[英]Scala compile error - could not find implicit value for evidence parameter of type
[英]enable implicit import for runtime type in scala check. “could not find implicit value for parameter”
我正在使用ScalaCheck
库和ScalaTest
在Scala中测试自己的自制Monoid
类
尝试实施DRY测试时,标题中出现隐式错误:
Error:(16, 12) could not find implicit value for parameter arbA: org.scalacheck.Arbitrary[A]
forAll { (a: A) =>
^
这是intAddition
Monoid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)
}
}
该代码可以编译,但在运行时会失败(运行时类型是否擦除?)。 我想在Scala中实现什么目标?
这段代码编译
不会的 您给出的错误是编译错误。 应该通过添加它抱怨的隐式参数来解决此问题:
def assertIdentityBehaviour[A](M: Monoid[A])(implicit arbA: Arbitrary[A]) = ...
// or equivalently, def assertIdentityBehaviour[A: Arbitrary](M: Monoid[A]) = ...
您仅使用A
调用 assertIdentityBehaviour
,而其参数可用,但是错误在于其定义 。
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