如何覆盖 ScalaCheck 的一些生成器以强制（自动）生成精炼类型？ 仅非空列表，例如

Question

I have a quite big structure of case classes and somewhere deep inside this structure I have fields which I want to refine, for example, make lists non-empty. Is it possible to tell ScalaCheck to make those lists non-empty using automatic derivation from scalacheck-magnolia project (without providing each field specifically)? Example:

import com.mrdziuban.ScalacheckMagnolia.deriveArbitrary
import org.scalacheck.Arbitrary
import org.scalacheck.Gen

case class A(b: B, c: C)
case class B(list: List[Long])
case class C(list: List[Long])

// I've tried:
def genNEL[T: Gen]: Gen[List[T]] = Gen.nonEmptyListOf(implicitly[Gen[T]])
implicit val deriveNEL = Arbitrary(genNEL)

implicit val deriveA = implicitly[Arbitrary[A]](deriveArbitrary)

But it's didn't worked out.

Answer 1

I'm not sure how to be generic, since I'm not familiar with getting automatic derivation for Arbitrary with scalacheck-magnolia. It seems like scalacheck-magnolia is good for deriving an Arbitrary for case classes, but maybe not for containers (lists, vectors, arrays, etc.).

If you want to just use plain ScalaCheck, you could just define the implicit Arbitrary for A yourself. Doing it by hand is some extra boilerplate, but it has the benefit that you have more control if you want to use different generators for different parts of your data structure.

Here's an example where an Arbitrary list of longs is non-empty by default, but is empty for B .

implicit val listOfLong =
  Arbitrary(Gen.nonEmptyListOf(Arbitrary.arbitrary[Long]))


implicit val arbC = Arbitrary {
  Gen.resultOf(C)
}

implicit val arbB = Arbitrary {
  implicit val listOfLong =
    Arbitrary(Gen.listOf(Arbitrary.arbitrary[Long]))
  Gen.resultOf(B)
}

implicit val arbA = Arbitrary {
  Gen.resultOf(A)
}

property("arbitrary[A]") = {
  Prop.forAll { a: A =>
    a.b.list.size >= 0 && a.c.list.size > 0
  }
}