使用cats.Contravariant类型类

Question

我试图将我的Contravariant类型类与语法一起应用，但遇到了找不到它的问题。这是我目前所拥有的：

import cats._
import cats.implicits._

object Test {
  type Foo[A] = A => Unit
  private val f: Foo[String] = (_: String) => ()
  implicit val cvar: Contravariant[Foo] = null

  private val FF: Foo[Int] = f.contramap((i: Int) => //error: value contramap is not a member of Foo
    String.valueOf(i)
  )
}

我不明白。我提供了隐式Contravariant[Foo]，但是语法无论如何都不会被应用。怎么了？

Answer 1

错误是我没有扩展ContravariantSyntax。删除隐式和混合在其中的工作如预期：

import cats._
import cats.syntax.ContravariantSyntax

object Test extends ContravariantSyntax{
  type Foo[A] = A => Unit
  private val f: Foo[String] = (_: String) => ()
  implicit val cvar: Contravariant[Foo] = null
  private val FF: Foo[Int] = f.contramap((i: Int) =>  //compiles - Ok!
    String.valueOf(i)
  )
}