Question

获取：

Error:(20, 5) could not find implicit value for parameter console: example.Console[F]
    new NameThing().program
Error:(20, 5) not enough arguments for constructor NameThing: (implicit evidence$1: cats.Monad[cats.package.Id], implicit console: example.Console[cats.package.Id])example.NameThing[cats.package.Id].
Unspecified value parameter console.
    new NameThing().program

不确定为什么。

我正在尝试了解Tagless Final的单元测试

我有：

case class FullName(first:String, last:String)

trait Console[F[_]] {
  def prompt:F[Unit]
  def read:F[String]
  def display(fullName: FullName):F[Unit]
}

class NameThing[F[_]: Monad](implicit console: Console[F]) {

  def program: F[Unit] = for {
    _ <- console.prompt
    rawName <- console.read
    fullName = parse(rawName)
    _ <- console.display(fullName)
  } yield ()

  def parse(rawName:String):FullName = {
    val parts = rawName.split(" ")
    FullName(parts(0), parts(1))
  }
}

在我的单元测试中，我有：

   case class TestEnv()
   type Test[A] = Reader[TestEnv, A]

   implicit object TestConsole extends Console[Test] {
      override def prompt: Test[Unit] = Reader(_ => Unit)
      override def read: Test[String] =  Reader(_ => "Joe Bloggs")
      override def display(fullName: FullName): Test[Unit] = Reader(_ => Unit)
    }

    new NameThing().program

完整代码：https://bitbucket.org/jameskingconsulting/scala-effects/src/master/

Answer 1

尝试

new NameThing[Test]().program

类型参数似乎被推断为cats.Id而不是Test。

在Scala Tagless Final，隐式，单元测试中找不到隐式

1 个答案: