Solutions to the ScalaCheck problem that shrinking failing values may generate invalid values, because the constraints of the generator are not respected. This is for using ScalaCheck from within ScalaTest.

Shrinking.scala

import org.scalatest._
import prop._
import org.scalacheck.Arbitrary._
import org.scalacheck.Gen

/**
 * Solutions to the ScalaCheck problem that shrinking failing values may generate 
 * invalid values, because the constraints of the generator are not respected.
 *
 * See also http://stackoverflow.com/questions/20037900/scalacheck-wont-properly-report-the-failing-case
 * and http://code.google.com/p/scalatest/issues/detail?id=14
 */
class Shrinking extends PropSpec with PropertyChecks with ShouldMatchers {

  def odd(i: Int) = i % 2 == 1

  // Old solution - use whenever{} clause within the property

  val odds: Gen[Int] = for {
    n <- arbitrary[Int]
    if (odd(n))
  } yield n

  property("fails with big numbers") {
    forAll(odds) { n =>
      whenever(odd(n)) {
        if (n % 2 == 0) throw new Exception("Argh") // should never happen
        if (n > 1000) (n / 2 + n / 2 should be(n)) else n should be(n)
      }
    }
  }

  // New solution - use suchThat() postcondition on the generator
  // See https://github.com/rickynils/scalacheck/commit/2d92eb6

  val odds2: Gen[Int] = arbitrary[Int].suchThat(odd)

  property("fails with big numbers v2") {
    forAll(odds2) { n =>
      if (n % 2 == 0) throw new Exception("Argh") // should never happen
      if (n > 1000) (n / 2 + n / 2 should be(n)) else n should be(n)
    }
  }

  // Alternative solution - disable shrinking entirely (for Ints)
  // From http://blog.knutwalker.de/2014/01/fun-with-scalatests-propertychecks.html
  // In pure ScalaCheck you can use forAllNoShrink() but this is not exposed in ScalaTest
  {
    import org.scalacheck.Shrink
    implicit val noShrink: Shrink[Int] = Shrink.shrinkAny

    property("fails with big numbers, shrinking disabled") {
      forAll(odds) { n =>
        if (n % 2 == 0) throw new Exception("Argh") // should never happen
        if (n > 1000) (n / 2 + n / 2 should be(n)) else n should be(n)
      }
    }
  }

}