Question

喂！：）

我不愿意一直跑到stackoverflow论坛来获得一些帮助，以帮助我解决另一个陌生的scala / cats问题。问题是：似乎没有真正有用的文档，只有一些毫无价值的-至少对我来说-复制行。

能否请您指出一些有用的文档？一些 real 代码？不只是REPL中的行？

在这里，我只是尝试使用scala / cats Eq和Show类型类... 我到底在做什么错了？

课程：

package org.hudelundpfusch.utilites.decisions.data

import cats.Show
import cats.kernel.Eq

case class Fact[+T <: Any](name: String, value: T)
  extends Equals {

  override def canEqual(that: Any): Boolean = that match {
    case _: Fact[_] => true
    case _          => false
  }

  override def equals(other: Any): Boolean = other match {
    case that: Fact[_] =>
      (that canEqual this) &&
        name == that.name &&
        value == that.value
    case _ => false
  }

  override def hashCode(): Int = {
    val state = Seq(name, value)
    state.map(_.hashCode()).foldLeft(0)((a, b) => 31 * a + b)
  }

  override def toString = s"Fact(name=$name, value=$value)"

}

case object Fact {

  implicit val factEq: Eq[Fact[_]] = Eq.fromUniversalEquals[Fact[_]] // Neither of this works

//  implicit def factEq: Eq[Fact[_]] = new Eq[Fact[_]] {
//    def eqv(x: Fact[_], y: Fact[_]): Boolean = (x != null, y != null) match {
//      case (true, _)  => x.equals(y)
//      case (_, true)  => y.equals(x)
//      case _          => true
//    }
//  }

  implicit def factShow[T]: Show[Fact[T]] = (t: Fact[T]) => t.toString // Example calls for 'implicit val factShow[Fact[_]]' but that doesn't work

}

最大的惊喜：

package org.hudelundpfusch.utilites.decisions

import cats._
import cats.data._
import cats.syntax._
import cats.implicits._
import cats.implicits.eq
import com.typesafe.scalalogging.LazyLogging
import org.hudelundpfusch.utilites.decisions.data.Fact
import org.hudelundpfusch.utilites.decisions.data.Fact._
// Tried to import everything that came to my mind to make the stuff working

object Fuddel
  extends App
    with LazyLogging {

  logger.info("Let's start to fuddel!")
  this.fuddel()
  logger.info("Enough with fuddling!")

  def fuddel(): Unit = {
    val fact1: Fact[String] = Fact[String]("FactName", "FactValue")
    println(s"${fact1.show}")
    val fact2: Fact[String] = Fact[String]("FactName", "FactValue")
    println(s"${fact2.show}")

    println(s"${fact1.equals(fact2)}")
    println(s"${fact1 == fact2}")
//    println(s"${fact1 === fact2}") // Not resolved...According to the repl example this should work with implicits imported
    println(s"${fact1 eq fact2}") // False? Oh joy! Thanks to the great repl example!
  }

}

所以，请问有没有毫无价值的文档？

预先感谢

有比我更好的一天

亚历克斯

Answer 1

1。。这里编译很好（我删除了程序包名称和日志记录依赖项）：

import cats.Show
import cats.kernel.Eq

case class Fact[+T](name: String, value: T) extends Equals {

  override def canEqual(that: Any): Boolean = that match {
    case _: Fact[_] => true
    case _          => false
  }

  override def equals(other: Any): Boolean = other match {
    case that: Fact[_] => true // TODO: replaced, irrelevant
    case _ => false
  }

  override def hashCode(): Int = {
    val state = Seq(name, value)
    state.map(_.hashCode()).foldLeft(0)((a, b) => 31 * a + b)
  }

  override def toString = s"Fact(name=$name, value=$value)"
}

case object Fact {
  implicit def factEq[A]: Eq[Fact[A]] = Eq.fromUniversalEquals[Fact[A]]
  implicit def factShow[T]: Show[Fact[T]] = (t: Fact[T]) => t.toString
}

请注意，通用量化而不是factEq[A]中的通配符。然后在Fuddel.scala中：

import cats.syntax.show._
import cats.syntax.eq._
import Fact._

object Fuddel
  extends App {

  this.fuddel()

  def fuddel(): Unit = {
    val fact1: Fact[String] = Fact[String]("FactName", "FactValue")
    println(s"${fact1.show}")
    val fact2: Fact[String] = Fact[String]("FactName", "FactValue")
    println(s"${fact2.show}")

    println(s"${fact1.equals(fact2)}")
    println(s"${fact1 == fact2}")
    println(s"${fact1 === fact2}")
    println(s"${fact1 eq fact2}")// must be false, different instances
  }

}

请注意，eq是Scala中每个对象可用的方法，无法覆盖它。

2。。我建议您阅读Welsh, Gurnell "Scala with Cats"。 Scaladoc也很好，但是除非阅读cats库中有关包和隐式组织的介绍性章节，否则您无法有效地导航它。

斯卡拉猫：尝试对“ Eq”特征使用“ ===”语法时存在隐式问题

1 个答案: