[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
庫和ScalaTest
在Scala中測試自己的自制Monoid
類
嘗試實施DRY測試時,標題中出現隱式錯誤:
Error:(16, 12) could not find implicit value for parameter arbA: org.scalacheck.Arbitrary[A]
forAll { (a: A) =>
^
這是intAddition
Monoid的實現:
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
}
...
}
和測試套件:
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)
}
}
該代碼可以編譯,但在運行時會失敗(運行時類型是否擦除?)。 我想在Scala中實現什么目標?
這段代碼編譯
不會的 您給出的錯誤是編譯錯誤。 應該通過添加它抱怨的隱式參數來解決此問題:
def assertIdentityBehaviour[A](M: Monoid[A])(implicit arbA: Arbitrary[A]) = ...
// or equivalently, def assertIdentityBehaviour[A: Arbitrary](M: Monoid[A]) = ...
您僅使用A
調用 assertIdentityBehaviour
,而其參數可用,但是錯誤在於其定義 。
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.