在 Scala 检查中为运行时类型启用隐式导入。 "could not find implicit value for parameter"

Question

我正在使用 ScalaCheck 库和 ScalaTest

在 Scala 中测试我自己的 home-brewed Monoid 类

在尝试实施 DRY 测试时，我在标题中收到隐式错误：

Error:(16, 12) could not find implicit value for parameter arbA: org.scalacheck.Arbitrary[A]
    forAll { (a: A) =>
       ^

这里是 intAddition 幺半群的实现：

trait Monoid[A] {
  def op(a1: A, a2: A): A
  def zero: A
}

object Monoid {
...
  val intAddition: Monoid[Int] = new Monoid[Int] {
    override def op(a1: Int, a2: Int): Int = a1 + a2
    override def zero: Int = 0
  }
...
}

和测试套件：

import org.fpinscala.monoids.Monoid._
import org.fpinscala.testutils.UnitSpec
import org.scalatest.prop.PropertyChecks
import org.scalacheck.Arbitrary._

import scala.language.implicitConversions

class MonoidSpec extends UnitSpec with PropertyChecks {

  def assertIdentityBehaviour[A](M: Monoid[A]): Unit = {
    import M._
    forAll { (a: A) =>
      op(zero, a) should be(a)
      op(a, zero) should be(a)
    }
  }

  behavior of "intAdditionMonoid"

  it should "obey identity laws" in {
    assertIdentityBehaviour(intAddition)
  }
}

此代码编译但在运行时失败（运行时类型擦除？）。我想在 Scala 中实现什么？

Answer 1

This code compiles

没有；你给出的错误是编译错误。它应该通过添加它抱怨的隐式参数来修复：

def assertIdentityBehaviour[A](M: Monoid[A])(implicit arbA: Arbitrary[A]) = ...
// or equivalently, def assertIdentityBehaviour[A: Arbitrary](M: Monoid[A]) = ...

您仅调用 assertIdentityBehaviour A 参数可用，但错误在其 定义中.

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