scala - 正数的性质不应缩减为负数

Question

在 ScalaCheck 中，我有一个属性测试，它使用一个正整数生成器，当它失败时 ScalaCheck 将收缩为一个非正值。

收缩应该有助于找到最少的失败案例。缩小到所需范围之外的值是令人困惑和无益的。这是一个已知错误，请参阅 ScalaCheck issue #129 Gen.suchThat not respected by shrinking

是否可以在范围内定义我自己的收缩实例，以便它只会收缩为正整数？

例子

这是一个最小的属性测试:

class ShrinkProp extends Properties("Shrink") {
  property("posNum[Int]") = {
    Prop.forAll(Gen.posNum[Int]) { _: Int =>
      Prop.falsified
    }
  }
}

这通常会导致 ScalaCheck 将参数缩小为零:

[info] Done compiling.[info] ! Shrink.posNum[Int]: Falsified after 0 passed tests.[info] > ARG_0: 0[info] > ARG_0_ORIGINAL: 1[info] Failed: Total 1, Failed 1, Errors 0, Passed 0

Or worse, it could on occasion shrink to a negative value:

[info] ! Shrink.posNum[Int]: Falsified after 5 passed tests.[info] > ARG_0: -1[info] > ARG_0_ORIGINAL: 3[info] Failed: Total 1, Failed 1, Errors 0, Passed 0

Disable shrinking

One solution is shutting off shrinking with forAllNoShrink:

class ShrinkProp extends Properties("Shrink") {
  property("posNum[Int]") = {
    Prop.forAllNoShrink(Gen.posNum[Int]) { _: Int =>
      Prop.falsified
    }
  }
}

结果是没有收缩:

[info] ! Shrink.posNum[Int]: Falsified after 0 passed tests.[info] > ARG_0: 1[info] Failed: Total 1, Failed 1, Errors 0, Passed 0

Adding guards

Another alternative is adding a guard to the test, so that he shrunk values or just skipped:

import Prop.BooleanOperators

class ShrinkProp extends Properties("Shrink") {
  property("posNum[Int]") = {
    Prop.forAll(Gen.posNum[Int]) { x: Int =>
      (x >=  1) ==> Prop.falsified
    }
  }
}

是否有替代方法来禁用收缩和添加守卫？

Answer 1

ScalaCheck 中没有正整数收缩器。您必须自己编写。

概览

Shrink 需要定义为属性测试范围内的隐式。然后 Prop.forAll 将找到正确的 Shrink 类(如果它在范围内并且具有未通过测试的值的适当类型签名)。

从根本上说，Shrink 实例是一个将失败值 x 转换为“收缩”值流的函数。它的类型签名大致是:

trait Shrink[T] {
  def shrink(x: T): Stream[T]
}

可以用伴生对象的apply方法定义一个Shrink，大致是这样的:

object Shrink {
  def apply[T](s: T => Stream[T]): Shrink[T] = {
    new Shrink[T] {
      def shrink(x: T): Stream[T] = s(x)
    }
  }
}

答案:收缩正整数

正整数的收缩器是一个Stream，它通过将值减半来收缩以通过二进制搜索找到最小的失败案例，但在达到零之前停止:

class ShrinkProp extends Properties("Shrink") {

  implicit val posIntShrinker: Shrink[Int] = Shrink { x: Int =>
    Stream.iterate(x / 2) { x: Int =>
      x / 2
    }.takeWhile { x: Int =>
      x > 0 // Avoid zero.
    }
  }

  property("posNum[Int]") = {
    Prop.forAll(Gen.posNum[Int]) { _: Int =>
      Prop.falsified
    }
  }
}

证明失败有效:

[info] ! Shrink.posNum[Int]: Falsified after 6 passed tests.[info] > ARG_0: 2[info] > ARG_0_ORIGINAL: 4[info] Failed: Total 1, Failed 1, Errors 0, Passed 0

Even better, you could write a property to verify your shrinker behaves as it should:

property("posIntShrinker") = {
  Prop.forAll { x: Int =>
    val shrunk = Shrink.shrink(x)
    Prop.atLeastOne(
      (x >= 2) ==> shrunk.size > 0,
      (x <= 1) ==> shrunk.isEmpty
    )
  }
}

[info] + Shrink.posIntShrinker: OK, passed 100 tests.[info] Failed: Total 1, Failed 0, Errors 0, Passed 1

写一个通用的正数 Shrink 会很好，它可以收缩其他类型的数字，比如 Long、浮点类型和 BigDecimal.