Arbitrary functions fail when the Gen instance on their return type sometimes fails

[nrinaudo]

This means, among other things, that any Arbitrary instance that relies on Gen.suchThat cannot be used to generate arbitrary functions.

As an example:

import org.scalacheck._

implicit val nonZero: Arbitrary[Int] = Arbitrary(Arbitrary.arbitrary[Int].suchThat(_ != 0))
val prop = Prop.forAll { (f: String => Int) => f("foo"); true }

And then:

scala> prop.check
java.lang.StackOverflowError
  ...

See alexarchambault/scalacheck-shapeless#50 for a far more insidious example.

[non]

So, just to be clear, the generator has to fail many times (by default 100) before the function generator fails. See pureApply and doPureApply here: https://github.com/rickynils/scalacheck/blob/master/src/main/scala/org/scalacheck/Gen.scala#L73.

I think this is the only reasonable behavior for a generator that can be filtered (it is roughly the same behavior as the original QuickCheck's). By contrast, @xuwei-k's ScalaProps library does not even include suchThat or filter, which is a cleaner design (it removes the possibility of generator exhaustion during tests and also ensures function generators can't fail).

My sense is that ScalaCheck-Shapeless wasn't just failing some of the time, but a significant fraction of the time (e.g. 50% or more) which seems like it needed to be fixed anyway.

[alexarchambault]

@non Per the source, doApply is only called once. The retries parameters is useless here.

[alexarchambault]

As @nrinaudo's example illustrates, it only takes one failure to make Gen.function1 fail:

import org.scalacheck._

implicit val nonZero: Arbitrary[Int] = Arbitrary(Arbitrary.arbInt.arbitrary.suchThat(_ != 0))
val prop = Prop.forAll { (f: String => Int) => f("foo"); true }

prop.check

gives

! Exception raised on property evaluation.
> ARG_0: <function1>
> Exception: org.scalacheck.Gen$RetrievalError: couldn't generate value
org.scalacheck.Gen.loop$1(Gen.scala:57)
org.scalacheck.Gen.doPureApply(Gen.scala:58)
org.scalacheck.Gen.pureApply(Gen.scala:72)
org.scalacheck.GenArities$$anonfun$function1$1$$anonfun$1.apply(GenArities.
  scala:15)

[non]

OK! Yes, there is a bug/typo, the method should read:

  def doPureApply(p: Gen.Parameters, seed: Seed, retries: Int = 100): Gen.R[T] = {
    @tailrec def loop(r: Gen.R[T], i: Int): Gen.R[T] = {
      if (r.retrieve.isDefined) r
      else if (i > 0) loop(doApply(p, r.seed), i - 1)
      else throw new Gen.RetrievalError()
    }
    loop(doApply(p, seed), retries)
  }

I've confirmed that with this change the property passes.

I'll open a PR to fix this. Thanks for being persistent and reporting it!

[peter-empen]

I'd appreciate your publishing this fix for Scala 2.11. Thanks a lot.