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[英]Extending generic type with Ordered trait makes sbt compiler issue 'diverging implicit expansion for type' error
[英]Scalacheck issue with higher kinds : diverging implicit expansion for type Arbitrary
我已經定義了一個monad類型的類,並且試圖通過scalacheck驗證其規律。
我有以下錯誤:
diverging implicit expansion for type org.scalacheck.Arbitrary[(A, Box[B])]
我的scalacheck代碼如下:
class OptionMonadSpec extends MonadSpec[String, String, String, Option](Monad.optionMonad)
abstract class MonadSpec[A, B, C, Box[_] : ClassTag](monad: Monad[Box])
(implicit boxArb: Arbitrary[Box[A]], aArb: Arbitrary[A], bArb: Arbitrary[B], cArb: Arbitrary[C])
extends Properties(s"Monad for ${classTag[Box[_]]}") {
property("left identity") = forAll { (f: (A => Box[B]), a: A) =>
val boxA: Box[A] = monad.pure(a)
monad.flatMap(boxA)(f) == f(a)
}
property("right identity") = forAll { box: Box[A] =>
monad.flatMap(box)(monad.pure) == monad
}
property("associativity") = forAll { (f: (A => Box[B]), g: (B => Box[C]), box: Box[A]) =>
val boxB: Box[B] = monad.flatMap(box)(f)
monad.flatMap(boxB)(g) == monad.flatMap(box) { a =>
val boxB: Box[B] = f(a)
monad.flatMap(boxB)(g)
}
}
}
我是否想念隱式任意類型的東西?
這是我的單子:
trait Monad[Box[_]] extends Functor[Box] {
def pure[A](a: A): Box[A]
def flatMap[A, B](boxA: Box[A])(f: A => Box[B]): Box[B]
}
object Monad {
implicit val optionMonad = new Monad[Option] {
override def pure[A](x: A): Option[A] = Some(x)
override def flatMap[A, B](boxA: Option[A])(f: A => Option[B]) = boxA.flatMap(f)
override def map[A, B](boxA: Option[A])(f: A => B) = boxA.map(f)
}
}
謝謝
您在范圍內有一個隱式Arbitrary[Box[A]]
,但是沒有一個用於Arbitrary[Box[B]]
(Scalacheck需要為A => Box[B]
創建一個)或為Arbitrary[Box[C]]
(稍后會要求)。
一種更原則的方法是創建類似
trait Arbitrary1[F[_]] {
def liftArb[A](arb: Arbitrary[A]): Arbitrary[F[A]]
}
並提供Arbitrary1[Box]
但在調用forAll
時需要更加明確。
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