import cats.implicits._

找不到解决此问题的方法并使用<+> 调用路由类。也许是intellij问题？有人可以帮我吗？

Answer 1

为 build.sbt 添加这个 scala 选项

scalacOptions += "-Ypartial-unification"

Answer 2

也许你应该阅读http://eed3si9n.com/herding-cats/import-guide.html

https://blog.softwaremill.com/9-tips-about-using-cats-in-scala-you-might-want-to-know-e1bafd365f88 建议 2)

<+> 来自cats.syntax.semigroupk._。

以下代码编译

import cats.effect.{Async, ExitCode, IO}
import cats.syntax.semigroupk._
import org.http4s.HttpRoutes
import org.http4s.dsl.Http4sDsl
import org.http4s.syntax.kleisli._
import scala.language.higherKinds

class App {

  class FirstRoutes[F[_] : Async](service: FirstService[F]) extends Http4sDsl[F] {
    def routes: HttpRoutes[F] = HttpRoutes.of[F] {
      ???
    }
  }

  class SecondRoutes[F[_] : Async] extends Http4sDsl[F] {
    def routes: HttpRoutes[F] = HttpRoutes.of[F] {
      ???
    }
  }

  trait FirstService[F[_]]

  /*override*/ def run[F[_]: Async](args: List[String]): IO[ExitCode] = {
    val someService: FirstService[F] = ???
    for {
      _ <- {
        val app = {
          //...
          val firstRoutes = new FirstRoutes[F](someService)
          val secondRoutes = new SecondRoutes[F]
          (firstRoutes.routes <+> secondRoutes.routes).orNotFound
        }
        IO(???)
      }
    } yield ExitCode(1)
  }
}