无法找到参数半群的隐式值

Question

我只是尝试编译以下代码：

import cats.Monoid
import cats.instances.boolean._ // for Monoid
import cats.instances.int._ // for Monoid
import cats.instances.list._ // for Monoid
import cats.instances.string._ // for Monoid
import cats.syntax.apply._ // for imapN
import cats.syntax.semigroup._

case class Cat2(name: String, yearOfBirth: Int, favoriteFoods: List[String])

object FancyFunctor {

  val tupleToCat: (String, Int, List[String]) => Cat2 =
    Cat2.apply _

  val catToTuple: Cat2 => (String, Int, List[String]) =
    cat => (cat.name, cat.yearOfBirth, cat.favoriteFoods)

  implicit val catMonoid: Monoid[Cat2] = (
    Monoid[String],
    Monoid[Int],
    Monoid[List[String]]
  ).imapN(tupleToCat)(catToTuple)

  def main(args: Array[String]): Unit = {

    val garfield = Cat2("Garfield", 1978, List("Lasagne"))
    val heathcliff = Cat2("Heathcliff", 1988, List("Junk Food"))

    val a = garfield |+| heathcliff

  }

}

我收到了错误消息：

[info] Compiling 1 Scala source to /home/developer/Desktop/scala/catssemigroupal/target/scala-2.12/classes ...
[error] /home/developer/Desktop/scala/catssemigroupal/src/main/scala/io/khinkali/FancyFunctor.scala:25:22: could not find implicit value for parameter semigroupal: cats.Semigroupal[cats.kernel.Monoid]
[error]   ).imapN(tupleToCat)(catToTuple)
[error]                      ^
[error] one error found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 1 s, completed Feb 24, 2018, 10:18:01 PM

我究竟做错了什么？

Answer 1

使用 cats 1.0.1 进行编译：

import cats.Monoid
import cats.instances.boolean._ // for Monoid
import cats.instances.int._ // for Monoid
import cats.instances.list._ // for Monoid
import cats.instances.string._ // for Monoid
import cats.syntax.apply._ // for imapN
import cats.syntax.semigroup._ // for |+|
import cats.instances.invariant._ // for  catsSemigroupalForMonoid: InvariantSemigroupal[Monoid] 

object FancyFunctor {

  /*
  val tupleToCat: (String, Int, List[String]) => Cat2 =
    Cat2.apply _

  val catToTuple: Cat2 => (String, Int, List[String]) =
    cat => (cat.name, cat.yearOfBirth, cat.favoriteFoods)

  */

  case class Cat2(name: String, yearOfBirth: Int, favoriteFoods: List[String])
  implicit val catMonoid: Monoid[Cat2] = (
    Monoid[String],
    Monoid[Int],
    Monoid[List[String]]
  ).imapN(Cat2.apply)(c => Cat2.unapply(c).get)

  def main(args: Array[String]): Unit = {

    val garfield = Cat2("Garfield", 1978, List("Lasagne"))
    val heathcliff = Cat2("Heathcliff", 1988, List("Junk Food"))

    val a = garfield |+| heathcliff

  }

}

如何仅使用浏览器（不使用 IDE）快速查找：

• 在文档中搜索Semigroupal（缺少的类型类）
• 在结果页面上Monoid使用浏览器的文本搜索进行搜索
• 单击返回的隐式方法Semigroupal[Monoid]，
• 点击定义类的链接：InvariantMonoidalInstances
• 寻找线性子类，选择比“全部”更精确的东西

提示：过度杀伤方法cats.implicits._适用于您的代码。

作为奖励：您不必重新定义Cat2.apply和Cat2.unapply，它们会自动为每个case类提供。