[英]Scalacheck issue with higher kinds : diverging implicit expansion for type Arbitrary
I've defined a monad type class and I'm trying to verify its law with scalacheck. 我已经定义了一个monad类型的类,并且试图通过scalacheck验证其规律。
I have the following error : 我有以下错误:
diverging implicit expansion for type org.scalacheck.Arbitrary[(A, Box[B])]
My scalacheck code is the following : 我的scalacheck代码如下:
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)
}
}
}
Did I miss soemthing in the implicit Arbitrary types ? 我是否想念隐式任意类型的东西?
Here is my monad : 这是我的单子:
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)
}
}
Thanks 谢谢
You have an implicit Arbitrary[Box[A]]
in scope, but you don't have one for Arbitrary[Box[B]]
(which Scalacheck needs to create one for A => Box[B]
) or for Arbitrary[Box[C]]
(which it'll ask for afterwards). 您在范围内有一个隐式Arbitrary[Box[A]]
,但是没有一个用于Arbitrary[Box[B]]
(Scalacheck需要为A => Box[B]
创建一个)或为Arbitrary[Box[C]]
(稍后会要求)。
A more principled approach would be to create something like 一种更原则的方法是创建类似
trait Arbitrary1[F[_]] {
def liftArb[A](arb: Arbitrary[A]): Arbitrary[F[A]]
}
and provide Arbitrary1[Box]
but it would require being more explicit when calling forAll
. 并提供Arbitrary1[Box]
但在调用forAll
时需要更加明确。
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